PART III.—THE METAMORPHOSES OF INSECTS

We have seen that the embryo rapidly passes through extraordinary changes of form, and now, after hatching, especially in the insects with a complete metamorphosis, the animal continues to undergo striking changes in form, in adaptation to different modes of life.

The life of a winged insect, such as a butterfly, fly, or bee, may be divided into four stages: the embryo, or egg state, the larva, pupa, and imago,—the term _metamorphosis_ being applied to the changes after birth, or post-embryonic stages of life. The transformations of the more specialized orders of insects involve wonderful changes of form, which are only paralleled in other types of animals by the metamorphoses of the echinoderms, of certain worms, and of the Crustacea, as well as by those of the frog. An insect, such as a butterfly or bee, during its post-embryonic life lives, so to speak, three different lives, having distinct bodily structures and existing under quite dissimilar surroundings and habits; so that a caterpillar is practically a different animal from the pupa, and the latter from the imago, with different organs, the appendages and other structures being so modified as to be, so far as regards their functions, radically different. These changes of functions or of habits have also been plainly enough the exciting cause of the divergency in structure of what fundamentally is one and the same organ, the change having been brought about by adaptation of the same organs to quite different uses.

The changes are not only observable in the body and its appendages, but also in the internal organs, and consequently are both structural and physiological. The term _larva_, as applied to the first stage of animals, is a very variable and indefinite one, that of insects in general being a much more highly organized animal than the larva of a worm, starfish, or crustacean.

_a._ The nymph as distinguished from the larval stage

As there is no marked difference between the different stages of the young in the insects with an incomplete metamorphosis (Heterometabola), the chief difference being the possession of the rudiments of wings and the absence of a resting stage, the terms _larva_ and _pupa_ are in reality scarcely applicable to them, and we much prefer the term _nymph_, first proposed by Lamarck for the active “pupa” of Orthoptera, Hemiptera, the Odonata and Ephemeridæ, and adopted in part by many. Indeed, in the more generalized and older orders, the larval and pupal stages are not differentiated, though the term _larval_, in its general sense, will probably always be used; just as we speak of the larval stages of worms, echinoderms, or Crustacea.

Eaton in his elaborate work on the Ephemeridæ employs the term _nymph_ to designate all the aquatic or early stages in the development of the young after hatching, and he urges that the old-fashioned usage of _larva_ and _pupa_ seem scarcely worth retention. “Nymphs are young which live an active life, quitting the egg at a tolerably advanced stage of morphological development and having the mouth-parts formed after the same main type of construction as those of the adult insect.” The word _nymph_ is used in the same sense by McLachlan, and by Cabot. Calvert also applies the term _nymph_ “to the stage of odonate existence between the egg and the transformation into the imago.” On the other hand, Brauer applies the term _nymph_ to the pupa of holometabolous insects. For larval Hyatt proposes the term _nepionic_.

_b._ Stages or stadia of metamorphosis

The intervals or periods between the moults or ecdyses of caterpillars and other eruciform larvæ are called stages or _stadia_; thus, as most caterpillars moult four times, we have five stages or stadia, or stage (stadium) I to V. As observed by Sharp, there is, unfortunately, no term in general use to express the form of the insect at the various stadia; “entomologists say, ‘the form assumed at the first moult,’ and so on.” Hence he adopts a term suggested by Fischer,[86] and calls the insect as it appears after leaving the egg the first _instar_, and what it is after the first moult the second instar, and so on; hence the pupa, or chrysalis, which assumed that condition after moulting five times would be the sixth instar, and the butterfly itself would be the seventh instar.

_c._ Ametabolous and metabolous stages

In the Synaptera development is direct, the young differing neither in form, structure, or habits from the adult. Hence they are said to be _ametabolous_. Since there is an absence of even a tendency to a partial metamorphosis, it is evident that the insects have not inherited a tendency to undergo a transformation, but that it is an adaptation induced in the hexapod type after the first winged insects appeared, and which became more marked in the more specialized insects and at a period comparatively late in geological history, _i.e._ perhaps at or soon after the beginning of the Carboniferous period.[87]

The transformations of the pterygote insects vary greatly in degree, and it is difficult to draw the line between the grades. Those in which the adults differ from the freshly hatched young only or mainly in having wings are generally said to have an incomplete or gradual metamorphosis. There is no inactive, resting, or pupal stage, and the wings are acquired only after successive moults. Insects with an incomplete metamorphosis are the Orthoptera, Dermaptera, Platyptera (Mallophaga, Plecoptera, Corrodentia, Embidæ), Ephemeridæ, Odonata, Thysanoptera, and Hemiptera, with the exception of the male Coccidæ, in which there is a resting or sub-nymph stage. As regards the number of moults in the Synaptera, Grassi states that in Campodea there is a single fragmentary ecdysis, while Sommers tells us that _Macrotoma plumbea_ sheds its skin throughout life, even after attaining its full size.

As an example of the partial metamorphosis of the hemimetabolous insects we may select that of the locust, in which there are five moults and six stages (instars), as seen in Fig. 558, five of which are nymphal. In the first two stages there are no rudiments of wings, these appearing after the second moult. Besides the acquisition of wings there are slight differences after each moult, both in structure and color, besides size, so that we may always recognize the comparative age and the particular stage of growth of any individual.[88]

We have watched the development of _Melanoplus spretus_ from the egg to the imago, and examined thousands of specimens which show the six stages. On the other hand, European authors differ as to whether there are three, four, or five moults in the migratory locust.[89] It is not improbable that, as is the case with many other insects, the number of moults may vary according to the temperature and food, variation in these agencies causing either retardation or rapidity in development.

Those with a complete metamorphosis are said to be _metabolous_ or _holometabolous_. (Lang.)

Leach[90] in 1815 gave the name of Ametabola to insects without, and Metabola to insects with a metamorphosis.

Latreille (1831) called insects with an incomplete metamorphosis _homotenous_ (which means similar to the end of life), and those with a complete metamorphosis, _polymorphous_. For the different degrees of metamorphosis of insects he employed two terms: for the incomplete degree, _metamorphosis dimidia_, and for the total or pupal, _metamorphosis perfecta_.

Westwood in his Introduction to the Modern Classification of Insects (1839), taking into account the relation of the larva with the imago, divided insects into two divisions: the _Heteromorpha_, or those in which there is no resemblance between the parent and its offspring, and _Homomorpha_, in which the larva resembles the imago, except in the absence of wings.

From the point of view of the degree of metamorphosis, insects have been divided into _Heterometabola_ and _Metabola_.

I. _Heterometabola._—This group may be divided as follows:

1. _Manometabola_,[91] embracing those forms with a slight or gradual metamorphosis, but which are active in all the stages, without any resting stage. The orders passing through this degree of metamorphosis are the following: Orthoptera, Dermaptera, Platyptera, Thysanoptera, and Hemiptera (Coccidæ excepted).

In all these groups, the only external differences of importance between the freshly hatched nymph and the adult is the presence of wings. The chief difference internally is the complete development of the sexual glands.

It should be observed, however, that in the last nymph stage of the Thysanoptera the articulations of the limbs are enveloped by a membrane and the wings enclosed in short fixed sheaths; the antennæ are turned back on the head, and the insect, though it moves about, is much more sluggish than in the other state. (Haliday.) Hence here we have a close approach to the following degree.

2. _Heremetabola_,[92] including those forms with a gradual though slight or incomplete metamorphosis, but with a quiescent or resting stage at the close of the nymph life. Lang has emphasized this stage, calling attention to the fact that the fore legs of the nymph of the 17–year Cicada, which lives underground on the roots of trees, are thick and adapted for digging. The transition from the nymph to the winged adult is signalized by the decided change in form of the fore legs, as well as by the acquisition of the wings. “The last larval stage is, then, what is called _quiescent_, _i.e._ the organization of the imago develops within the chrysalis at the expense of the accumulated reserve material.” (Lang.) There seems to be a resting stage, when the insect does not perhaps suck the sap from the roots, and awaits in its chamber its approaching change to the imago; but we should scarcely apply the term _pupa_ to this stage, though the antennæ of the freshly hatched larva are larger and longer than in the fully grown nymph and are distinctly 8–jointed.

3. _Hemimetabola._—In this division, so named by Brauer, the changes are more marked, though there is no truly inactive pupa-like stage. The orders are Perlaria (Plecoptera), Odonata, and Plectoptera (Ephemeridæ). The freshly hatched nymphs of these three groups are much alike in shape, that of Perlidæ, and indeed most of the Platyptera, being more generalized, unless we except that of Chloëon; all closely recall Campodea, and are therefore in the Campodea-stage. These nymphs are indeed more generalized than the freshly hatched nymph of Blattidæ, or any other of the orders mentioned except the Platyptera, to which perlids belong. They all have feet, and the body is more or less flattened. (Fig. 560.)

II. _Holometabola._—In this division we have for the first time a true larva, and a pupa stage as distinguished from the imago. Moreover, the insect at each stage is distinguished by radical differences in form, surroundings, and in the nature of the food, while the pupa is inactive, usually immovable, and incapable of taking any food, and is often protected by a cocoon spun by the larva. The holometabolous orders are the Neuroptera, Coleoptera, Mecoptera, Trichoptera, Lepidoptera, Siphonaptera, Diptera, and Hymenoptera.

As we have among worms, echinoderms, and Crustacea certain exceptional species in a metamorphic group whose metamorphosis is suppressed, their development being direct, so there is in pterygote insects, though in a very much less degree, cases of direct development. In the wingless cockroaches such as Pseudoglomeris, etc., of the tribe of Periphæriides, in some of which, however, the males are winged, and in the Hemiptera, occur wingless forms such as the lice and bed-bug. The Mallophaga are all wingless, while certain Dermaptera (Chelidura, Anisolabis) are also apterous. The absence of wings in such cases is due to disuse from parasitism, or to a life under stones or in cracks and fissures, where the insects are driven to avoid their enemies, and hence do not need wings. The growth of wings and consequently the development of a metamorphosis is suppressed, so that, as Lang says, “in contrast to the original ametabola of the Apterygota, we have here an _acquired ametabola_.”

It is rare that, after the rudiments of wings have once appeared in the very young, they should disappear in the late nymph stage; this is, however, said by Walsh to be the case with the Ephemerid Bætisca (Fig. 440). This is a case of retardation in an acquired ametabolesis.

THE LARVA

The term _larva_ is peculiarly applicable to the young of the holometabolous orders. The name (Latin, _larva_, a mask) was first given to the caterpillar because it was thought by the ancients to mask the form of the perfect insect. Swammerdam supposed that the larva contained within itself “the germ of the future butterfly, enclosed in what will be the case of the pupa, which is itself included in three or more skins, one over the other, that will successively cover the larva.” What led to his conception of the nature of these changes was probably his observations on the semitransparent larva of the gnat, in which the body and limbs of the pupa can be partially seen; for Weismann has shown that the great Dutch observer’s belief that the pupal and imaginal skins were in reality already concealed under that of the larva is partially founded in fact. Swammerdam states: “I can point out in the larva all the limbs of the future nymph, or Culex, concealed beneath the skin,” and he also observed beneath the skin of the larvæ of bees, just before pupating, the antennæ, mouth-parts, wings, and limbs of the adult. But, as we shall see farther on, the discovery by Weismann in the larva of the germs of the imago has completely changed our notions as to the nature of metamorphosis, and revolutionized our knowledge of the fundamental processes concerned in the change from larva to pupa, and from pupa to imago.

Not only are the larvæ of each order of insects characteristic in form, so that the grub or larva of beetles is readily distinguished from those of other orders, or the maggot of flies from the apodous larva of wasps and bees, but within the limits of the larger orders there is great diversity of larval forms, showing that they are the result of adaptation to their surroundings. This is especially the case with the larvæ of the Coleoptera, Lepidoptera, Diptera, and Hymenoptera.

In general, the larvæ of insects may be divided into two types,—the _Campodea-form_, or campodeoid, sometimes called thysanuriform, and the _eruciform_.

_a._ The Campodea-form type of larva

This is the most primitive and generalized type of larva (Fig. 560). A Campodeoid larva is one nearest in general shape to Campodea, the form which we have seen to be the nearest allied to the probable ancestor of the insects, and it also resembles the nymphs of the heterometabolous insects, before the appearance of their rudimentary wings.

Brauer, in 1869,[93] first suggested that the larvæ of a great number of insects may be traced back to Campodea and Iapyx. The Campodea-form larva is active, with a more or less flattened body, well developed mandibulate mouth-parts, and usually long legs. The nearest approach to the form of Campodea is the freshly hatched nymph of cockroaches (Blattidæ), Forficula, Perlidæ, Termitidæ, Psocidæ, Embidæ, Ephemeridæ, Odonata, especially the more generalized Agrionidæ, the nymphs of Hemiptera, the larvæ of certain Neuroptera, the active pedate larvæ of the more generalized Coleoptera, such as those of Carabidæ, Cicindelidæ, Dyticidæ, etc., and the first larva (instar) of Stylopidæ and Meloidæ (Fig. 560, _d_).

While the Campodea-shape is retained throughout nymphal life, of the orders above mentioned the Neuroptera and Coleoptera alone have a true resting pupal stage.

It should also be observed that great changes in the form of the nymph occur within the limits of the Orthoptera; the nymph of all the families except that of the Blattidæ, evidently the most generalized and primitive, being more or less specialized, while the nymphs of the other orders all vary in degree of specialization and modification. The process of adaptation once begun went on very rapidly, as it has in many other orders of insects, as well as in animals of other phyla.

_b._ The eruciform type of larva

Brauer also sagaciously pointed out that “a larger part of the most highly developed insects assume another larva form, which appears not only as a later acquisition, through adaptation to certain definite conditions, but also arises as such before our eyes. The larvæ of Lepidoptera, of saw-flies, and Panorpidæ show the form most distinctly, and I call this the caterpillar form (_Raupenform_). That this is not the primitive form, but one later acquired, we see illustrated in certain beetles. The larvæ of Meloë and of Sitaris, in their fully grown conditions, possess the caterpillar form, but the new-born larvæ of these genera show the Campodea-form. The last form is lost as soon as the larva begins its parasitic mode of life.... The larger part of the beetles, the Neuroptera (in part), the bees and flies (the last with the most degraded maggot form), possess larvæ of this second form.” In 1871 we adopted these views, giving the name _eruciform_ to this type of larvæ, and afterwards Lubbock adopted Brauer’s views. Brauer considered that the eruciform larva was the result of living a stationary semi-parasitic life on plants, in carrion, or burrowing in the trunks and branches or leaves and buds of trees, where they do not have to move about in search of their food. The change from the Campodea-form to the eruciform larva is a process of degeneration and often of atrophy of the limbs, and, in the footless forms of dipterous and hymenopterous insects, of the gnathites, accompanied by a tendency of the body to become more or less cylindrical.

The first steps in the origination of the eruciform larva were apparently taken in the order Neuroptera, as restricted by Brauer and by myself, where, though the larvæ are campodeoid, there is a true resting pupal stage. The most generalized larval form is perhaps that of the Sialidæ (Fig. 560, _l_), in which the body tends to be slightly cylindrical, though the legs are long, and the gnathites well developed for seizing and biting their living prey. The terrestrial larvæ of the Hemerobiidæ, though modifications of the sialid larval form, are considerably specialized in adaptation to their active carnivorous habits. But the life-history of Mantispa, where there are two larval stages, gives us plainly enough the key to the mode in which the complete metamorphosis was brought about. The larva, born a true Campodea-like form, with large, long, 4–jointed legs, has a structure which would enable it to move about freely after its prey, beginning at once to live a sedentary life in the egg-sac of a spider; before the first moult it loses the use of its legs, while the antennæ are partly aborted. The result is that, owing to this change of habits and surroundings from those of its active ancestors, it changes its form, and the fully grown larva becomes cylindrical, with small slender legs, and, owing to the partial disuse of its jaws, acquires a small, round head.

Its antennæ, mouth-parts, and legs not only retarded in growth, but retrograding and becoming vestigial, the body meanwhile becoming fat and cylindrical, an apparent acceleration of growth goes on within, with probably an enlargement of the intestine and fat-body, and thus the pupal form is perfected while the larva is full-fed and quiescent. It is not improbable that in the primitive neuropteron, as the result of a mode of life like that of Mantispa, the quiescent life of the later stages graduated into a quiescent, inactive pupal life, allowing the changes going on in the internal organs to result in a complete metamorphosis, which was transmitted to the later Neuroptera, thus making the complete metamorphosis a fixed, normal condition. It thus appears that a change of habits and of food, and more especially the fact that the nymph became so surrounded with an abundance of food close at hand that it did not have to run actively about and seize it in a haphazard manner, were the factors bringing about a change from the Campodea-form nymph to the eruciform larva, thus inducing a hypermetamorphosis.

The larvæ of the Mecoptera (Panorpidæ, Fig. 562, _b_) are still more caterpillar-like, and besides their cylindrical body, rounded head, small short gnathites, small thoracic legs, they have what appear to be 2–jointed legs to each of the nine abdominal segments, and the close resemblance to caterpillars is farther carried out by the presence of a pair of prothoracic spiracles, none existing on the other two thoracic segments.

In the Meloidæ (Fig. 560, _d_) and Stylopidæ the first larval stage is Campodea-form; the changes will be described in the subsequent section on Hypermetamorphosis, and while these cases of change from a campodeoid to an inactive eruciform larva are very salient, if we compare the graduated series of larval forms throughout the order of Coleoptera, as represented by the illustrations in Fig. 561, we shall see that in nearly, if not each, case the form of the boring or mining, or bark or bud or seed-inhabiting grub is the result of a change of habit and commissariat from active predaceous larvæ, like those of the Carabidæ and other adephagous families, together with those of the Staphylinidæ, with their flat body, big mandibles, and well-developed maxillæ, to the cylindrical bodies of such larvæ as those of Dermestes and Anthrenus, which live a more sedentary life, to the root-feeding wire-worm or elaterid larvæ, and scarabæid grubs, onward to the phytophagous Chrysomelidæ, with the mining and boring buprestids and cerambycids,—in all these forms we see a gradual atrophy of the legs, which is fully carried out in the vermiform or maggot-like larva of the weevils. These changes throughout the members of the entire order are epitomized in the life-history of the Meloidæ, in which there are three typical forms of larva: the Campodea-form (triungulin stage), eruciform (second or carabidoid stage), and vermiform (coarctate) larva.

In the Lepidoptera the eruciform, pedate type is adhered to throughout the order, with the rare exception of the nearly apodous mining larva of Prodoxus (Fig. 563, _a_), Phyllocnistis, and Nepticula, which have no thoracic legs, and the limacodid larvæ, whose abdominal legs are totally aborted, while the thoracic ones are much reduced (Fig. 564).

In the Hymenoptera the phytophagous forms are eruciform, while by the agency of the same factors as already mentioned, _i.e._ a sedentary or parasitic life and abundance of food within constant reach, the larvæ lose their legs and become vermiform.

In the Diptera, which are the most highly specialized of insects, the maggot or vermiform shape, and absence of any legs, prevails throughout the order, though the eucephalous larvæ show their origin from a primitive eruciform type of larva. The highly specialized larvæ of the Culicidæ and Simuliidae are undoubtedly related to the earliest and most generalized types, while the maggots of the parasitic flies (Tachinidæ) and other muscids are later degradational forms, and the result of adaptation induced, as in the previous cases, by a sedentary or parasitic mode of life, living as they do immersed in an abundance of rich nitrogenous food, with the result that the mouth-parts have become atrophied by disuse, while the limbs have become entirely aborted, though the thoracic imaginal discs develop normally in the embryonic or pre-larval stages.

It appears, therefore, highly probable that the metamorphoses of insects are the result of the action of change of conditions, just as the polymorphism of Termites is with little doubt the result of differences of food and other conditions. These matters will be farther discussed under the head of Causes of Metamorphosis.

LITERATURE ON ANCESTRY OF INSECTS, ETC.

=Müller, Fritz.= Für Darwin, 1869, pp. 144, 67 Figs.

=Brauer, Friedrich.= Betrachtung ueber die Verwandlung der Insekten im Sinne der Descendenz-Theorie. (Verhandlung d. k.k. zool. bot. Gesell. Wien., 1869, 1 Taf., pp. 1–20.)

=Packard, A. S.= Amer. Naturalist, iii, p. 45, March, 1869.

—— Proc. Boston Soc. Nat. Hist., xiv, 1870, p. 61.

—— Amer. Nat., iv, Feb. 1871, p. 756; v, 1871, pp. 52, 567.

—— Embryological Studies. (Memoirs Peabody Acad. Sc. Salem, 1871–72.)

—— Our common insects, 1873, Chapter on Ancestry of Insects, pp. 175–178.

—— Third Report U. S. Ent. Commission, 1883, pp. 295–304.

=Lubbock, John.= On the origin of insects. (Journ. Linn. Soc., London, xl, 1873.)

—— Origin and metamorphoses of insects. (Nature, 1873 [in book form, 1874], pp. 108, 66 Figs.)

=Mayer, Paul.= Ueber Ontogenie and Phylogenie der Insekten. (Jena. Zeitschr. Wissens., x, 1876, pp. 125–221, 4 Taf.)

=Hyatt, A., and Arms, J. M.= Insecta. (Bost. Soc. Nat. Hist. Guides for science-teaching, viii.) Boston, 1890, pp. 300, 13 Pls., 223 Figs.

_c._ Growth and increase in size of the larva

The rapidity of growth and enormous increase in size in early life is especially noticeable in caterpillars and other phytophagous larvæ. The latest observations are those of Trouvelot on _Telea polyphemus_. When this silkworm hatches, it weighs 1⁄20 of a grain.

When

10 days old it weighs ½ a grain, or 10 times the original weight. 20 days old it weighs 3 grains 60 times the original weight. 30 days old it weighs 31 grains 620 times the original weight. 40 days old it weighs 90 grains 1800 times the original weight. 56 days old it weighs 207 grains 4140 times the original weight.

“When,” he says “a worm is 30 days old, it will have consumed about 90 grains of food; but when 56 days old, it is fully grown and has consumed not less than 120 oak leaves, weighing ¾ of a pound; besides this it has drank not less than ½ an ounce of water. So the food taken by a single silkworm in 56 days equals in weight 86,000 times the primitive weight of the worm. Of this about ¼ of a pound becomes excrementitious matter, 207 grains are assimilated, and over 5 ounces have evaporated.”[94]

Dandolo stated that the Asiatic silkworm (_Bombyx mori_) weighs on hatching not over 1⁄100 of a grain, but when fully grown about 95 grains. During this period, therefore, it has increased 9500 times its original weight, and has eaten 60,000 times its weight of food. Newport thought this estimate of the amount of food was a little too great. But comparing it with Trouvelot’s estimate for the American silkworm, which weighs when hatched five times as much, it would not appear to be so. Newport found that the larva of _Sphinx ligustri_ at the moment of leaving the egg weighs about 1⁄80 of a grain, and when fully fed 125 grains, so that in the course of 32 days it increases about 9976 times its original weight. This proportion of increase is exceeded by the larva of _Cossus ligniperda_, which, boring in the trunks of trees, remains about three years in the larva state, and increases, according to Lyonet, to the amount of 72,000 times its first weight.

Newport adds that those larvæ in which the proportion of increase is the greatest, are usually those which remain longest in the pupa state, as in the silkworm. “Thus Redi observed in the maggots of the common flesh-flies a rate of increase amounting to about 200 times the original weight in 24 hours, but the proportion of increase in these larvæ does not at all approach that of the Sphinx and Cossus.” From his observations on the larva of one of the wild bees (_Anthophora retusa_) Newport believes that this is also the case with the Hymenoptera. The weight of the egg of this insect is about 1⁄150 of a grain, and the average of a full-grown larva 68⁄10 grains, so that its increase is about 1020 times its original weight; “which compared with that of the Sphinx of medium size, is but as 1 to 9¾, and to a Sphinx of maximum size only as 1 to a little more than 11.”

The growth is most rapid after the last moult. “Thus a larva of _Sphinx ligustri_, which at its last change weighed only about 19 to 20 grains, at the expiration of eight days, when it was fully grown, weighed nearly 120 grains.” (Newport.)

_d._ The process of moulting (ecdysis)

Insects periodically shed the exoskeleton, together with the chitinous lining of their internal organs of ectodermal origin, which thus sloughed off are called the _exuvia_. The process in the locust has been described by Riley.[95] It occupies from half to three-quarters of an hour (Fig. 565). This process has naturally, from the ease with which it can be observed, been most frequently examined in the Lepidoptera, though careful and detailed observations of the inner and outer changes are still greatly needed, especially in other orders. In the caterpillar of most moths, especially one of the more generalized bombycine moths, on slipping out of its egg-shell the head is of enormous size as compared with the body, but the latter soon fills out after the creature has eaten a few hours; the head, of course, does not during this time increase in size, and the larvæ of different instars may be exactly distinguished, as Dyar has shown, by the measurements of the head.

Before the caterpillar moults, it stops feeding, and the head is now small compared with the body; the head of the second instar is now large, situated partly under the much-swollen prothoracic segment, and pushes the head of the first instar forward.

Newport has well described the mode of shedding the skin in _Sphinx ligustri_, and his detailed description will apply to most lepidopterous larvæ.

The whole body is wrinkled and contracted in length, and there are occasionally powerful contractions and twitchings of its entire body; the skin becomes dry and shrivelled, and is gradually separated from the new and very delicate one of the next instar beneath. After several powerful efforts of the larva the old skin cracks along the middle of the dorsal surface of the mesothoracic segment, and by repeated efforts the fissure is extended into the 1st and 3d segment, while the covering of the head divides along the vertex and on each side of the clypeus. “The larva then gradually presses itself through the opening, withdrawing first its head and thoracic legs, and subsequently the remainder of its body, slipping off the skin from behind like the finger of a glove. This process, after the skin has once been ruptured, seldom lasts more than a few minutes. When first changed the larva is exceedingly delicate, and its head, which does not increase in size until it again changes its skin, is very large in proportion to the rest of the body.” (Art. Insecta, etc.)

Trouvelot’s account is more detailed and an advance on that of Newport’s view. He explicitly states, and we know that he was a very close observer, that the old skin is detached by “a fluid which circulates between it and the worm.” His account is as follows: The polyphemus worm, like all other silkworms, changes its skin five times during its larval life. The moulting takes place at regular periods, which comes around about every 10 days for the first four moultings, while about 20 days elapse between the fourth and fifth moulting. The worm ceases to eat for a day before moulting, and spins some silk on the vein of the under surface of a leaf; it then secures the hooks of its hind legs in the texture it has thus spun, and there remains motionless; soon after, through the transparency of the skin of the neck, can be seen a second head larger than the first, belonging to the larva within. The moulting generally takes place after four o’clock in the afternoon; a little before this time the worm holds its body erect, grasping the leaf with the two pairs of hind legs only; the skin is wrinkled and detached from the body by a fluid which circulates between it and the worm; two longitudinal bands are seen on each side, produced by a portion of the lining of the spiracles, which at this moment have been partly detached; meanwhile the contractions of the worm are very energetic, and by them the skin is pulled off and pushed towards the posterior part; the skin thus becomes so extended that it soon tears just under the neck, and then from the head. When this is accomplished the most difficult operation is over, and now the process of moulting goes on very rapidly. By repeated contractions the skin is folded towards the tail, like a glove when taken off, and the lining of the spiracles comes out in long white filaments. When about one-half of the body appears, the shell still remains like a cap, enclosing the jaws; then the worm, as if reminded of this loose skull-cap, removes it by rubbing it on a leaf; this done, the worm finally crawls out of its skin, which is attached to the fastening made for the purpose. Once out of its old skin, the worm makes a careful review of the operation, with its head feeling the aperture of every spiracle, as well as the tail, probably for the purpose of removing any broken fragment of skin which might have remained in these delicate organs. Not only is the outer skin cast off, but also the lining of the air-tubes and intestines, together with all the chewing organs and other appendages of the head. After the moulting, the size of the larva is considerably increased, the head is large compared with the body, but 8 or 10 days later it will look small, as the body will have increased very much in size. This is a certain indication that the worm is about to moult. Every 10 days the same operation is repeated. From the fourth moulting to the time of beginning the cocoon the period is about 16 days. (Amer. Naturalist, i, pp. 37, 38.)

Little has been recorded as to the exact mode of casting the larval skin in Coleoptera. Slingerland states that _Euphoria inda_ when pupating sheds the larval skin off the anal end in the same way as in caterpillars, while in _Pelidnota punctata_ the larval skin splits down the whole length of the back, retains the larval shape, and forms a covering for the pupa which remains inside. (Can. Entomologist, xxix, p. 52.) The old larval skin in the Coccinellidæ and certain Chrysomelidæ is retained crumpled up at the end of the body, while in Dermestes, Anthrenus, etc., it cloaks the pupa.

Not only is the integument, with its hairs, setæ, and other armatures, as well as the cornea or facets of the eyes, shed, but also all the lining or intima of those internal organs which have been originally derived by an ingrowth or invagination of the ectoderm are likewise cast off, with the probable exception, of course, of the mid-intestine, which is endodermal in its origin. Even so early an observer as Swammerdam noticed that the internal lining of the alimentary canal comes away with the skin. He states that the larva of _Oryctes nasicornis_ sheds both the lining of the colon, and of the smaller as well as the larger branches of the tracheæ.

Careful observations are still needed on the internal changes at ecdysis of most insects. Newport seems to have observed more closely than any one else, notwithstanding the great number who have reared caterpillars but have not carefully observed these points, the extent of the process internally. He informs us: “The lining of the mouth and pharynx, with that of the mandibles, is detached with the covering of the head, and that of the large intestines with the skin of the posterior part of the body, and besides these also the lining of the tracheal tubes. The lining of the stomach itself, or the portion of the alimentary canal which extends from the termination of the œsophagus to the insertion of the so-called biliary vessels, is also detached, and becomes completely disintegrated, and appears to constitute part of the _meconium_ voided by the insect on assuming its imago state.” (Art. Insecta, p. 876.) Newport states on another occasion that he had “noticed the remarkable circumstance [now explained by the fact that the mid-intestine is of endodermal origin] that the mucous lining of the true ventriculus was not cast off with the rest, but was discharged with the fecula.”[96] Burmeister also observed that the smaller tracheæ as well as the internal tunic of the colon of Libellulæ are shed.

In the apodous larvæ of Hymenoptera which live in cells, as we have observed in those of Bombus, during the process of moulting, the delicate skin breaks away in shreds, probably owing to the tension due to the unequal growth of the different parts of the body. “Thus after the skin beneath has fully formed, shreds of the former skin remain about the mouth-parts, the spiracles, and anus. Upon pulling upon these, the lining of the alimentary tube and tracheæ can be drawn out, sometimes, in the former case, to the length of several lines.”[97] We then added, “As all these internal systems of vessels are destined to change their form in the pupa, it may be laid down as a rule in the moulting of insects and Crustacea, that the lining of the internal organs, which is simply a continuation of the outer tegument, or arthroderm, is, in the process of moulting, sloughed off with that outer integument.” We have satisfied ourself that in the larvæ of the Lepidoptera (_e.g._ Datana) the tracheæ at the time of ecdysis undergo a complete histolysis, and arise _de novo_ from hypodermal cells, the so-called spiral threads originating from elongated peritracheal nuclei. (See p. 449, Fig. 412.) This is undoubtedly also the case with the salivary ducts, which are strengthened and rendered elastic by tænidia like those of tracheæ. As the urinary tubes are diverticula of the proctodæum, itself an ectodermal invagination, they may also, though not lined with a chitinous intima, be renewed. With little doubt the intima of the ducts of poison, spinning, and most, if not all the other glands, though certainly the dermal glands, is exuviated. We have found that the lobster in moulting sheds, besides the skin with the most delicate setæ, the lining of the proventriculus, and the apodemes of the head and thorax, hence it is most probable that the tentorium of the head of insects as well as the apodemes and phragmas of the thorax are exuviated.

The formation of the inner skin, or that of any succeeding stage (instar), is due to the secretion of the structureless chitinous layer by the cells of the hypodermis, during the process of histogenesis. These cells at this time are very active, and the formation of the new layer of chitin arrests the supply of nourishment to the old skin, so that it dries, hardens, and with the aid of the fluid thrown out at this time separates from the new chitinous layer secreted by the hypodermis.

Mention of this fluid, which Newport was the first to observe, and which he says causes the separation of the old from the underlying fresh integument of the caterpillar, recalls a passage in Hatchett-Jackson’s Studies in the morphology of the Lepidoptera, which we quote on a succeeding page, where he calls attention to the formation of such a liquid, which in the reptiles facilitates the process of moulting, adding, “Whether such is the case with the moult of the caterpillar, I do not know.” Is it not also possible that the growth of the setæ or tubercles on the cuticle of the caterpillar may likewise serve to loosen and detach the overlying skin about to be cast off? After writing the foregoing, we find that Miall and Denny have suggested that the setæ of the cockroach probably serve the same purpose as the casting-hairs of the crayfish and reptiles.

It is well known that in the crayfish and in lizards the skin is first loosened by the growth of temporary hairs or setæ, which locally grow inward from the old cuticle and push the skin away when it is shuffled off by the movements of the body, jaws, and limbs, as well as the body in general.[98]

Such spines arise in the pupa of many insects, for Verhoeff finds that the spines and teeth of pupal fossorial and other Hymenoptera, as well as Coleoptera, function as moulting-processes for loosening and pushing off the last larval skin, rather than for locomotion. He also claims that the spines of the pupa of the dipterous Anthrax are both for locomotion and for boring, especially the spines on the head and tail. He therefore divides these pupal spines into helcodermatous (boring or tearing) and locomotor spines.

Gonin has fully confirmed Newport’s discovery of the exuvial fluid. He states that during pupation the outside of the pupa, especially the parts of the head and thorax “is coated with a viscous liquid secreted by special glands.” The parts only harden subsequent to pupation after exposure to the air (p. 41). His observations were made under the direction of Professor Bugnion, who kindly writes us:—

“M. Gonin has proved the formation of a liquid which passes under the cuticle at the time of the last moult and facilitates exuviation. We think that this liquid is secreted by large cells (unicellular glands) which we see especially on the surface of segments 1–3. These cells form part of the hypodermis, and their pores open under the cuticle.”

In a subsequent letter enclosing a sketch kindly made for me by M. Gonin (Fig. 566), Professor Bugnion writes me Aug. 24, 1897, regarding the functions of the large hypodermal cells (_l. hy_), as follows: “It seems to me, in fact, after having again examined the sections, that the function of these cells is not sufficiently elucidated. Indeed these cells occur only in the section passing through the 1st segment, between the head and 1st thoracic segment. It would seem, if these cells supply the liquid which lubricates the surface at the time of ecdysis, that they should be spread over the entire surface of the body. Moreover, these cells have no distinct orifice, and although there is seen at times to issue streams of a substance (coagulated by the reagents), they cannot be compared with true unicellular glands like those of the epidermis of fishes, amphibians, etc.

“On the other hand, if it is the blood which oozes out on the surface (according to your hypothesis), it would seem that the loss of blood would cause the death of the larva. I believe then it is due to the secretion of the hypodermis which spreads over the whole surface when the cells are still soft (not yet hardened from contact with the air). At all events, there is a liquid spread over the surface; it is this liquid which glues the wings and the legs to the body at the moment the caterpillar issues from the rent in its skin. If at this instant we plunge the pupa in the water the liquid is dissolved, and the feet, wings, etc., are not glued to the body.”

Dr. T. A. Chapman also writes us: “There is no question about the existence of a fluid between the two skins at moulting. In hairy larvae the hairs are always wet at first, or if the skin be renewed rather more quickly than the larva does it naturally, the wetness of both surfaces is obvious. I do not know the nature of the fluid, but it is related to that which hardens into the dense pupal case, and also hardens in a less degree the skin of the larva. I suppose it must contain some chitin in a soluble form. If a newly cast larva skin be taken, there is no difficulty in extending the shrivelled mass to its full length and dimensions, but if a short time elapses, this chitin hardens, and the skin cannot be extended after soaking in water, alcohol, ammonia, or any other solvent I have tried.”

It has been stated that there is a subimaginal pellicle in Lepidoptera, but as Dr. Chapman writes me, “what has been observed has been some of the inner pupal dissepiments, such as the pupal cases of the under wings,” etc. They may be observed in the head of the tineid pupæ, and other small moths. We have thought that the delicate, purplish, powdery layer left in the cast shells of the pupæ of saturnians, Catocalæ, and other moths, might possibly be such a pellicle, but this view has been dispelled by the following statement of Professor Bugnion in a letter answering an inquiry whether he had noticed such a pellicle.

“A liquid which is secreted in a few minutes at the time of the last moult, forms in drying a yellowish layer spotted with black (in _Pieris brassicæ_). This layer extends around the entire pupa, and serves both to protect it and to glue together the wings, legs, etc., in their new position. The dried liquid on the surface of the pupa, and by means of which the appendages are glued to the surface, very likely corresponds to the pellicle of which you speak.” The newly exposed integument is at first pale and colorless, but soon assumes the hues peculiar to the species, and the insect, at first exhausted, after a short rest becomes active.

E. Howgate has noticed under the microscope peculiar internal movements in a small immature transparent geometrid while moulting. “Each separate segment,” he says, “commencing at the head, elongated within the outer skin, whilst the next ones remained in their former state. Each segment in its turn behaved in this curious manner until the last was reached, when the motion was reversed and proceeded toward the head, when it was again reversed.... The whole proceeding appeared as if the larva was gliding within itself, segment after segment, the outer skin remaining as if held by the other segments, whilst the particular one in motion freed itself within. After remaining motionless for a short interval, the skin near the head swelled and burst, open at the back.... Presently out comes the head of the new caterpillar, pushing forward the old one.... After a short struggle the new true legs appear, pushing off and treading under foot the old ones. Then by violent wriggling movements the abdominal legs were extricated. Then all is clear, and the larva, which is quite exhausted, coils itself up and literally pants for breath.” (The Naturalist, November, 1885, No. 124, p. 366, quoted in Psyche, iv, p. 327, 1887.)

Since the worms and most other ametabolous invertebrates are not known to moult their integument, the body steadily increasing in size without frequent changes of skin, it seems that growth may go on and still be accompanied by considerable changes in shape of the body without change of skin. Frequent ecdyses appear, then, to be the result of the great and sudden changes of the body, necessitated by the adaptation of the animal to new or unusual conditions of life. In young Daphnia, a cladocerous crustacean, as many as eight moults were observed in a period of 17 days, and spiders frequently moult even after reaching their full size. The swollen bodies of the gravid female of Gastrophysa, Meloë, or of Termites, and of the honey ant show that the skin can stretch to a great extent, but in the metamorphoses of Crustacea and of insects, whose young are more or less worm-like or generalized in form, with fewer segments and appendages, or with appendages adapted for quite different uses from those of mature life, the necessity for a change of skin is seen to be necessary for mechanical reasons. Hence Crustacea and insects moult most frequently early in life, when the changes of form are most thoroughgoing and radical, while simple growth and increase in size are most rapid at the end of larval life, as seen both in shrimps and crabs, and in insects.

The hibernating caterpillars of certain butterflies are known to moult once oftener than those of the summer brood. Mr. W. H. Edwards has discussed the subject with much detail. “There seems,” he says, “to be a necessity with the hibernators of getting rid of the rigid skin in which the larva has passed the winter; that is, if the hibernation has taken place during the middle stages, as it does in Apatura and Limenitis. In these cases very little food is taken between the moult which precedes hibernation and the one which follows it, and the larva while in lethargy is actually smaller than before the next previous moult. The skin shrinks, and has to be cast off before the awakened larva can grow. Those species (observed) whose larva moults five times in the winter brood require but four moults during the summer.” He adds that while the larva is in lethargy, it is actually smaller than before the next previous moult. Dr. Dyar writes: “I think there is no doubt about the number of stages of arctian larvæ. They seem to have a great capacity of spinning out their life-history by interpolated stages (as regards width of head). I think it is because so many of them hibernate, and only a single brood extends throughout the season.” (Psyche iii, p. 161.)

On the other hand, it is difficult to understand why the caterpillars of arctians moult so frequently, nearly twice as often as in most other caterpillars, though the changes of form and armature are so slight.

Dr. Chapman also writes me: “Arctians resemble bears (Arctos), polar and others, in having long hairs to protect them during winter, and are, in fact, typically hibernators. Many of them have to half-hibernate, having warmth enough to keep them awake, but not enough food for growth, but their tissues, at least the chitinous ones of the cutis, and also probably, and perhaps especially, of the alimentary canal, become old and effete, and require the rejuvenescence acquired by a moult. Other smooth-skinned hibernators have similar capabilities.”

Chapman has shown in his paper on Acronycta that these caterpillars of this genus illustrate how larvæ may lose a moult, and they do so to acquire a sudden change of plumage.

=The number of moults in insects of different orders.=—It will be seen from the data here presented that the number of moults is as a rule greatest in holometabolic insects with the longest lives, and that an excessive number of ecdyses may at times be due to some physical cause, such as lack of food combined with low temperature.

In Campodea there is a single fragmentary moult (Grassi), while the Collembola (_Macrotoma plumbea_) shed their skin throughout life. (Sommer.)

In the winged insects, especially Lepidoptera, the number of moults is dependent on climate. Insects of wide distribution growing faster in warmer climates consequently shed their skins oftener; for example, the same species may moult once oftener in the southern than in the northern States, as in the case of _Callosamia promethea_, which in West Virginia is double-brooded. Hibernating larvæ moult once oftener than those of the summer brood. (W. H. Edwards.) Weniger by rearing the larvæ of _Antheræa mylitta_ and _Eacles imperialis_, and which, when reared under normal conditions, actually have six stages, found that when reared in a warm moist atmosphere of about 25° C. they have but five stages, _i.e._ moult but four times. In the hot and moist climate of Ceylon, _A. mylitta_ has but five stages. (Psyche, v, p. 28.)

Among Orthoptera Acrydians moult five times; _Diapheromera femorata_ but twice (Riley); a katydid (_Microcentrum retinervis_) moults four times (Comstock). _Mantis religiosa_, according to Pagenstecher, moults seven times, having eight stages, including that before the amnion is cast, but the first “moult” being an exuviation of the amnion, the number of stages is seven. Cockroaches (_Periplaneta americana_) are said by Marlatt to “pass through a variable number of moults, there being sometimes as many as seven.”

In the Homoptera there are, in general, from two to four moults; thus in Typhlocyba there are five stages, and in Aphis at least three, and in Psylla four during the nymphal state. Psocus has four. Riley states that the nymph of the female coccid, _Icerya purchasi_, sheds its skin three times, and that of the male twice. Notwithstanding its slow growth, Riley says, the 17–year Cicada moults oftener than once a year, and the number of larval stages probably amounts to 25 or 30 in all. The bed-bug sheds its skin five times; and with the last moult appear the minute wing-pads characteristic of the adult. In _Conorhinus sanguisuga_ there are “at least two larval stages and pupal stages.” (Marlatt.)

In the dragon-flies moulting occurs, Calvert thinks, many times, since the rudiments of wings are said by Poletaiew to only appear in odonate nymphs after the third or fourth moult.

In the May-fly, Chloëon, the number of ecdyses is 20. The neuropterous _Ascalaphus_ (Helecomitus) _insimulans_ of Ceylon moults three times before pupating. Among the Mecoptera Felt has shown that _Panorpa rufescens_ moults seven times.

In Coleoptera the normal or usual number is not definitely known; Meloë moults five times, but this is a hypermetamorphic insect; _Tribolium confusum_ has been carried by Mr. Chittenden through seven moults. _Phytonomus punctatus_, the clover-leaf weevil, moults three times, according to Riley, who has observed that _Dermestes vulpinus_ passes through seven larval stages.

In the breeding jars, with plenty of food and a constant temperature of from 68° to 78° F., the larvæ cast their 1st skin in from four to nine days, the great majority moulting at seven days. Under the same conditions the 2d skin was cast at from four to seven days, the majority moulting at six days; the 3d skin at from three to six days, the majority moulting at five days; and the 4th skin at from three to six days, the majority moulting at five days; the 5th skin at from five to seven days, and the 6th skin at six days. There are thus seven larval stages. (Report for 1885, p. 260.)

Riley has ascertained that by rearing isolated larvæ of _Tenebrio molitor_, one after being kept nearly a year had moulted 11 times, when it died. A second larva, hatched June 5, had moulted 12 times by June 10 of the following year, (1877), when it also died. Of _T. obscurus_ three larvæ were reared to the imago state. One moulted 11 times by Aug. 30 of the same year, pupated Jan. 20, 1877, and finally became a beetle Feb. 7, 1877. The other two both moulted 12 times, and reached the imago stage Feb. 18 and March 9, respectively. “All were, as nearly as possible, under like conditions of food and surroundings, and in all cases the moult that gave the pupa is not considered among the larval moults.”

Two larvæ of the museum pest (_Trogoderma tarsale_) were kept by Riley in a tight tin box with an old silkworm cocoon. “They were half-grown when placed in the box. On Nov. 8, 1880, there were in the box 28 larva skins, all very much of a size, the larva having apparently grown but little. The skins were removed and the box closed again as tightly as possible. Recently, or after a lapse of two years, the box was again opened and we found one of the larvæ dead and shrivelled up; but the other was living and apparently not changed in appearance. There were 15 larva skins in the box. He could not tell when the one larva died, but it is certain that within a little more than three and a half years, two larvæ shed not less than 43 skins, and that one larva did not, during that time, appreciably increase in size. We know of no observations which indicate the normal or average length of life, or number of moults in either Tenebrio or Trogoderma, but it is safe to assume from what is known, in these respects, of allied species, that in both the instances here referred to, but particularly in the case of Trogoderma, development was retarded by insufficient nutrition, and that the frequent moulting and slow growth resulted therefrom, and were correlated.”[99] Further observations such as these are greatly needed.

Of the Siphonaptera the common cat and dog flea (_Pulex serraticeps_) moults three times before pupating. (Howard.)

In Lepidoptera the usual or average number of moults is four, but the number varies considerably, the greatest number yet known occurring in _Phyrrarctia isabella_, which, Dr. Dyar informs me, moults 10 times.

From Dyar’s observations it appears that there are usually five larval stages, but six and seven stages are not infrequent, while there are seven in _Seirarctia echo_, eight in _Ecpantheria scribonia_, Scepis, and Apatelodes, and nine and ten in arctians, while the European _Nola centonalis_ moults nine times, other species of this genus shedding their skins six times. (Buckler.) (Psyche, v, pp. 420–422.) _Callosamia promethea_ appears, as a rule, to moult but three times. _Orgyia antiqua_ was found by Hellins to moult from three to five times. Riley found that in _O. leucostigma_ the males moult four times, the female four, but sometimes five times, while Dyar states that in _O. gulosa_ the male larvæ moult three or four times, the female always four times; in _O. antiqua_, however, there are six stages, and in the female seven. Lithocolletis, Chambers thinks, as a rule, moults eight times, and Comstock thinks that _L. hamadryadella_ casts its skin seven or eight times.

In the blow-fly (Calliphora) Leuckart and Weismann have inferred at least two moults, while Weismann suspected that there are as many as four. In _Musca domestica_ we have observed that the larva moults three times; in Œstridæ there are three larval stadia. (Brauer.) In Corethra there are four larval moults, and Miall thinks there are probably as many in Chironomus. Passing to the phytophagous Hymenoptera, there are three moults or four larval stages in _Nematus erichsonii_, but Dyar informs us that less than four stages in saw-fly larvæ is very rare, that he has only one record of less than five, and that that is doubtful; “five for nematid, six and seven for others, is certainly the rule. The highest I have is the indication of 11 stages for _Harpiphorus varianus_, but this again is an inference only, and attended with doubt.” (Can. Ent., xxvii, p. 208.) In Bombus we have observed five different sizes of larvæ, and hence suppose the least number of ecdyses is five, while we are disposed to believe that this insect, as well as wasps and bees, in general shed their skins as many as eight times during their entire existence.

The honey-bee, Cheshire thinks, since he has found the old and ruptured pellicles, probably moults six times before it spins its cocoon, or passes into the semipupa condition. (Bees and Bee-keeping, p. 20.)

As to the cause of the great number of moults in the arctians and in the beetles experimented with by Riley, it would seem that cold and the lack of food during hibernation were the agents in arctians, and starvation or the lack of food in the case of the beetles, such cause preventing growth, though the hypodermis-cells retained their activity.

=Reproduction of lost limbs.=—Here might be discussed the subject of the renovation or renewal of maimed or lost limbs, or the reparation of other injuries. As is well known, the cœlenterates, echinoderms, and worms under certain circumstances multiply by self-division, or if artificially mutilated, the parts are gradually restored by cell-proliferation or histogenesis. It is so with the antennæ and legs of crustaceans as well as the digits and tail of salamanders. The experiments first made by Le Pelletier[100] on spiders, and later by Heineken,[101] and others after him, on different spiders, as well as on Orthoptera and Hemiptera (Blatta, Reduvius, etc.), have proved that antennæ and legs and other external parts which have been injured or shortened, or entirely cut off in young individuals, are replaced at the next, or after successive moults, though generally in diminished size. This does not usually occur in adult life, and the process of reparation of lost parts is apparently due to the active growth of the cells of the parts affected during the process of moulting, when the histolysis of the maimed or diseased parts is succeeded by the rapid development of new cells, not only of the hypodermis, but also of the more specialized tissues within. And this tends to prove that such histolysis and making over of the muscles and other structures within occur especially in all metamorphic insects, and also in ametabolous forms, though the process has been most thoroughly examined in the Diptera, where these changes are more marked.

Gonin has found that the thoracic legs of the caterpillar correspond only to the tarsi of the imago (Fig. 608). It results, he says, from this fact that in accordance with the observations of Réaumur (which were wrongly interpreted by Newport and Künckel D’Herculais) that the amputation of the legs of the larva does not involve the entire leg, but only the extremity of the leg of the imago.

=Formation of the cocoon.=—While the larvæ of many insects, as those of the butterflies, suspend themselves before transforming, and spin no cocoon, or dig into the earth for protection and to secure an immunity from too great changes of temperature, a large proportion of the larvæ of metabolous insects which lead an inactive pupal life, line their earthen cells with silk, or spin a more or less elaborate case of silk, called the _cocoon_. We have seen that the inactive pupa of the male scale-insects is covered by the scale itself, or even in one case the insect forms a true cocoon of fibres of wax. The aquatic larvæ of the Neuroptera and Coleoptera creep out of the water, and by the movements of their bodies make a rude earthen cell in the bank, while that of Donacia spins a dense, leathery cocoon (Fig. 567) in the earth. The larvæ of the Embiidæ are protected by a cocoon, which they renew at each moult. Coniopteryx spins an orbicular cocoon, the Hemerobiidæ a spherical, dense, whitish one. The Trichoptera transform within their larval cases, which thus serve as cocoons, as do certain case-bearing Lepidoptera, notably the Psychidæ.

The pupa of certain leaf-eating beetles (Chrysomelidæ), as well as the Coccinellidæ, Dermestidæ, Hister, etc., are usually protected by the cast larval skin, which is retained, forming a rude shelter. While many beetles spin an oval cocoon (Gyrinus, Silphidæ), the wood-boring species make one of chips glued together, and that of Lucanus, which feeds on decayed wood, is lined with silk (Fig. 568). Anobium constructs a silken cocoon, interweaving the fine particles of its thin castings; the larvæ of weevils also usually spin silken cocoons.

The larval skin of the coarctate Diptera is retained as a protection for the soft-bodied pupa within, the old larval skin separating from the integument of the semipupa. To this cocoon-like covering of the coarctate pupa we have restricted the term _puparium_, originally used by Kirby and Spence to designate the pupa. The puparium is usually cylindrical or barrel-shaped, rounded at each end.

In the _Diptera cyclorhapha_, or common house and flesh flies, etc., the puparium remains in vital connection, by means of four tracheæ, with the enclosed pupa, which escapes from the case through a curved seam or lid at the anterior end and not by a slit in the back, as do the orthoraphous families, represented by the horse-fly (Tabanidæ, Asilidæ, Fig. 570), etc., where in some cases the obtected pupa remains within the loose envelope formed by the old larval skin, which Brauer calls a false puparium. The dry, hard puparium is burst open at the cephalic end when the fly emerges, by means of the frontal vesicle, which is distended with fluid (Fig. 571).

The exact mode of spinning the cocoon by caterpillars has been carefully observed by L. Trouvelot in the case of the polyphemus silkworm.

“When fully grown, the worm, which has been devouring the leaves so voraciously, becomes restless and crawls about the branches in search of a suitable place to build up its cocoon; before this it is motionless for some time, holding on to the twig with its front legs, while the two hind pair are detached; in this position it remains for some time, evacuating the contents of the alimentary canal until finally a gelatinous, transparent, very caustic fluid, looking like albumen, or the white of an egg, is ejected; this is a preparation for the long catalepsy that the worm is about to fall into. It now feels with its head in all directions, to discover any leaves to which to attach the fibres that are to give form to the cocoon. If it finds the place suitable, it begins to wind a layer of silk around a twig, then a fibre is attached to a leaf near by, and by many times doubling this fibre and making it shorter every time, the leaf is made to approach the twig at the distance necessary to build the cocoon; two or three leaves are disposed like this one, and then fibres are spread between them in all directions, and soon the ovoid form of the cocoon distinctly appears. This seems to be the most difficult feat for the worm to accomplish, as after this the work is simply mechanical, the cocoon being made of regular layers of silk united by a gummy substance. The silk is distributed in zigzag lines of about one-eighth of an inch long. When the cocoon is made, the worm will have moved his head to and fro, in order to distribute the silk, about 254,000 times.

“After about half a day’s work, the cocoon is so far completed that the worm can hardly be distinguished through the fine texture of the wall; then a gummy resinous substance, sometimes of a light-brown color, is spread all over the inside of the cocoon. The larva continues to work for four or five days, hardly taking a few minutes of rest, and finally another coating is spun in the interior, when the cocoon is all finished and completely air tight. The fibre diminishes in thickness as the completion of the cocoon advances, so that the last internal coating is not half so thick and so strong as the outside ones.” (Amer. Naturalist, i, p. 86.)

The mode of spinning the cocoon of an ichneumon (Microgaster) parasitic on Philampelus has been well described by John P. Marshall, as follows:—

The first appearance of the parasite is represented in Fig. 572, 1. A warty excrescence appears on the back of the caterpillar, which slowly emerges until it is seen to be a larva enclosed in a delicate transparent membrane, as represented in 2. This it soon succeeds in bursting, and, rising to its full length, balances itself a moment as in 3, then, bending double, it ejects from its mouth a glairy liquid, which instantly changes to silk, and fastens the posterior end to the skin of the caterpillar, as shown in 4, side view. It now begins to spin its cocoon by attaching a silken thread to the silky mass by which it had previously fastened itself to the caterpillar, and forming a series of loops of uniform size, first from right to left, and then back again from left to right, as represented in the front view, 5, and better in the enlarged view, _5^a_, the arrow heads showing the direction in which the head of the larva moved while forming the loops. The ends of the series, numbered 1, 2, 3, 4, are fastened to the edges of the ventral side of the body, which thus serves as a measure of the width of the cocoon, and also acts as a support for the frail fabric in the first stages of spinning. After the larva has fastened the fabric as far up on its ventral surface as it can, conveniently, it then begins to spin free, as shown in the side view, 6, where it is represented as just completing the first half of its cocoon, which resembles in form a slipper. This accomplished, the larva ceases to spin for the time being, bends its head, as in 7, towards its ventral surface, and pushes the half cocoon free from its body. The form of the silken fabric enables it to stand unsupported, while the larva, sliding its head down to the base, holds on firmly until it swings its posterior end into the toe of the slipper.

Figure 572, 8, shows it in the act of changing end for end, and in 9 the larva is seen erect, beginning at the base to complete the other half of its cocoon; 10 shows the larva contracting its body as it spins upward for about half the length of the cocoon, when it again changes end for end, as shown in 11, where it is beginning at the upper part to unite the two sides, finally enclosing itself as represented in 12.

It may now be seen, under the microscope, through the meshes of its cocoon actively engaged in lining the interior with layers of very fine silk ejected from its mouth in great abundance. One half of the cocoon is first lined by a forward and back movement of its head, and then reversing its position, it lines the other half in a similar manner.

In one case the larva was disengaged from the skin of the caterpillar, after beginning its cocoon. It, however, began again, and spun a portion while lying on the table. This was removed, when it began a third time, and completed its cocoon.

In about 10 days the insect made its appearance through a hole in the upper end, as represented in 13. The top was eaten off in a perfect circle and hung by a few threads, so as to resemble a lid as it was thrown back.

One caterpillar observed had between 300 and 400 cocoons on its back and sides, and another was dissected after more than 30 larvæ had escaped, and 130 were discovered in the soft integuments of the back.

The figures from 1 to 13 are magnified five diameters, but in order to observe the spinning of the cocoon a power of 50 is required. (Amer. Naturalist, xii, pp. 559, 560.)

Certain differences observed by W. A. Buckhout in a Microgaster parasitic on the different species of Macrosila, are referred to in the same volume, p. 752.

While those chalcidid larvæ which feed internally on their host, as a rule, transform into naked, more or less coarctate pupæ, Howard states that the larvæ of Copidosoma, Bothriothorax, Homalotylus, and perhaps others, which are much crowded within their host, cause a marked inflation of the body of the latter (Figs. 573, 574). The nature of this cocoon-like cell, and how it is produced, is unknown. “Its structure shows it not to be silk, nor yet the last larval skin of the parasite, and whether it is an adventitious tissue of the host-larva or a secretion of the parasite, or is explicable upon other grounds, I cannot say.”

The silken cocoon of an aphidiid ichneumon has been found by Miss Murtfeldt, and also by Dr. Riley, under a rose aphid in which it had lived, and referred by Howard to the genus Praon (Fig. 575).

=Sanitary conditions observed by the honey-bee larva, and admission of air within the cocoon.=—Cheshire has observed that after the larva of the honey-bee has spun its cocoon or silken lining of its cell, it observes the following means of preserving cleanliness. The food given to the larva, especially during the latter part of the growing period, contains much pollen, the cases of the grains of which consist of cellulose, which is indigestible.

“These cases, with other refuse matters, collect in quantity within the bowel, which becomes distended, since it has no opening. The imprisoned larva, having little more than enough room for turning, must be freed of these objectionable residua.... In a word, the larva turns its head upon its stomach, and pushes the former towards the base of the cell until its position is reversed, the tail being outwards; and, thus placed, it laps up all residue of food, especially from its old clothes previously referred to, until they are dried, and practically occupy no space. It now throws up its stomach and bowel, with all their contents, and without detaching them from its outer skin, which is moulted as before, but in this instance to be pressed against the cell, so as to form for it an interior lining. The dejectamenta of the bowel in this way lie between the cast skin and cell-wall (as seen at _e_, Fig. 577), and so the larva remains absolutely unsoiled. It now turns its head and resumes its old position, joining its cocoon to the edges of its last cast skin, so that its habitation is relined, it is cleansed, and air can still pass to it through the imperceptible openings left by the bees in the sealing. This point is of radical importance, since breathing is carried on pretty rapidly during the latter part of its subsequent transformations, the absorbed oxygen permitting then of a production of heat, and causing also considerable diminution in weight.”

As to the passage of air into the bee’s cocoon, Cheshire states that before the cocoon can be built, a cover, technically called sealing, is put over the larva by its nurses. These covers are made of pollen and wax, and are pervious to the air. They are more convex and regular in form than those sealing in the honey.[102]

THE PUPA STATE

The word _pupa_ is from the Latin meaning baby. Linnæus gave it this name from its resemblance to a baby which has been swathed or bound up, as is still the custom in Southern Europe. The term _pupa_ should be restricted to the resting inactive stage of the holometabolous insects.

Lamarck’s term _chrysalis_ was applied to the complete or obtected pupa of Lepidoptera and of certain Diptera, and _mumia_, a mummy, to the pupæ of Coleoptera, Trichoptera, and most Hymenoptera. Latreille (1830) also restricted the term pupa to the “oviform nymph,” or puparium, of Diptera. Brauer applies the term _nymph_ to the pupa of metabolous insects.

The typical pupa is that of a moth or butterfly, popularly called a chrysalis. A lepidopterous pupa in which the appendages are more or less folded close to the body and soldered to the integument, was called by Linnæus a _pupa obtecta_; and when the limbs are free, as in Neuroptera, Mecoptera, Trichoptera, and the lepidopterous genus Micropteryx it is called a _pupa libera_ (Fig. 579). When the pupa is enclosed in the old larval skin, which forms a pupal covering (puparium), the pupa was said by Linnæus to be _coarctate_. The pupa of certain Diptera, as that of the orthoraphous families, is nearly as much obtected as that of the tineoid families of moths, especially as regards the appendages of the head; the legs being more as in _pupæ liberæ_ (Fig. 580).

The male Coccid anticipates the metabolous insects in passing through a quiescent state, when, as Westwood states, it is “covered by the skin of the larva, or by an additional pellicle.” The body appears to be broad and flat, the antennæ and fore legs resting under the head, while the two hinder pairs of legs are appressed to the under side of the body. There is but a slight approach to the pupa libera of a metabolous insect.

Riley states that the male larva of _Icerya purchasi_ forms a cocoon waxy in character, but lighter, more flossy, and less adhesive than that of the female egg-cocoon. It melts and disappears when heated, proving its entirely waxy nature. When the mass has reached the proper length, the larva casts its skin, which remains in the hind end of the cocoon, and pushes itself forward into the middle of the cocoon. The pupa (Fig. 581) is of the same general form and size as the larva. All the limbs are free and slightly movable, so that they vary in position, though ordinarily the antennæ are pressed close to the side, as are the wing-pads; the front pair of legs are extended forward. “If disturbed, they twist and bend their bodies quite vigorously.” The pupa state lasts two or three weeks. A similar pupa is that of _Icerya rosæ_. (Riley and Howard.)

The metamorphosis of _Aspidiotus perniciosus_ is of interest. The male nymph differs much after the first moult from the female, having large purple eyes, while the female nymph loses its eyes entirely. It passes into what Riley terms the _pro-pupa_ (Fig. 582, _b_), in which the wing-pads are present, while the limbs are short and thick. The next stage is the “true pupa” (Fig. 582, _c_, _d_), in which the antennæ and legs are much longer than before. There is no waxy cocoon, but only a case or scale composed of the shed larval skin, i.e. “with the first moult the shed larval skin is retained beneath the scale, as in the case of the female; with the later moultings the shed skins are pushed out from beneath the scale,” and when they transform into the imago they “back out from the rear end of their scale.”

The pupæ of Coleoptera and of Hymenoptera, though there is, apparently, no near relationship between these two orders, are much alike in shape, and, as Chapman pertinently suggests, those of both orders are helpless from their quiescence, and hence have resorted for protection to some cocoon or cell.

But it is quite otherwise with the pupæ of Lepidoptera and Diptera, which vary so much in adaptation to their surroundings, and hence afford important taxonomical and phylogenetic characters. This, as regards the Lepidoptera, was almost wholly overlooked until Chapman called attention to the subject, and showed that the pupæ had characters of their own, of the greatest service in working out the classification, and hence the phylogeny, of the different lepidopterous groups. We have, following the lead of Chapman, found the most striking confirmation of his views, and applied our present knowledge of pupal structures to dividing the haustellate Lepidoptera into two groups,—Paleolepidoptera and Neolepidoptera.

The pupæ of the Neuroptera, Coleoptera, and Hymenoptera differ structurally from the imago, in the parts of the head and thorax being less differentiated. Thus in the head the limits or sutures between the epicranium and clypeus, and the occiput and gula, are obscurely marked, while the tergal and pleural sclerites of the imago are not well differentiated until the changes occurring just before the final ecdysis.

It is easy, however, to homologize the appendages of the pupæ with those of the imago of all the holometabolous orders except in the case of the obtected pupa of the Lepidoptera (and probably of the obtected dipterous pupæ), where the cephalic appendages are soldered together.

That the appendages of the lepidopterous pupa are, as generally supposed, merely cases for those of the imago has been shown by Poulton to be quite erroneous. He says: “If we examine a section of a pupal antenna or leg (in Lepidoptera), we shall find that there is no trace of the corresponding imaginal organ until shortly before the emergence of the imago. In the numerous species with a long pupal period, the formation of imaginal appendages within those of the pupa is deferred until very late, and then takes place rapidly in the lapse of a few weeks. This also strengthens the conclusion that such pupal appendages are not mere cases for the parts of the imago, inasmuch as these latter are only contained within them for a very small proportion of the whole pupal period.” On the other hand, Miall and Hammond claim that there is a strong superficial contrast as to the formation of the imaginal organs, between Lepidoptera and tipularian Diptera, the appendages, wings, and compound eyes being substantially those of the imago. “With the exception of the prothoracic respiratory appendages and the tail-fin, there is little in the pupa of Chironomus which does not relate to the next stage.”

The exact homology of the “glazed eye” of the lepidopterous pupæ and of the parts under the head, situated over the maxillæ, is difficult to decide upon, and these points need farther examination. In the dipterous pupa it is interesting to observe that the halteres are large and broad, which plainly indicates that they are modified hind wings. The number and arrangement of the spiracles is different in pupæ from those of the larva and imago.

There are also secondary adaptive structures peculiar to the pupa, which are present and only of use in this stage. These are the thoracic, spiracular, or breathing appendages of the aquatic Diptera (Fig. 583), the various spines situated on the head or thorax, or on the sides, or more often at the end of the abdomen, besides also the little spines arranged in more or less circular rows around the abdominal segments, the cocoon-breaker, and the cremaster of many pupæ.

In the pupa of certain Diptera, there is a terminal cremaster-like spine, as in that of _Tipula eluta_ (Fig. 584), _Tabanus lineola_ (Fig. 585), besides adminicula or locomotive spines like those of lepidopterous pupæ (Fig. 580, _a_, _b_, _c_).

The pupæ of Coleoptera are variously spined or hairy (Fig. 586). Those of Hydrophilus and of Hydrobius are provided with stout spines on the prothorax and abdomen which support the body in its cells, so that, as Lyonet first showed, though surrounded on all sides by moist earth, it is kept from contact with it by the pupal spines; other pupæ of beetles, such as that of the plum weevil, which is also subterranean, possess similar spines. The abdomen of many coleopterous pupæ, such as those of Carabidæ, end in two spines, to aid them in escaping from their cells in wood or in the earth; others have stiff bristles, and others spines along each side of the abdomen (Fig. 586). All these structures are the result of a certain amount of activity in what we call quiescent pupæ, but most of these are for use at the end of pupal life, at the critical moment when by their aid the insect escapes from its cocoon or subterranean cell, or if parasitic, bores out of its host.

If we are to account for the causes of their origin, we are obliged to infer that they are temporary deciduous structures due to the need of support while the body is subjected to unusual strains and stresses in working its way out of its prison in the earth, or its cell within the stems and trunks of plants and similar situations. They are pupal inheritances or heirlooms, and well illustrate the inheritance of characters acquired during a certain definite, usually brief, period of life, and transmitted by the action of synchronous heredity.

The pupæ of certain insects are quite active, thus that of Raphidia, unlike that of Sialis, before its final ecdysis regains its activity and is able to run about. (Sharp, p. 448.)

_a._ The pupa considered in reference to its adaptation to its surroundings and its relation to phylogeny

The form of the pupa is a very variable one, as even in Lepidoptera it is not entirely easy to draw the line between a pupa libera and a pupa obtecta (Fig. 578); and though the period is one of inactivity, yet when they are not in cocoons or in the earth in subterranean cells, their form is more or less variable and adapted to changes in their surroundings. Even in the obtected pupa of butterflies, there is, as every one knows, considerable variability of shape and of armature, which seems to be in direct adaptability to the nature of their environment. Scudder has well shown that in certain chrysalids, such as those of the Nymphalidæ, which are variously tuberculated, and hang suspended by the tail, and often hibernate, these projections serve to protect the body. All chrysalids with projections or ridges on different parts of the body, being otherwise unprotected, move freely when struck by gusts of wind, hence “the greater the danger to the chrysalis from surrounding objects, the greater its protection by horny tubercles and roughened callous ridges.” The greater the protection possessed in other ways, as by firm swathing or a safe retreat, the smoother the surface of the body and the more regular and rounded its contours. The tendency to protection by tubercles is especially noticeable in certain South American chrysalids of nymphalid butterflies. This response to the stimuli of blows or shocks is also accompanied by a sensitiveness to the stimulus of too strong light.

Previously Scudder[103] had made the important suggestion that the smooth crescent-shaped belt of the “glazed eye” or “eyepiece” of chrysalids is, as an external covering of the eye, midway between that of the caterpillar and the perfect insect, and he asks: “May it not be a relic of the past, the external organ of what once was? And are we to look upon this as our hint that the archaic butterfly in its transformations passed through an _active_ pupal stage, like the lowest insect of to-day, when its limbs were unsheathed, its appetite unabated?” etc. Scudder also shows that “the expanded base of the sheath covering the tongue affords protection also to the palpi which lie beneath and beside the tongue.”

All this tends to show the importance of studying the structure of the pupa, in order to ascertain how the pupal structures have been brought about, with the final object of discovering whether the pupæ of the holometabolic insects are not descended from active nymphs, and if so, the probable course of the line of descent.

_b._ Mode of escape of the pupa from its cocoon

“In all protected pupæ,” as Chapman says, “the problem has to be faced, how is the imago to free itself from the cocoon or other envelope protecting the pupa.” In the Coleoptera and Hymenoptera the imago becomes perfected within the cocoon or cell, as the case may be, and as Chapman states, “not only throws off the pupal skin within the cocoon, but remains there till its appendages have become fully expanded and completely hardened, and then the mandibles are used to force an outlet of escape,” and he calls attention to the fact that “in many cases, even in some entire families, they are of no use whatever to the imago except in this one particular,” and he cites the Cynipidæ as perhaps the most striking instance of this circumstance.

In those Neuroptera which spin a silken cocoon, _e.g._ the Hemerobiidæ, the Trichoptera, and in Micropteryx (Fig. 588), the jaws used by the pupa for cutting its way out of the cocoon are even larger in proportion than in the pupa of caddis-flies (Fig. 588), being of extraordinary size.

In Myrmeleon the pupa pushes its way half out of the cocoon, and then remains, while the imago ruptures the skin and escapes (Fig. 589, _a_).

Thus in the Neuroptera and Trichoptera we have already established the more fundamental methods of escape from the cocoon, which we see carried out in various ways in the more generalized or primitive Lepidoptera.

The most primitive method in the Lepidoptera of escaping from the cocoon seems to be that of Micropteryx.

“In this genus,” says Chapman, “though it is nominally the pupa that escapes from the cocoon, it is in reality still the imago, the imago clothed in the effete pupal skin. To rupture the cocoon it uses not its own jaws, but those of the pupal skin, energizing them, however, in some totally different way from ordinary direct muscular action, their movements being the result of the vermicular movements of the pupa, acting probably by fluid pressure on the articular structure of the jaws, by some arrangement not altogether different perhaps from the frontal sac of the higher Diptera. In the Micropteryges the jaws of the pupa not only rupture the cocoon, but appear to be the most active agents in dragging the pupa through the opening in the cocoon and through any superincumbent earth, being merely assisted by the vermicular action of the abdominal segments, and we find in accordance with this circumstance that the pupal envelope is still very thin and delicate, and has little or no hardening or roughness by which to obtain a leverage against the walls of the channel of escape.” (Trans. Ent. Soc. London, 1896, pp. 570, 571.)

Some sort of a beak or hard process, more or less developed, according to Chapman, adapted for breaking open the cocoon exists in nearly all the Lepidoptera with incomplete pupæ (_pupæ incompletæ_), except the limacodid and nepticulid section. “In all these instances the pupa emerges from the cocoon precisely as in the Micropteryges, that is, the moth it really is that emerges, but does so encased in the pupal skin. To achieve this object, it seems to have been found most efficient to have three, four, or five abdominal segments capable of movement, but to have the terminal sections (segments) soldered together.”

This cocoon-breaker, as we may call it, is especially developed in _Lithocolletis hamadryadella_. As described by Comstock, it forms a toothed crest on the forehead which enables it to pierce or saw through the cocoon.

“Each pupa first sawed through the cocoon near its juncture with the leaf and worked its way through the gap, by means of the minute backward-directed spines upon its back, until it reached the upper cuticle of the leaf. Through this cuticle it sawed in the same way that it did through the cocoon. The hole was in each case just large enough to permit the chrysalis to work its way out, holding it firmly when partly emerged. When half-way out it stopped, and presently the skin split across the back of the neck and down in front along the antennal sheaths, and allowed the moth to emerge.”[104]

We have observed and figured the cocoon-breaker in Bucculatrix, Talæporia (Fig. 590, _a_), Thyridopteryx, and Œceticus, and rough knobs or slight projection answering the purpose in Hepialidæ, Megalopyge, Zeuzera, and in Datana.[105] See also the spine on the head of _Sesia tipuliformis_ (Fig. 578).

The imago of the attacine moths cuts or saws through its cocoon by means of a pair of large, stout, black spines (_sectores coconis_), one on each side of the thorax at the base of the fore wings (Fig. 591), and provided with five or six teeth on the cutting edge (_C_, _D_).

Our attention[106] was drawn to this subject by a rustling, cutting, and tearing noise issuing from a cocoon of _Actias luna_. On examination a sharp black point was seen moving to and fro, and then another, until both points had cut a rough irregular slit, through which the shoulder of the moth could be seen vigorously moving from side to side. The hole or slit was made in one or two minutes, and the moth worked its way at once out of the slit. The cocoon was perfectly dry. The cocoon-cutter occurs in all the American genera, in _Samia cynthia_, and is large and well marked in the European _Saturnia pavonia-minor_ and _Endromis versicolora_. In _Bombyx mori_ the spines are not well marked, and they are quite different from those in the Attaci. There are three sharp points, being acute angles of the pieces at the base of the wing, and it must be these spines which at times perform the cutting through of the threads of the cocoon described by Réaumur, and which he thought was done by the facets of the eyes. It is well known that in order to guard against the moths cutting the threads, silkraisers expose the cocoon to heat sufficient to destroy the enclosed pupa. In Platysamia the cocoon-cutters, though well developed, do not appear to be used at all, and the pupa, like that of the silkworm and other moths protected by a cocoon, moistens the silk threads by a fluid issuing from the mouth, which also moistens the hairs of the head and thorax, together with the antennæ. It remains to be seen whether these structures are only occasionally used, and whether the emission of the fluid is not the usual and normal means of egress of the moth from its cocoon. Dr. Chapman remarks that throughout the obtected moths “there are many devices for breaking through the cocoon: specially constructed weak places in the cocoon, softening fluid, applied by the moth, assisted by special appliances of diverse sorts, such as in Hybocampa[107] and Attacus,” etc.

As to the fluid mentioned above, Trouvelot states that it is secreted during the last few days of the pupa state, and is a dissolvent for the gum so firmly uniting the fibres of the cocoon. “This liquid is composed in great part of bombycic acid.” (Amer. Naturalist, i, p. 33.)

The pupa of the dipterous genus Sciara (_S. ocellaris_ O. S.) resembles a tineid pupa, and before transforming emerges for about two-thirds of its length from the cocoon; the pupa-skin remaining firmly attached in this position.[108]

Certain hymenopterous pupæ are provided with temporary deciduous conical processes. Thus we have observed in the pupa of _Rhopalum pedicellatum_ two very prominent acute tubercles between the eyes (_h_, Fig. 592). As the cocoon is very slight, these may be of use either in extracting itself from the silken threads or in pushing its way along before emerging from the tunnel in the stem of plants. (See also p. 611.)

_c._ The cremaster

Although this structure is in general confined to lepidopterous pupæ, and is not always present even in them, since it is purely adaptive in its nature, yet on account of its singular mode of development from the larval organs, and the accompanying changes in the pupal abdomen, it should be mentioned in this connection. The cremaster is the stout, triangular, flattened, terminal spine of the abdomen, which aids the pupa in working its way out of the earth when the pupa is subterranean, or in the pupa of silk-spinning caterpillars its armature of secondary hooks and curved setæ enables it to retain its hold on the threads of the interior of its cocoon after the pupa has partially emerged from the cocoon, restraining it, as Chapman well says, “at precisely that degree of emergence from the cocoon that is most desirable.” He also informs us that while in the “_pupæ incompletæ_ the cremaster is attached to an extensible cable, which always allows some emergence of the pupa, in the pupæ obtectæ there is no doubt but that in such cases as the Ichthyuræ, Acronyctæ, and many others, it retains the pupal case in the same position within the cocoon that the living pupa occupied; this is also very usually the case in the Geometræ and in the higher tineids (my pyraloids).”

In many of the more generalized moths there is no cremaster (Micropteryx, Gracilaria, Prodoxus, Tantura, Talæporia, Psychidæ, Hepialidæ, Zeuzera, Nola, Harrisina), though in Tischeria and Talæporia (Fig. 590, but not in Solenobia) and Psychidæ, two stout terminal spines perform the office of a cremaster, or there are simply curved setæ on the rounded, unarmed end of the abdomen, as in Solenobia.

In the obtected Lepidoptera, for example in such a group as the Notodontidæ, where the cremaster is present, though variable in shape, it may from disuse, owing to the dense cocoon, be without the spines and hooks in Cerura, or the cremaster itself is entirely wanting in Gluphisia, and only partially developed in Notodonta. In the butterflies whose pupæ are suspended (Suspensi), the cremaster is especially well developed. Reference might here be made to the temporary pupal structures in certain generalized moths, which take the place of a cremaster, such as the transverse terminal row of spines in Tinea, the two stout spines in Tischeria, and the dense rough integument and thickened callosities of the pupal head and end of abdomen of Phassus, which bores in trees with very hard wood; also the numerous stout spines at the end and sides of the abdomen in Ægerians. These various projections and spines, besides acting as anchors and grappling hooks, in some cases serve to resist strains and blows, and have undoubtedly, like the armature in the larvæ and imagines of other insects, arisen in response to intermittent or occasional pressure, stresses, and impacts.

=Mode of formation of the cremaster and suspension of the chrysalis in butterflies.=—We are indebted to Riley[109] for an explanation of the way the cremaster has originated, his observations having been made on species of over a dozen genera of butterflies (Suspensi).

He shows that the cremaster is the homologue of the suranal plate of the larva.[110] The preliminary acts of the larva have been observed by various authors since the days of Vallisneri, _i.e._ the larva hanging by the end of the abdomen, turning up the anterior part of the body in a more or less complete curve, and the skin finally splitting from the head to the front edge of the metathoracic segment, and being worked back in a shrivelled mass toward the point of attachment. The critical feat, adds Riley, which has most puzzled naturalists, is the independent attachment of the chrysalis and the withdrawal from and riddance of the larval skin which such attachment implies. Réaumur explained this in 1734 by the clutching of the larval skin between sutures of the terminal segments of the chrysalis, and this is the case, though the sutures act in a somewhat different way.

Before pupation the larva spins a mass or heap of silk, the shape of which is like an inverted settee or a ship’s knee, and “one of the most interesting acts of the larva, preliminary to suspension, is the bending and working of the anal parts in order to fasten the back of the (suranal) plate to the inside of the back of the settee, while the crotchets of the legs are entangled in the more flattened position or seat.”

In shedding the larval skin, the following parts are also shed, and have some part to play in the act of suspension: _i.e._ 1st, the tracheal ligaments (Fig. 593, _tl_), or the shed tracheæ from the last or 9th pair of spiracles; 2d, the rectal ligament (Fig. 593, _rl_), or shed intestinal canal; 3d, the Osborne or retaining membrane (_membrana retinens_, Fig. 593, _mr_), which is the stretched part of the membrane around the rectum and in the anal legs, and which is intimately associated with the rectal ligament.

The structures in the chrysalis are, first, the cremaster, with its dorsal (Fig. 594, _dcr_) and ventral (_vcr_) ridges, and the cremastral hook-pad (_chp_), said by Riley to be “thickly studded with minute but stout hooks, which are sometimes compound or furnished with barbs, very much as are some of our fishing-hooks, and which are most admirably adapted to the purpose for which they are intended.”

Secondly, there are the other structures, viz., the sustainers (_sustentors_), two projections which Riley states “homologize with the soles (_plantæ_) of the anal prolegs, which take on various forms (3), but are always directed forward so as easily to catch hold of the retaining membrane.” These sustentors are, however, as Jackson[111] has shown, and as we are satisfied, the vestiges of the anal legs.

Thirdly, the sustentor ridges, which, as Riley states, may be more or less obsolete in some forms, in Paphia (Fig. 596, _B_) and Limenitis form “quite a deep notch, which doubtless assists in catching hold of the larval skin in the efforts to attach the cremaster.”

“It is principally,” adds Riley, “by the leverage obtained by the hooking of the sustainers in the retaining membrane, which acts as a swimming fulcrum, that the chrysalis is prevented from falling after the cremaster is withdrawn from the larval skin. It is also principally by this same means that it is enabled to reach the silk with the cremastral hook-pads.”

“Dissected immediately after suspension, the last abdominal segment of the larva is found to be bathed, especially between the legs and around the rectum, in an abundance of translucent, membranous material.”

“An hour or more after suspension the end of the forming chrysalis begins to separate from the larval skin, except at the tip of the cremaster (Fig. 597, _b_). Gradually the skin of the legs and of the whole subjoint (10th segment) stretches, and with the stretching, the cremaster elongates, the rectal piece recedes more and more from the larval rectum, and the sustentor ridges diverge more and more from the cremaster, carrying with them, on the sustainers, a part of the soft membrane.” The rectal ligament will sustain at least 10 or 12 times the weight of the chrysalis. That of Apatura seems to rely almost entirely on the rectal ligament, assisted by the partial holding of the delicate larval skin.

FORMATION OF THE PUPA AND IMAGO IN THE HOLOMETABOLOUS INSECTS (THE DIPTERA EXCEPTED)

We have seen that in the incomplete metamorphosis, although there may be as many as five, and possibly seven moults, and in Chloëon as many as 20, and in _Cicada septemdecim_ perhaps 25 or 30, there is but a slight change of form from one stage to another, and no period of inactivity. And this gradual outer transformation is so far as yet known paralleled by that of the internal organs, the slight successive changes of which do not differ from those observed in the growth of ametabolous insects. With the growth of the internal organs there probably goes on a series of gradual regenerative processes, and Korschelt and Heider state that we may venture to assume that each changed cell or group of cells which have become exhausted by the exercise of the functions of life are reabsorbed and become restored through the vital powers of the tissues, so that as the result there goes on a constant, gradual regeneration of the organs.

While the Hemiptera have only an incomplete metamorphosis, the males of the Coccidæ are, as shown by O. Schmidt, remarkable for passing through a complete or holometabolous development, with four stages, three of which are pupal and inactive. Hence, as Schmidt observes, there is here a hypermetamorphosis, like that of the Meloidæ, Stylopidæ, etc.

Shortly before the end of the larval stage of the male appear the imaginal buds of the eyes, legs, and wings. In the 2d or 1st pupal stage there is an atrophy of the antennæ and legs. On the other hand, at this stage the female completes its metamorphosis.

The rudiments of the wings arise on the edge of the dorsal and ventral side of the 2d thoracic segment, and this, we would remark, is significant as showing a mode of origin of the wings intermediate between that of the manometamorphic and holometamorphic insects. (See pp. 137–142.) While Schmidt could not ascertain the exact structure of the imaginal buds, he says “in general the process of formation of the extremities is exactly as Weismann has described in Corethra.” The two later pupal stages are “as in other metabolic insects.” (See p. 690, Fig. 637.)

Thus far the internal changes in the metamorphosis of the Coleoptera have not been thoroughly studied. They are less complete than in the other holometabolous insects, the differences between the larva and imago being much less marked than in the more specialized orders, and so far as known all the larval organs pass, though not without some great changes, directly into the imaginal ones, the only apparent exception being the mid-intestine, which, as stated by Kowalevsky, undergoes a complete transformation during metamorphosis. The following account, then, refers almost wholly to the Lepidoptera, Hymenoptera, and Diptera.

_a._ The Lepidoptera

The first observations on the complete metamorphosis of insects which were in any way exact were those of Malpighi, in 1667, and of Swammerdam, in 1733. While the observations of Swammerdam, as far as they extended, were correct, his conclusions were extraordinary. They were, however, accepted by Réaumur and by Bonnet, and generally held until the time of Herold in 1815, and lingered on for some years after. The rather famous theory of incasement (“_emboîtement_”) propounded by Swammerdam was that the form of the larva, pupa, and imago preëxisted in the egg, and even in the ovary; and that the insects in these stages were distinct animals, contained one inside the other, like a nest of boxes, or a series of envelopes one within the other, or, to use his own words: “_Animal in animali, seu papilio intra erucam reconditus._”

This theory Swammerdam extended to the whole animal kingdom. It was based on the fact that by throwing the caterpillar, when about to pupate, in boiling water, and then stripping off the skin, the immature form of the butterfly with its appendages was disclosed. Malpighi had previously observed the same fact in the silkworm, perceiving that before pupation the antennæ are concealed in the head of the larva, where they occupy the place previously taken by the mandibular muscles; also that the legs of the moth grew in those of the larva, and that the wings developed from the sides of the worm.

Even Réaumur (1734) remarked: “Les parties du papillon cachées sous le fourreau de chenille sont d’autant plus faciles à trouver que la transformation est plus proche. Elles y sont neanmoins de tout temps.” He also believed in the simultaneous existence of two distinct beings in the insect. “Il serait très curieux de connaître toutes les communications intimes qui sont entre la chenille et le papillon.... La chenille hache, broye, digere les aliments qu’elle distribué au papillon; comme les mères préparent ceux qui sont portés aux fœtus. Notre chenille en un mot est destineé à nourrir et à defendre le papillon qu’elle renferme.” (T. i, 8^e Mémoire, p. 363.)

Lyonet (1760), even, did not expose the error of this view that the larva enveloped the pupa and imago, and, as Gonin says, it was undoubtedly because he did not use for his dissections of the caterpillar of Cossus any specimens about to pupate. Yet he detected the wing-germs and those of the legs, stating that he presumed the bodies he saw to be the rudiments of the legs of the moth (p. 450).

Herold, in his work on the development of the butterfly (1815), was the first to object to this erroneous theory, showing that the wings did not become visible until the very end of larval life; that as the larval organs disappear, they are transformed or are replaced by entirely new organs, which is not reconcilable with a simple putting off of the outer envelope. The whole secret of metamorphosis, in Herold’s opinion, consisted in this fact, that the butterfly in the larva state increases and accumulates a supply of fat until it has reached the volume of the perfect state; then it begins the chrysalis period, during which the organs are developed and take their definite form.[112] (Abstract mostly from Gonin.) Still the old ideas prevailed, and even Lacordaire, in his Introduction à l’Entomologie published in 1834, held on to Swammerdam’s theory, declaring that “a caterpillar is not a simple animal, but compound,” and he actually goes so far as to say that “a caterpillar, at first scarcely as large as a bit of thread, contains its own teguments threefold and even eightfold in number, besides the case of a chrysalis, and a complete butterfly, all lying one inside the other.” This view, however, we find is not original with Lacordaire, but was borrowed from Kirby and Spence without acknowledgment. These authors, in their Introduction to Entomology (1828), combated Herold’s views and stoutly maintained the old opinions of Swammerdam. They based their opinions on the fact, then known, that certain parts of the imago occur in the caterpillar. On the other hand, Herold denied that the successive skins of the pupa and imago existed as germs, holding that they are formed successively from the “_rete mucosum_,” which we suppose to be the hypodermis of later authors. In a slight degree the Swammerdam-Kirby and Spence doctrine was correct, as the imago does arise from germs, _i.e._ the imaginal disks of Weismann, while this was not discovered by Herold, though they do at the outset arise from the hypodermis, his _rete mucosum_. Thus there was a grain of truth in the Swammerdam-Kirby and Spence doctrine, and also a mixture of truth and error in the opinions of Herold.

The real nature of the internal changes wrought during the process of metamorphosis was first revealed by Weismann in 1864. His discovery of the germs of the imago (imaginal buds) of the Diptera, and his theory of _histolysis_, or of the complete destruction of the larval organs by a gradual process, was the result of the application of modern methods of embryology and histology, although his observations were first made on the extremely modified type of the Muscidæ or flies, and, at first, he did not extend his view to include all the holometabolous insects. Now, thanks to his successors in this field, Ganin, Dewitz, Kowalevsky, Van Rees, Bugnion, Gonin, and others, we see that metamorphosis is, after all, only an extension of embryonic life, the moults and great changes being similar to those undergone by the embryo, and that metamorphosis and alternation of generations are but terms in a single series. Moreover, the metamorphoses of insects are of the same general nature as those of certain worms, of the echinoderms, and the frog, the different stages of larva, pupa, and imago being adaptational and secondary.

While the changes in form from the larva to the pupa are apparently sudden, the internal histogenetic steps which lead to them are gradual. In the Lepidoptera a few days (usually from one to three) before assuming the pupa stage, the caterpillar becomes restless and ceases to take food. Its excrements are now hard, dry, and, according to Gonin, are “stained carmine red by the secretions of the urinary tubes.” Under the microscope we find that they are almost exclusively composed of fragments of the intestinal epithelium. These red dejections were noticed by Réaumur, and afterwards by Herold, and they are sure indications of the approach of the transformations. It now wanders about, and, if it is a spinner, spins its cocoon, and then lies quietly at rest while the changes are going on within its body. Meanwhile, it lives on the stores of fat in the fat-body, and this supply enables it to survive the pupal period.

The amount of fat is sometimes very great. Newport removed from the larva of _Cossus ligniperda_ 42 grains of fat, being more than one-fourth of the whole weight of the insect, he adds that the supply is soon nearly exhausted during the rapid development of the reproductive organs, “since, when these have become perfected, the quantity that remains is very inconsiderable.”

Although the larval skin of a lepidopterous insect is suddenly cast off, the pupa quickly emerging front it, yet there are several intermediate stages, all graduating into each other. If a caterpillar of a Clisiocampa, which, as we have observed, is much shortened and thickened a day or two before changing to a pupa, is hardened in alcohol and the larval skin is stripped off, the semipupa (pro-nymph, pro-pupa of different authors) is found to be in different stages of development, and the changes of the mouth-parts are interesting, though not yet sufficiently studied.

Newport attributes the great enlargement and changes in the shape of the thoracic segments of the larva of _Vanessa urticæ_ at this time, to the contraction or shortening of the muscles of the interior of those segments, “which are repeatedly slowly extended and shortened, as if the insect were in the act of laborious respiration.” This, he adds, generally takes place at short intervals during the two hours immediately preceding the change to the pupa, and increases in frequency as that period approaches. He thus describes the mode of moulting the larval skin: “When the period has arrived, the skin bursts along the dorsal part of the 3d segment, or mesothorax, and is extended along the 2d and 4th, while the coverings of the head separate into three pieces. The insect then exerts itself to the utmost to extend the fissure along the segment of the abdomen, and, in the meantime, pressing its body through the opening, gradually withdraws its antennæ and legs, while the skin, by successive contortions of the abdomen, is slipped backwards, and forced towards the extremity of the body, just as a person would slip off his glove or his stocking. The efforts of the insect to get entirely rid of it are then very great; it twirls itself in every direction in order to burst the skin, and, when it has exerted itself in this manner for some time, twirls itself swiftly, first in one direction, then in the opposite, until at last the skin is broken through and falls to the ground, or is forced to some distance from it. The new pupa then hangs for a few seconds at rest, but its change is not yet complete. The legs and antennæ, which when withdrawn from the old skin were disposed along the under surface of the body, are yet separate, and do not adhere together as they do a short time afterwards. The wings are also separate and very small. In a few seconds the pupa makes several slow, but powerful, respiratory efforts; during which the abdominal segments become more contracted along their under surface, and the wings are much enlarged and extended along the lateral inferior surface of the body, while a very transparent fluid, which facilitated the slipping off of the skin, is now diffused among the limbs, and when the pupa becomes quiet dries, and unites the whole into one compact covering.”

=The changes in the head and mouth-parts.=—The changes of form from the active mandibulate caterpillar to the quiescent pupa, and then to the adult butterfly, are, as we have seen, in direct adaptation to their changed habits and surroundings, and they differ greatly in details in insects of different orders. In many Lepidoptera and certain Diptera the pupa and imago are without the mandibles of the larva, and, instead, the 1st maxillæ in the former order, and the 2d maxillæ in the latter, are highly developed and specialized. The changes in the shape of the head, with the antennæ, the latter rudimentary in the larvæ of the two orders named, are noteworthy, and will be referred to under those orders. The same may be said of the thorax with the legs and wings, and the abdomen with the ovipositor. Every part of the body undergoes a profound change, though in the Coleoptera, Trichoptera, and the more generalized and primitive Diptera, each segment and appendage of the larva are directly transformed into the corresponding parts of the pupa, and subsequently of the imago. We shall see, however, beyond, that this general statement does not apply to the Hymenoptera, in which there is a process of cephalization or transfer of parts headward, peculiar to that order.

=The change in the internal organs.=—These were especially, as regards the nervous system, first carefully examined and illustrated by that great English entomotomist, Newport, and those of the reproductive organs by Herold as early as 1815. A glance at the figures (598–604), reproduced from Newport’s article Insecta, will show the changes wrought especially in the digestive and nervous systems of Sphinx and Vanessa, his account of the alterations in the muscles having already been quoted. As the pupal form is much nearer to that of the imago than of the larva, so the digestive canal is seen to be nearly as much differentiated in the pupa as in the imago, though the reservoir (“sucking-stomach”) of the imago is not indicated in the pupa. These changes are such as occur in an insect which is enormously voracious as a larva, and which often, passing through a period of complete inactivity, taking no food at all, finally becomes an insect which needs to suck in only a minimum quantity of water or nectar, and which practically abstains from all food. The head and genital glands also, as well as the urinary vessels, are nearly the same. On the other hand, the salivary glands have undergone, in the imago, a thoroughgoing reduction.

The changes undergone by the nervous system of _Sphinx ligustri_ and _Vanessa urticæ_ have been described by Newport with fulness of detail. An abstract of his observations on _Vanessa urticæ_, which undergoes its changes in June in 14 days, and in August in eight days, we will now give, in part verbatim, the subject being rendered much clearer by his figures, which are reproduced.

During the last larval stage, certain changes have already taken place in different parts of the cord, which shows that they had been a long time in progress. Besides the lateral approximation of the cords, the first change consists in a union of the 11th and 12th ganglia, the latter one being carried forwards; these two ganglia being entirely separate before the 3d moult.

Two hours after the larva of _Vanessa urticæ_ has suspended itself in order to pupate, the brain is not yet enlarged, but the subœsophageal ganglion is nearly twice its original size and the ganglia behind are nearer together. “A little while before the old larval skin is thrown off there is great excitement throughout the body of the insect.” About half an hour (Fig. 603, 2) before this occurs the alary nerves and the cerebral, 2d, 3d, 4th, and 5th ganglia are slightly enlarged, and the 1st subœsophageal ganglion very considerably. Immediately after the insect has entered the pupa state (Fig. 603, 3), all the ganglia are brought closer together. One hour after (Fig. 603, 4) pupation the cerebral ganglia are found to be more closely united, the 4th and 5th ganglia are nearer, and the distance between the remaining ganglia is also reduced.

Seven hours after pupation there is a greater enlargement of the cerebral ganglia, optic nerves, and ganglia and cords of the future thoracic segments.

At 12 hours (Fig. 603, 5) the 5th pair of ganglia has almost completely coalesced with the cord and the 4th; at 18 hours (Fig. 603, 6) the whole of the ganglia, cords, and nerves have become more enlarged, especially those of the wings, while the 4th and 5th ganglia of the cords have now so completely united as to appear like an irregular elongated mass. At 24 hours (Fig. 604, 7) the 4th and 5th ganglia are completely united, the 5th being larger than the 4th. At 36 hours (Fig. 604, 8) the optic nerves have attained a size almost equal to that of the brain. The 1st subœsophageal ganglion now forms, with the cerebral ones, a complete ring around the œsophagus, the crura having almost disappeared. The 6th ganglion has now disappeared, but the nerves arising from it remain. At 48 hours (Fig. 604, 9) the cord is straight instead of being sinuous, and the 7th ganglion has disappeared, while the thoracic ganglia are greatly enlarged. At the end of 58 hours the 2d and 3d thoracic ganglia have united, and the double ganglion thus formed is only separated from the large thoracic mass composed of the 4th, 5th, and part of the 6th ganglia, by the short but greatly enlarged cords which pass on each side of the central attachment of the muscles. “The optic and antennal nerves have nearly attained their full development, and those numerous and most intricate plexus of nerves in the three thoracic segments of the larva form only a few trunks, which can hardly be recognized as the same structures. The arrangement of the whole nervous system is now nearly as it exists in the perfect insect. The whole of these important changes are thus seen to take place within the first three days after the insect has undergone its metamorphosis; and they precede those of the alimentary canal, generative system, and other organs, which are still very far from being completed, and indeed, as compared with the nervous system, have made but little progress.” (Art. Insecta, pp. 962–965.)

The initial steps and many of the subsequent internal changes escaped the notice of Newport and others of his time, and it was not until the epoch-making work of Weismann on the ultimate processes of transformation of Corethra and of Musca, that we had an adequate knowledge of the subject.

Weismann (1864) was the first to show for the Muscidæ and Corethra that the appendages, wings, and other parts of the imago originate in separate, minute, cellular masses called imaginal disks, buds, or folds (histoblasts of Künckel). These imaginal buds, which arise from the hypodermis, being masses of indifferent cells, are usually present in the very young larva, and even in the later embryonic stages. It has been shown that such imaginal buds exist for each part of the body, not only for the appendages and wings (p. 126), but for the different sections of the digestive canal. During the semipupal stage these buds enlarge, grow, and at the same time there is a corresponding destruction of the larval organs. The process of destruction is due to the activity of the blood corpuscles or leucocytes (phagocytes), the larval organs thus broken up forming a creamy mass, the buds from which the new organs are to arise resisting the attacks of the virulent leucocytes, which attach themselves to the weakened tissue and engulf the pigments (see p. 422). The two processes of destruction of the larval organs (histolysis) and the building up of the imaginal organs (histogenesis) go hand in hand, so that the connection of the organs in question in most cases remains entirely continuous; while the last steps in the destruction of the larval organs only take place after the organs of the imago have assumed their definite shape and size. Other observers have corroborated and confirmed his statements and observations, Gonin extending them to the Lepidoptera and Bugnion to the Hymenoptera.

It is a pity that the observations, such as were set on foot by Weismann, were not first made on the Trichoptera and Lepidoptera, which are much more primitive and unmodified forms than the Diptera, but mistakes of this nature have frequently happened in the history of science.

The latest and most detailed researches are those of J. Gonin on the metamorphoses of _Pieris brassicæ_, made under the direction of Professor E. Bugnion. They fill an important gap in our knowledge, and show that the Lepidoptera transform in nearly the same manner as described by Weismann in Corethra. We give the following condensed account of Gonin’s observations.

On opening a caterpillar entering on the semipupa state (Fig. 605), the relative position of the germs (imaginal buds or folds) of the wings and of the legs are seen.

These imaginal buds in a more advanced stage are seen in our sections of a tineid larva (Figs. 606, 607).

The number of 12 imaginal buds found by Weismann in the thorax of Muscidæ does not occur in Lepidoptera, since, as in the Hymenoptera (Bugnion), the dorsal buds of the prothoracic segment are wanting. Gonin finds in Pieris that the ventral buds of the three thoracic segments are each represented by several distinct folds attached to the femoro-tibial bud and to the tarsal joints.

The imaginal buds serve in some cases for the formation of new organs (wings, legs of insects with apodous larvæ); in others for the growth and the transformation of organs already existing (legs, antennæ, 1st and 2d maxillæ of Lepidoptera).

As to the peripodal sac or hypodermic envelope which contains the imaginal bud, a portion persists and is regenerated, while the other part becomes useless and is detached under the form of débris, as shown by Weismann, Viallanes, and Van Rees in the Muscidæ. On this point Gonin disagrees with Dewitz, who stated that the walls of the wing-sacs are not destroyed, but are only gradually withdrawn at the time of pupation, in order to allow the orifice to distend and let the wing pass out to the exterior.

The portion of the sac which persists (basal portion, peripheral pad of Bugnion, or annular zone of Künckel) serves at first to attach the appendage, while forming, to the hypodermis of the larva, then afterwards to more or less completely regenerate the adjoining portion of the integument. In this way the hypodermis of the thorax is partially, that of the head is almost entirely, replaced by the imaginal epithelium which proliferates at the base of the appendages,[113] while that of the abdominal segments persists, at least in a modified way, and only undergoes (at the end of the pupal period) transformations as regards the appearance of the scales and pigment.

=The wings.=—The imaginal buds of the wings do not participate in the larval moults. Gonin has observed, contrary to Dewitz, that their surface only forms a cuticle towards the end of the last larval stage.

The network of fine tracheæ of the wing-bud is drawn out at the time of pupation with the internal cuticle of the large tracheæ. The permanent tracheæ of the wing have already appeared at the time of the 3d moult under the form of large rectilinear trunks, the position of which corresponds afterwards to that of the veins, but they are not filled with air until the time of pupation. There are from eight to ten of these tracheæ in each wing (Fig. 159), and they give rise in the pupa to a new system of fine tracheæ (tracheoles) which replaces that of the larva. (For further details the reader is referred to pp. 126–137.)

=Development of the feet and the cephalic appendages.=[114]—In the apodous larvæ of Diptera and Hymenoptera the rudiments of the legs are, like those of the wings, developed within hypodermal sacs; at times they remain there up to the end of larval life, but sometimes they appear early at the surface. This origin of the legs, thanks to Weismann, Künckel, and Van Rees, is well known in the Diptera; in the Hymenoptera it has been proved to be the case with ants by Dewitz, and in Encyrtus by Bugnion. As for the Lepidoptera our knowledge that the legs of the imago arise from the six thoracic legs of the caterpillar, up to the date of Gonin’s paper has not been in advance of that of Malpighi and Swammerdam.

Réaumur, moreover, was supposed to have furnished the proof, having from his experiments concluded that “if the legs of the pupa appear longer and larger than those of the caterpillar wherein they were contained, it is because they were folded and squeezed.” (8^e Mém., p. 365.)

This explanation of Réaumur’s has been generally accepted. Graber (Die Insekten, p. 506) accepted it, after examining microscopic sections of the legs, and Künckel averred that “Réaumur, having, in certain caterpillars, completely cut off one of the thoracic legs, had concluded that the butterfly which came from it lacked the corresponding member.” (Rech. sur l’org. et dév. des volucelles, p. 160.)

Newport, it is true, denied this disappearance of the legs, but did not wish to put himself in opposition to received ideas, and supposed that the member cut off was partly reformed in the imago.

Künckel believes that he has found a better solution in his theory of histoblasts or imaginal buds; in his opinion, “Réaumur and Newport are both right,” but “when Réaumur cut off a caterpillar’s leg, he at the same time removed the histoblast, the rudiment of the leg of the butterfly. When Newport repeated this experiment, he simply mutilated the histoblast without completely destroying it: in the first case, the adult insect was born with one leg less; in the second case, it appeared with an atrophied foot.”

“So ingenious an explanation,” says Gonin, “is not necessary.” To prove that the experiments of the two savants are not contradictory, it would have been sufficient to cite, as Künckel did not do, the exact words of Réaumur, for he having cut from a caterpillar “more than half of three of the thoracic legs on the same side,” says he found that the chrysalis had “the three limbs on one side _shorter_ than the corresponding ones on the other side.” The same operation repeated on a somewhat younger caterpillar again showed in the chrysalis three maimed limbs, “so that they could not be said to be entirely absent. These results are like those of Newport; the interpretation only was faulty, as we shall attempt to prove.”

The real relations of the adult legs to the larval legs are thus shown by Gonin.

“If we carefully strip off the skin of a caterpillar near the time of pupation (Fig. 608), we see that the extremity only of the legs of the imago is drawn out of the larval legs; the other parts are pressed against each side of the thorax: near the ventral line a small pad represents the coxa and the trochanter; the femur and the tibia are distinctly recognizable, but soldered to each other and only separated by a slight furrow; they form by their union a very acute knee or bend. The femur is movable on the pad-like coxa, the tibia continues without precise limits with the extremity concealed in the larval legs. The three divisions of the latter do not appear to have any relation with the live joints of the perfect state. Under the microscope the rudiment appears very strongly plaited at the level of the tarsus, much less so in the other regions. A large trachea penetrates into the femur with some capillaries; reaching the knee it bends into the tibia at a sharp curve, but does not become truly sinuous in approaching the extremity. It is then the tarsus especially which is susceptible of elongation; it may, on being withdrawn, give rise to the illusion that the whole organ is disengaged from the larval leg.

“This disposition is, we believe, not known. It gives the key to the experiments of Réaumur and of Newport.

“Even when we cut off the limb of the caterpillar at its base, we only remove the tarsus of the imago; the femur and the tibia remain intact. From an evident homology Réaumur has erroneously concluded that there is an identity. His opinion, classical up to this day, that the limb of the butterfly is entirely contained in the leg of the caterpillar, has been found to be inexact and should be abandoned.”

=Embryonic cells and the phagocytes.=—Up to the last larval stage the legs do not offer, says Gonin, any vestige of an imaginal germ, but they contain a great number of embryonic cells (Fig. 145, _ec_). They are almost always collected around a nerve or trachea; sometimes they are independent, and sometimes retained in the peritoneal sheath, seeming to arise by proliferation from this sheath. Some thus contribute to the lengthening of the tracheal branches or nerves, and the others, becoming detached, form leucocytes or phagocytes. They are very numerous in the legs, at the beginning of the 4th stage, but are disseminated some days later throughout the whole cavity of the body. At the time of histolysis they attack the larval tissues and increase in volume at their expense; in return they serve for the nutrition of the imaginal parts and exercise no destructive action on them. Van Rees agrees with Kowalevsky in comparing these attacks of the embryonic cells, sometimes victorious and sometimes impotent, to the war which the leucocytes wage against both the attenuated and the virulent bacteria.

=Formation of the femur and of the tibia, transformation of the tarsus.=—Capillary tracheæ appear in the leg at the same time as in the wing. They arise from the end of a tracheal trunk near the base of the limb on the dorsal and convex side. After the 3d moult the hypodermis thickens near this place; in a few days a pad is formed there and then a large imaginal bud with a circular invagination. These buds were noticed by Lyonet, who supposed them to be “les principes des jambes de la phalène.” Nerves and a tracheal branch penetrate into the femoro-tibial bud and form a small bay or constriction which marks the point of junction of the femur with the tibia, and the body-cavity remains in direct communication with the end of the limb.

The tarsus undergoes a series of changes; the surface is folded in a very complicated way; at the level of each articulation, but only in the internal and concave region of the leg, is developed a deep fold; on one side there is a hypodermic thickening, on the other a simple leaf of the envelope, which afterwards joins at its base with the parietal hypodermis, and then two leaves are destroyed with the large cells of the setæ. The internal part and end of the tarsus are then reconstituted with the elimination of the débris, while the external and convex region undergoes direct regeneration.

The coxa and trochanter are derived from the base of the larval leg, and only the 1st pair are well separated from the base of the thorax. One or two days before pupation the femoro-tibial bud, after having, until now, preserved its antero-posterior direction, is placed transversely as regards the larva, then becoming directed obliquely forward. This rotatory movement of the coxa may be attributed to the great extension of the fore wings, which push before them the two first pairs of legs. The last pair in their turn are simply covered by the hind wings and are but slightly displaced. This new position of the legs is that of the imago: the knee of the 1st pair is situated in front of the tarsus; that of the 2d a little outward; that of the 3d pair is directed backward. (Gonin.)

=The antennæ.=—These appendages also have the same relation with those of the caterpillar as in the case of the legs, the larval appendages being only the point of departure of the imaginal growth. Weismann has observed in Corethra how at the approach of each moult an invagination like the finger of a glove allows the antenna to elongate from its base. The process, says Gonin, is identical in the caterpillar of Pieris. At the last moult the invagination is so pronounced that it is not effaced with the renewal of the chitinous integument. Several days later it again begins to grow larger. As the imaginal bud gradually sinks into the cavity of the head, it presses back the hypodermic wall and thus forms an envelope around it. Its base, widely opened, gives admission to the nerves, besides capillaries and sometimes a large trachea.

As soon as it reaches the posterior region of the head, the antenna in lengthening becomes folded and describes the great curves which led Réaumur to compare it to a ram’s horn (Fig. 613). The leaf of the envelope thickens in the interior and all around the base of the organ. Its ultimate rôle is closely like that of the two other hypodermic formations. It is at the outset this layer of cells which in the larva supports the ocelli. This layer, hidden on each side under the parietal region, thickens and regenerates, forming a circular pad which becomes more prominent and finally assumes the form of the compound eye of the imago.

Finally, this layer gives rise to a conical prolongation (Fig. 612, _c_), which after exuviation appears as a tuft of long hairs, and is called by Gonin the crest (cimier, Fig. 612), which is characteristic of the pupæ of Pieridæ. It is only differentiated towards the end of the 4th larval stage in a median depression of the vertex. It is an imaginal bud in the most general sense of the word.

On each side the base of the antenna comes in contact with the germ of the crest. The envelopes approach each other, and their thickened part constitutes with the ocular disks a new cephalic wall. The head of the butterfly thus marked off is triangular; all the larval parts remaining out of this area then disappear. The muscles and the nerves are resorbed by histolysis, then the external part of the imaginal envelopes and the old parietal hypodermis, reduced very thin and degenerated, is detached in shreds. The antenna becomes external throughout its whole extent. The transformation is in this case, then, almost as complete as in the thorax of Diptera or Hymenoptera. It is necessitated by the change of form and of volume of the head. The region of the ocelli persists unchanged almost alone from the larva to the imago also. The limit is not well marked between the portion which is the replacement or direct renovation of the epithelium.

=Maxilla and labial palpi.=—The development of the tongue (1st maxillæ) is so like that of the antennæ that it scarcely needs description. Beginning at the last moult, the hypodermic contents of the maxillæ is withdrawn in the cephalic cavity under the form of a hollow bud whose base is turned inward. The invagination remains less distinct than in the antennæ; it does not even reach to the anterior part of the œsophagus. The two symmetrical halves of the tongue approach each other and are thrice folded. When the caterpillar stops feeding, they each curve in in the form of an S, remaining lodged under the floor of the mouth (Fig. 613, _t_).

Underneath are to be seen two other buds, which by an identical process become the labial palpi (Figs. 614, 615, _p_).

At the anterior part of the head, where the organs are very close together, the envelopes form several folds without any further use (Fig. 615, _r_). The two leaves then fuse together and decay as at the surface of the tarsus.

Finally, in the mandibles and the labrum, there is only a cellular thickening without any invagination.

=Process of pupation.=—Notwithstanding the great number of persons who have reared Lepidoptera, close and patient observations as to the exact details are still needed. Gonin, who has made the closest observations on Pieris, pertinently asks why the antennæ, which are appendages of the head, are visible in the abdominal region, and why the tongue (maxillæ) is extended between the legs as far as the 3d abdominal segment. To answer these questions he made a series of experiments. Selecting some caterpillars which were about to pupate, he produced an artificial metamorphosis by removing the cuticula in small bits. Exposing the appendages in this way, they preserved the position which they are seen to take during growth. Each wing appeared within the limits of the segment from which it grew out (Fig. 610), not extending beyond, as it does in the normal pupa, so that Réaumur was wrong in saying that “the wings are here gathered on each side into a kind of band, which is large enough to lie in the cavity which is between the 1st and 2d segment.” (8^e Mém., p. 359.)

All these parts are coated with a viscous fluid secreted by special glands, which hardens after pupation upon exposure to the air. So long as the parts are soft, they can easily be displaced. Gonin drew one of the antennæ like a collar around the head, and one half of the tongue upon the outer side of the wing.

“When pupation is normal, the integument splits open on the back of the thorax, and the pupa draws itself from before backwards. Owing to the feeble adherence which the chitinous secretion gives it, it draws along with it the underlying organs. The legs, antennæ, the two halves of the tongue (maxillæ) retained by their end, each in a small chitinous case, can only disengage themselves from it when in elongating they have acquired a sufficient tension. The curves straighten out and the folds unbend. The chitinous mask of the head in withdrawing from the larval skin follows the ventral line; the tongue and labial palpi free themselves from its median part; the antennæ disengage themselves from the two lateral scales. Between these different appendages a space is left on the surface of the head for the eyes, and on the thorax for the legs. These are not completely extended on account of the lack of freedom of the femoro-tibial articulation; the femur preserves its direction from behind forwards, and the knee in the two first pairs remains at the same height. The wings overlie them and cover the under side of the two basal abdominal segments; their surfaces in becoming united increase much in size.”

As the chitinous frame of each spiracle gradually detaches itself, we see a tuft of tracheæ passing out of the orifice. It is at this moment that the provisional tracheal system is cast off, and it is easy to see that the process is facilitated by the simultaneous elongation of all the appendages. The permanent tracheæ can follow this elongation because they are sinuous, and need only to straighten their curves. It is, however, not the same with the tracheoles, as we have seen in the case of the wings (p. 133), and their extension or stretching is thus explained by a very simple mechanism.

“The position which the organs assume in the chrysalis is not due to chance, everything is determined in advance, and the microscope shows us that the structure of the hypodermis is specially modified in all the parts which remain external. It is a fact well known to those who rear Lepidoptera that if this normal arrangement is disturbed there are few chances that the perfect insect will survive. A leg lifted up, or an antenna displaced, leaves a surface illy protected against external influences. Almost always this accident causes a drying of the chrysalis.

“Several interesting experiments may be cited as bearing on this subject. If during transformation the chitinous mask of the head is separated from the integument beneath, it is arrested half-way in its development, and the antennæ and tongue are not fully extended. When the case or skin of the caterpillar is drawn, not from before backward, but in the opposite direction, all the appendages of the thorax are placed perpendicularly to the body. Dewitz and Künckel d’Herculais, in stating that the skin of the caterpillar splits open along its whole length, show that they were ignorant of the mechanism; for it is precisely because the chitinous larval skin splits open only in front that it preserves sufficient adherence to the organs beneath to draw them after it in the direction of the abdomen.

“To only read modern authors, one would suppose that the mechanism of pupation had remained hitherto unknown. In reality, it did not escape the notice of Swammerdam or of Réaumur, both of whom have described it with care. The first attached too much importance to the flow of blood, the action of which would be rather to push the organs out than to extend them over the surface of the thorax; the second insists on the movements of the insect. This factor, very admissible in caterpillars, ‘whose under side is situated on a horizontal plane’ (iii, 9^e Mémoire, p. 395), cannot be invoked for those which suspend themselves by the tail, as in the genus Vanessa.” (Gonin.)

_b._ The Hymenoptera

In the Hymenoptera, Ratzeburg was the first to figure and describe the numerous intermediate stages between the larva and pupa, his subjects being the ants, Cynips, and Cryptus, which pass through five stadia before assuming the final pupal shape.

In the bees, as we have observed in the larvæ of Bombus (Proc. Bost. Soc. Nat. Hist., 1866), after hardening a series in alcohol of young in different stages of development, it will be found difficult to draw the line between the different stages since they shade insensibly into each other, those represented in Fig. 616 being selected stages. The head of the incipient semipupa distends the prothoracic segment of the larva whose head is pushed forward and the thoracic segments are much elongated, while the appendages and wings are well developed, and have assumed the shape of those of the pupa. Development both in the head and thorax begins in the most important central parts, and proceeds outwards to the periphery. During this period the “median segment,” or 1st abdominal, has begun to pass forward and to form a part of the thorax.

In what may be termed the 3d stage (Fig. 616, _C_), though the distinction is a very arbitrary one, the change is accompanied by a moulting of the skin, and a great advance has been made towards assuming the pupal form. The abdomen is very distinctly separated from the thorax, the propodeum being closely united with the thorax, and the head and thorax taken together are nearly as large as the abdomen, the latter now being shorter and perceptibly changed in form, more like that of the completed pupa, while there are other most important changes in the elaboration of the parts of the thorax, particularly the tergites, and of the head and its appendages. Meanwhile the ovipositor has been completed and nearly withdrawn within the end of the abdomen.

The next to study the transformations of the Hymenoptera was Ganin, who discovered the early remarkable pre-eruciform larvæ, as we may call them, of certain egg-parasites (Proctotrypidæ). He discovered the imaginal buds of the wings in the third larva of Polynema (Fig. 185), but his observations, and those of Ayers, need not detain us here, as they have little to do with the subject of the normal metamorphosis of the Hymenoptera, and will be discussed under the subject of Hypermetamorphosis.

To Bugnion we owe the first detailed account of the internal changes in the Hymenoptera, his observations being made on a chalcid parasite, _Encyrtus fuscicollis_, a parasite of Hyponomeuta. The apodous larva (Fig. 618) moults but once, the next ecdysis being at the time of pupation. It passes through a semipupal stage.

Bugnion observed in the larva of Encyrtus three pairs of lower thoracic or pedal imaginal buds, two pairs of upper or alary buds, a pair of ocular or oculo-cephalic buds destined to build up all the posterior part of the head, a pair of antennal buds, and three pairs of buds of the genital armature (ovipositor). He also detected the rudiments of the buccal appendages under the form of six small buds (Fig. 619), which do not invaginate, and are not surrounded by a semicircular pad. Also in the abdomen, behind each pair of stigmata, there is a group of hypodermic cells (Fig. 617), which, without doubt, correspond to the wing-buds, but are not differentiated into a central bud and its pad, and does not merit the name of imaginal bud. In fact, except the eye-buds, which are unlike the others, he only observed the imaginal buds of the legs, wings, and ovipositor. The antennal buds are, like those of the buccal appendages, without an annular zone.

The pedal buds were detected in the middle of larval life. They each form a central bud surrounded by a circular thickening. They gradually elongate and become tongue-like and somewhat bent; soon a linear opening or slit appears, forming the mouth of a cavity which communicates with that of the body, allowing the passage into them of the tracheæ, muscles, and nerves, and afterwards of the blood. Finally, the buds grow longer and slenderer, are bent several times, and show traces of the articulations; and soon under the old larval skin, now beginning to rise in anticipation of the moulting, we see the coxa, femur, tibia, and tarsus of the perfect insect, the tarsal joints not yet being indicated.

The wing-buds (_a^1_, _a^2_) appear at the same time as those of the legs, as racket-shaped masses of small cells situated directly behind the 1st and 2d pair of stigmata, in contact with the tissue ensheathed by the corresponding tracheal vesicle (Fig. 618). Afterwards they have exactly the form of those of the Lepidoptera (Fig. 619).

The proliferation of the hypodermis is not limited to the thorax, but takes place at corresponding points in the first seven abdominal segments. These abdominal agglomerations of cells do not give rise to true buds, but serve simply to reconstitute the hypodermis of the abdominal segments at the time of metamorphosis.

=Ocular or oculo-cephalic buds.=—The eye of insects, as is well known, is a modification of a portion of the integument, the visual cells being directly derived from the hypodermis, the cornea being a cuticular product of this last, like chitinous formations in general.

The ocular buds appear towards the end of larval life as a simple mass of hypodermic cells, and form a compact layer on the dorsolateral face of the prothoracic segment, and clothe the cephalic ganglion or brain like a skull-cap. The central portion only is destined to form the eye, while the peripheral pad, continuing to thicken, gives rise to a voluminous and rounded mass, which meets on the median line that of the opposite side, and forms the integument of all the posterior part of the head.

Bugnion also observed on the median line a group of small hypodermic cells which he regarded as the rudiment of the anterior ocellus, but he did not detect those of the posterior ocelli.

=The antennal buds.=—These appear at an early date under the cuticle of the head, as two distinct rounded cellular masses, with a central cavity, but no annular zone (Fig. 619, _f_). Each one grows longer in a transverse sense, and its summit, extended from the outer side, curves downward. It now forms a hollow tube folded at the end, and terminated by a disk whose centre is perforated (Fig. 619, _f_). Afterwards, when the larva is ready to transform, it grows longer, becomes folded on itself in its cavity, and, passing beyond on each side the limits of the larval head, encroaches on the prothoracic segment.

=The buds of the buccal appendages.=—Towards the end of the larval period, the buds of the mouth-parts appear as small digitiform projections, situated on each side and below the mouth. Formed of small epithelial cells pressed against each other, they are all directed anteriorly, and possess no furrow or pad.

The 2d maxillæ (labium) is formed of two separate parts. The imaginal buds of the lower lip appear on each side of the median line, with a fissure indicating the differentiation of the palpus. On each side are to be seen the 1st maxillary buds, bearing each a rudimentary palpus, and, farther in front, the buds of the mandibles.

=The buds of the ovipositor.=—The six stylets of the ovipositor arise from six small imaginal buds which become visible in the second half of the larval period, on each side of the median line, on the lower face of the three last segments (Fig. 620, _q^1_, _q^2_, _q^3_). The bud is differentiated into a central discoidal bud, a furrow, and a marginal, rather thick swelling or pad. Afterwards, these buds elongate and form small papilliform projections directed backwards (Fig. 621); but only during the pupal period do they, as already observed in Bombus, approach each other and assume their definite shape as an ovipositor.

Finally, Bugnion states that while metamorphosis in the Hymenoptera is less highly modified than in the Muscidæ, it is more marked than in the Coleoptera and Lepidoptera. In these orders the pupa moves the abdomen, but in Hymenoptera it is absolutely immovable throughout pupal life, as long as the integument is soft.

DEVELOPMENT OF THE IMAGO IN THE DIPTERA

The flies, particularly the Muscidæ and their allies (Brachycera), are the most highly modified of insects, their larvæ having undergone the greatest amount of reduction and loss of limbs, this atrophy involving even most of the head. The following account has been prepared in part from the works of Weismann, Ganin, Miall, and Pratt, but mostly from the excellent general summarized account given by Korschelt and Heider.

In the holometabolic orders of insects, with their resting pupal stage, during which no food is taken, the entire activity of life seems to be turned to deep-seated and complicated internal developmental processes. These inner changes involve an almost complete destruction of many organs of the larva, and their renewal from certain germs (the imaginal buds) already present in the larva, as will be seen in the highly modified Muscidæ. Only a few larval organs become directly transferred into the body of the pupa and imago. Such are the rudiments of the genital system. The heart also, and the central portion of the nervous system, suffer only slight and unimportant, almost trivial, internal changes. On the other hand, most of the other organs of the larva become completely destroyed: the hypodermis, most of the muscles, the entire digestive canal with the salivary glands; while their cells, under the influence of the blood corpuscles (leucocytes), which here act as phagocytes, fall into pieces, which are taken up by them and become digested. Simultaneously with this destructive, histolytic process, the new formation of the organs by the imaginal buds, already indicated in the embryo, is accomplished in such a way that the continuity of the organs in most cases remains unimpaired. This process of transformation can only be understood by considering that of the embryonal germs of the organs, (1) only a part is destined for the use of the larva in growth, and for the performance of certain functions which exhaust themselves during larval life, so that it is no more capable of farther transformation, and finally becomes destroyed; while (2) a second part of the embryonal germs or rudiments persists first in an undeveloped condition, as imaginal buds, in order to undertake during the pupa stage the regeneration of the organs.

Though Swammerdam knew that the rudiments of the wings were already present under the skin of the larvæ, we are indebted, for our present knowledge, to the thorough and profound investigations of Weismann on the metamorphosis of the Diptera, and also to the researches of Ganin and others who have worked on the pupæ of Muscidæ, in which the development is most complicated and modified. In the more generalized and primitive Diptera, such as Corethra, the processes of formation of the pupa and imago are much simpler than in the muscids and Pupipara. These processes are still simpler in the Lepidoptera and Hymenoptera, and for this reason we have given a summary of what has been done on these organs by Newport, Dewitz, and especially by Bugnion.

Our knowledge of this subject is still very imperfect, only the more salient points having been worked out, and, as Korschelt and Heider state, there is still lacking certain proof as to how far the relations of the internal changes known to exist in the Muscidæ also apply to other orders of insects, though it must be considered that in the pupa of Lepidoptera, Hymenoptera, perhaps also the Coleoptera, and we would add in the Neuroptera as well as the male Coccidæ, very similar metamorphic processes take place.

_a._ Development of the outer body-form

The form of the imago is completely marked out in the pupa, so that the transition from the pupa to the imago is comparatively slight and only depends on the modification and development of the parts already present.

In most cases the modification in question consists of the changes occurring during the passage from the larval form to the imago, the reformation of parts already present being most marked, while the new rudiments only participate in a limited way in the process. Thus, for example, the head of the caterpillar together with the antennæ and mouth-parts, also the thoracic limbs, pass directly and unchanged from the larva into the pupa. The compound eyes and the wings are, however, new formations, the latter arising from imaginal buds. The same is the case with many other Heterometabola, where the passage of the larva into the pupa in general is due to a transformation of parts already present. The changes in the brain, the fusion of certain ganglia of the ventral nervous cord, the changes in the abdomen, involving the reduction in the number of segments and the remodelling of the end of the body, and the formation of the ovipositor or sting, and in the higher Hymenoptera the transfer of the 1st abdominal segment to the thorax, and the origin of the genital armature,—all these should here be taken into account.

It should be observed that in every case where the larvæ are footless, as in Diptera, all the Hymenoptera except the phytophagous ones and certain coleopterous larvæ, the limbs of the imago stage are, in the earliest stages, indicated as new structures in the form of imaginal buds.

=Formation of the imago in Corethra.=—Corethra may serve as an example of such a relatively simple metamorphosis. Its larva belongs to the group of eucephalous dipterous larvæ. The head of the perfect insect is already indicated in the larva, and its parts, with certain modifications, pass directly into the pupa. The compound eyes, and this is a rare exception among the Holometabola, are present in the larva. On the other hand, the thoracic legs, the wings, and halteres are developed out of new rudiments which are present in the last larval stage, before pupation. Each thoracic segment has four of them, two ventral and two dorsal (Fig. 622); the ventral buds becoming the legs. Of the dorsal pairs, that of the mesothorax develops into wings, that of the metathorax into halteres, while from the corresponding rudiments of the prothorax in Corethra arise the stigma-bearing dorsal or respiratory processes of the pupa, and in Simulium a tuft of tracheal gills (Fig. 623, _ra_; see also Fig. 582).

These imaginal buds may be regarded as evaginations of the outer surface of the body. The only difference is that the buds of the appendages as a whole seem sunken below the level of the surface of the body, being situated at the bottom of an evagination, as in the buds of the head and trunk in the Pilidium larva of nemertean worms, and in the rudiments of the lower surface of the body of Echinus present in the pluteus larva.

The lumen of the invagination in which the appendages of Corethra (and other Holometabola) are situated is called by Van Rees the _peripodal cavity_, and the external sheath bordering it, which is naturally continuous with the hypodermis of the body, the _peripodal membrane_ (Fig. 636, _p_).

We must adopt the view that the rudiments of the appendages (imaginal buds) are from the first divided into ectodermal and mesodermal portions, which are derived from the corresponding germ-layers of the larva. The ectoderm of the rudiments of the appendages is continuous with the peripodal membrane, and through it with the hypodermis. Weismann was inclined to derive the organs (tracheæ, muscles, etc.) developing within the germs of the appendages from a hypertrophy of the neurilemma of a nerve passing down from within into the imaginal bud, and held that nerves and tracheal branches soon after passed into the inner surface of the imaginal bud. (Korschelt and Heider.)

When the imaginal buds of the appendages enlarge, then the peripodal membranes become correspondingly distended, and the limbs within assume a more or less crumpled position, and in Corethra are spirally twisted, while the rudiments of the wings are folded. The completion of the rudimentary limbs is accomplished simply by their passing out of the invagination in which they originated. The limbs thus gradually become free, the peripodal membrane is seen to reach the level of the rest of the hypodermis and become a part of it, and the base of the extremity is no longer situated in a cavity.

The internal organs of Corethra undergo but to a slight degree the destruction (histolysis) which is so thoroughgoing in the Muscidæ. Kowalevsky states that in the mid-intestine of Corethra a histolysis of the larval and reconstruction of the imaginal epithelium goes on in the same way as has been described in Musca. Most of the larval organs pass without histolytic changes directly over into those of the pupal and imaginal stages; the muscles in general are also unchanged, but those of the appendages and wings are made over anew. The last arise, according to Weismann, in the last larval stage from strings of cells which are already present in the embryo.

When we consider how insignificant the internal transformations are during the metamorphosis of the Tipulidæ, of which Corethra serves as an example, we can scarcely doubt that we here have before us conditions which illustrate the passage between an incomplete and a complete metamorphosis. Thus, among other things, should be mentioned the short duration of the pupa stage and the activity of the pupa, as also the early appearance of the germs of the compound eyes, a character which Corethra has in common with the Hemimetabola. (Korschelt and Heider.)

=Formation of the imago in Culex.=—In respect to the formation of the imaginal head, Culex is still more primitive than Corethra. Miall and Hammond find from Hurst’s partly unpublished descriptions and preparations that there are no deep invaginations for the compound eyes or antennæ of the imago.

“The compound eye forms beneath the larval eye-spots, and is at first relatively simple and composed of few facets. The number increases by the gradual formation of partial and marginal invaginations, each of which forms a new element. The imaginal antenna grows to a much greater length than that of the larval antenna, and its base is accordingly telescoped into the head, while the shaft becomes irregularly folded.[115] Culex, though more modified than Chironomus in many respects, _e.g._ in the mouth-parts, is relatively primitive with respect to the formation of the imaginal head, and shows a mode of development of the eye and antenna which we may suppose to have characterized a remote and comparatively unspecialized progenitor of Chironomus.”

=Formation of the imago in Chironomus.=—The development of the head of the imago of _Chironomus dorsalis_ has been discussed by Miall and Hammond. The invaginations which give rise to the head of the fly could not be discovered even in a rudimentary state until after the last larval moult.

“Weismann has given reasons for supposing that invaginated imaginal rudiments could not come into existence before the last larval moult in an insect whose life-history resembles that of Corethra or Chironomus. If the epidermis were invaginated in any stage before the ante-pupal one, the new cuticle, moulded closely upon the epidermis, would become invaginated also, and would appear at the next moult with projecting appendages like those of a pupa or imago. This is actually the way in which the wings are developed in some larval insects with incomplete metamorphosis. In Muscidæ the invaginations for the head of the imago have been traced back to the embryo within the egg,[116] but the almost total subsequent separation of the disks from the epidermis renders their development independent of the growth of the larval cuticle and of the moults that probably take place therein.”

The pupal and imaginal cuticles do not follow at all closely the larval skin, but, says Miall, become at particular places folded far into the interior. “The folds which give rise to the head of the fly are two in number and paired. They begin at the larval antenna on each side of the head, and gradually extend further and further backwards. The object of the folds is to provide an extended surface which can be moulded, without pressure from surrounding objects, into the form of the future head. On one part of each fold the facets of the large compound eyes are developed; another part gives rise to the future antenna, a large and elaborate organ, which springs from the bottom of the fold, and whose tip just enters the very short antenna of the larva. The folds for the head ultimately become so large that the larval head cannot contain them, and they extend far into the prothorax. Here a difficulty occurs. If the generating cuticle of the prothorax were also to be folded inwards, the future prothorax would take a corresponding shape. But the prothorax of the fly has a form dictated to it by the limbs which it bears and by the muscles to which it gives attachment. These call for a great reduction in its length, and a peculiar shape, which it is not here necessary to describe. It will be enough to realize that the epidermis of the future prothorax cannot be sacrificed to the folds which are to give rise to the head of the fly. All interference between the two developing structures is obviated by the provision of a transverse fold, which pushes into the prothorax from the neck, and forms a sort of internal pocket. The floor of the pocket forms two longitudinal folds, which prolong the folds originating in the larval head. The roof of the pocket shrinks up and forms the connection between the head and thorax of the fly. Ultimately the head-part is drawn out, leaving the prothoracic structures unaffected.”[117]

The development of the head of the fly of Chironomus appears, as Miall and Hammond state, to be intermediate between the groups Adiscota and Discota of Weismann; _i.e._ “between the types in which the parts of the head of the fly are developed in close relation to those of the larva, and the types in which deep invaginations lead apparently to the formation of similar new parts far within the body, the seeming independence of the new parts being intensified by thoroughgoing histolysis,” and they suggest that possibly types may be discovered intermediate between Chironomus and Muscidæ.

We are now prepared to consider the extremely complicated changes, in the Muscidæ, leaving out of consideration the origin of the wings from imaginal buds, which has already been discussed on pp. 126–137.

=Formation of the imago in Muscidæ.=[118]—In the flesh, and undoubtedly the house, and allied flies the germs or imaginal buds of the legs and wings arise in the same way as in Corethra. But in the Muscidæ, the buds are situated far within the interior of the body, the peripodal cavities appear closed, and the peripodal membrane stands in connection with the hypodermis merely by means of a delicate thread-like stalk. This connecting cord, which was first detected by Dewitz, and whose interpretation was entirely right, shows in its interior, as Van Rees proved, a narrow cavity.

Though the earliest stages in the development of imaginal buds in the embryo of the Muscidæ are still unknown, yet we shall not go far astray if we refer them, like the imaginal buds of Corethra, to hypodermal invaginations. We must, then, regard the stalk-like connection just mentioned as the long drawn-out neck of this invagination.

In general, the development of the appendages (Figs. 626, 627) goes on as described in Corethra. The rudiments of the legs enlarge and show at an early date the first traces of the later joints. They are so packed in the peripodal cavities that the single joints of the extremities appear as if pushed in “like the joints of a travelling cup.” (Van Rees.) The evagination of the completely formed buds of the limbs, which occurs on the first day after the beginning of pupation, goes on in such a way that the stalk of the imaginal bud (Figs. 626, _B_; 627, _B_) shortens, while its cavity widens so that the limbs finally, as in Corethra, pass out through the widely opened mouth of the peripodal invagination, which at the same time gradually completely disappears. The peripodal membrane is converted into a thickened part of the hypodermis in the region adjoining the base of the leg, and from this thickened hypodermal portion, the formation of the hypodermis of the entire imaginal thorax goes on, as the larval hypodermis is gradually destroyed.

We must here settle the question as to the first origin of the mesodermal portions of the rudiments of the appendages. We can already distinguish in the imaginal buds of the fully grown muscid larva a clear separation between an ectodermal and an inner mesodermal part. Ganin derived the mesodermal part through a sort of differentiation and separation of the innermost layer of the ectodermal part, and Van Rees has, in general, confirmed this view. Kowalevsky, on the other hand, is inclined to the view that the mesodermal part of the imaginal bud is derived from the embryonal cells of the mesoderm. He finds scattered throughout the mesoderm, under the hypodermis of the larva, so-called wandering cells (Fig. 632, _A_, _w_), which are different in appearance from the leucocytes and from the elements from which the formation of the mesodermal parts of the imaginal rudiments proceed. Kowalevsky is inclined to believe that there are in each segment rudiments of the imaginal mesoderm, but which are so delicate and indifferent that we cannot find them in the first stages of their origin. From these mesodermal imaginal rudiments the above-mentioned wandering cells of the mesoderm are derived, which afterwards come into connection with the ectodermal portion of the imaginal buds.

Still more complicated and difficult to understand is the development of the head-section of muscids. We must remember that in muscid larvæ the head-section exists in its most rudimentary form, being the result of extreme modification and degeneration. The small size of the head is also due to the fact that it is more or less retracted within the thoracic region. Then, as shown by the researches of Weismann, in the last embryonal stages, the forehead, mandibles, and the whole region of the head around the mouth invaginate and form a sunken cavity (Fig. 628, _p_), in which the chitinous supports of the hooks characteristic of muscid larvæ are soon developed. This sunken part of the head, at whose inner end is the œsophagus, is called by the not entirely appropriate name of “pharynx,” and it must at present be remembered that the hollow space thus named is not a part of the digestive canal. It is an invaginated section of the head, _and the formation of the head of the imago mainly depends on the evagination of this region_.

The first rudiments of the most important parts of the head (eyes, antennæ, and forehead), occur in the youngest larvæ as paired masses of cells which lie in the thorax next to the two halves of the brain (for this reason called by Weismann “brain-appendages”), which are from their first origin connected with the pharynx, and may be regarded as the imaginal buds of the head. These appear very soon in later stages in the shape of elongated sacs widening at the hinder end (Fig. 628, _A_ and _B_, _h_), which from their origin are to be regarded as evaginations of the pharynx. Very soon epithelial thickenings appear in the wall of this sac-shaped brain-appendage, in which the rudiments of the parts of the future head may be recognized.

Disk-shaped thickenings in the hinder widened part of the brain-appendage form the rudiments of the compound eyes, which therefore may be called the eye-buds. On the basal surface of the eye-buds is situated a nervous expansion which is connected by a nerve with the supraœsophageal ganglion. This nerve becomes the optic nerve of the perfect animal, while the optic ganglion is clearly separated from the brain.

In the anterior, more cylindrical or tube-like part of the brain-appendage we find the “frontal buds” (_ss_), on which the antennal rudiments (_at_) soon bud out, in exactly the same way as the rudiments of the limbs arise from the imaginal buds.

Originally (Fig. 628, _A_) the brain-appendages lie tolerably far behind in the thorax of the larva, so that they connect the hindermost part of the wall of the pharynx with the foremost section of the brain, which they surround in the form of a mushroom. Afterwards, however, subsequent to pupation, they move, together with the central nervous system, farther forward (_B_), whereby they (if we have correctly understood the descriptions of Weismann and Van Rees) laterally surround the pharynx with their anterior end, which is somewhat ventrally bent. At the same time, there becomes established a gradually widening communication (_B_, _o_) between the brain-appendage and the pharynx, which soon extends in the form of a lateral pharyngeal fissure along the entire length of the brain-appendage. As a result, the cavity of the brain-appendage and the pharynx so completely unite that the two soon form a single sac, the head-sac or vesicle (Fig. 6–9, _k_). The walls of this head-vesicle are the later head-wall, the most important parts of which can now be recognized (the antennæ, eyes, rudiments of the beak). It is now necessary that the head-vesicle (Fig. 629, +, +) be, by the eversion of the pharynx, turned outward in order that the head of the pupa may be completed. By this eversion of invaginated parts, the former mouth-opening of the pharynx becomes a neck-section (Fig. 629, +, +) by which head and thorax are now united. (Korschelt and Heider.)

The cause of the eversion of the head-vesicle, which Weismann directly observed, appears to be due to an increase of the inner pressure through a contraction of the hinder parts of the body. The anterior end of the œsophagus now becomes turned down ventrally corresponding to the conformation of the head of the imago.

It has been shown that the so-called pharynx is only an invaginated part of the outer surface of the larval head. The brain-appendage Korschelt and Heider consider to be the diverticulum of this invagination, in which the single parts of the body lie in an invaginated state. They may throughout be compared to the rudiments of the thoracic limbs. All these imaginal buds have been traced back to the invaginated parts of the outer surface of the body, _i.e._ the ectoderm.

It should be borne in mind that the process of development of the head of the highly-modified Muscidæ is much more complex than in the more primitive Diptera.

In their essay on the development of the head of the imago of Chironomus, Miall and Hammond arrange the dipterous types thus far examined, in the order of complexity of the invaginations which give rise to the head of the imago, in the following order:—

1. Culex. Relatively simple. Invaginations of the imaginal buds, shallow. 2. Corethra, Simulium. } Intermediate. 3. Chironomus, Ceratopogon. } 4. Muscidæ. Relatively complex. Invaginations deep, and apparently, but not really, unconnected with the epidermis.

_b._ Development of the internal organs of the imago

It has already been observed that most of the organs of muscid larvæ (and this applies to most Diptera, Lepidoptera, Coleoptera, and Hymenoptera) are destroyed through the action of leucocytes, and that their reformation is accomplished by definite groups of embryonal cells, the imaginal buds or folds. Destruction and rebuilding occur during the pupa stage in such a way that in many cases while this process is going on the continuity of the organs does not seem to be disturbed. These transformations especially concern the hypodermis, the digestive canal, the muscles, the fat-body, and the salivary glands.

The transformation of the tracheal system is only partial, being in part a simple process of regeneration through cell-division. Slighter changes affect the heart, the central nervous system, and the reproductive system (Fig. 630).

=The hypodermis.=—The hypodermis of the imago arises through an extension of the ectodermal part of the imaginal buds. We have already mentioned this for the thorax. As the appendages of the thorax in the pupa gradually attain perfection, the hypodermis layer spreads from the place of their insertion, the layer consisting of numerous small cells whose origin we must refer to the peripodal membrane. This layer continues to spread over the surface of the pupal thorax, while at the same time the area of the larval hypodermis, consisting of large cells, is seen to diminish. Hence the thin edge of the newly-formed hypodermis (Fig. 631, _hi_) slowly grows into the space between the superficial cuticula and the larval hypodermis (Fig. 632, _h_), so that at this place the old hypodermis undergoing destruction eventually lies on the inner side of the newly-formed epithelial layer (_B_). We therefore see from this that, during the replacement of the old hypodermis by the new, the continuity of the superficial epithelium is never interrupted. Since the edges of the two kinds of hypodermis overlap, the surface of the body is nowhere bare of epithelium. The dissolution of the larval hypodermis is accomplished under the influence of the leucocytes (Fig. 632, _k_), which attack the larval hypodermis-cells and absorb their contents piece by piece, and so fill themselves with bits of the hypodermis-cells and their nuclei; since these fragments have the shape of roundish granules, they were called by Weismann granule-balls. These granule-balls, which fill the body-cavity of the later pupal stage, are nothing else than the leucocytes (blood corpuscles) which have absorbed the fragments of tissue of the larval body.

It should here be said that the destruction of the larval tissues is not to be attributed to the previous death of the cells, but is the result of the action of the leucocytes on tissues which, though weakened in their vital power, are still living. While the completely healthy, active tissues, _i.e._ those of the imaginal buds, withstand the attacks of the leucocytes, the less healthy larval tissues are by the attacks of the leucocytes divided into fragments and eaten and digested by them. This process is most marked in the histolysis of the larval muscles. The destruction of most of the larval organs depends, therefore, on the capacity of the amœboid blood-corpuscles for taking food and on intracellular digestion, as was first shown by Metschnikoff, who has given to these leucocytes the name of “phagocytes.”

This process of histolysis goes on in the same way in the head and abdomen as in the thorax. In the abdomen, as Ganin first proved, there are in each of the eight segments of which it consists in the larva four small cellular islets or imaginal buds (Figs. 631, _hi_, 632, _i_), from which originate the new hypodermis.

Van Rees has lately found in the abdominal segments another pair of smaller imaginal buds. The four imaginal buds occurring in the last segment are situated close to each other, encircling the anal opening (Fig. 633, _ims_), and take part in the formation of the hind-intestine, the rectal pouches and rectal papillæ. To this segment also belong the two pairs of imaginal genital buds (rudiments of the external sexual organs) which were first found by Künckel d’Herculais in Volucella.

The newly formed hypodermis spreads rapidly over the outer surface of the body, so that hypodermal areas corresponding to the separate imaginal buds soon unite. Simultaneously with this completion of the definite epithelial layer the larval hypodermis becomes completely destroyed by the phagocytes.

=The muscles.=—A similar process of destruction by phagocytes affects the greater number of the larval muscles, except the three pairs of thoracic muscles employed in respiration, and which pass intact from the larva to the imago. Indeed, the dissolution of the muscles is the first process which occurs in the metamorphosis. The destruction of the larval muscles is accomplished in such a way that, a great number of leucocytes which have collected on the surface of the muscular fibres, press through the sarcolemma and enter within the muscular tissue, filling the spaces formed between them, By this means the muscles break up into a number of rounded particles which are taken into the interior of the leucocytes. Thus a collection of granule-balls arise from the muscles, which finally separate from each other and become scattered throughout the body-cavity of the pupa. In the same way as the muscular substance, the muscle-nuclei are taken up and digested by the phagocytes.

The imaginal muscles develop from the definitive mesoderm which has originated from the mesoderm of the imaginal buds (Fig. 632, _C_, _m_).

=The digestive canal.=—As in the hypodermis and muscles, the histolysis of the larval digestive tract and its new formation from separate imaginal buds go on simultaneously, so that the continuity of the process is not interrupted.

The imaginal buds of the much-shortened pupal digestive canal occur in the mid-intestine (stomach) in the form of numerous scattered groups of cells (Fig. 633, _ie_), and in the fore- and hind-intestine in the form of rings (_v_ and _h_) of imaginal tissue. The imaginal ring of the fore-intestine (_v_) lies in the region of the proventriculus (_pr_, compare Fig. 635, _im_), while that of the hind-intestine is situated directly behind the base of the urinary tubes. The regeneration of these two parts of the digestive canal is not entirely accomplished by these two rings, but the imaginal rudiments of the neighboring parts of the outer surface of the body also have a share in it. Thus it appears that the foremost part of the œsophagus is built up from the imaginal buds in the region of the mouth, while the imaginal buds surrounding the anus in the 8th abdominal segment (Fig. 633. _ims_) produce by invagination the rectal pouches, together with the rectal papillæ.

The formation of the mid-intestine (stomach) takes place in such a way that the island-like imaginal buds spread out by cell-multiplication over the outer or basal surface of the larval mid-intestinal epithelium (Fig. 634, _o_), until they finally unite, so as to form the wall of the imaginal mid-intestine (stomach). At the same time the entire larval epithelium (_e_) is cast in the interior and forms the so-called yellow body, which becomes surrounded by a layer of small cells and a jelly-like mass, and remains until its destruction in the pupal stomach. The larval muscular layer (_m_) remains intact as long as the imaginal mid-intestine is not fully developed, when it is attacked and destroyed by phagocytes. The final muscular layer arises from single cells lying on the outer surface of the imaginal buds (Figs. 633, _im_, 634, _m′_), which should be regarded as special imaginal cells of the mid-intestinal muscular layer.

The transformation of the fore-intestine is introduced by a degeneration of the proventriculus and sucking stomach. The proventriculus (Fig. 635, _pr_), which had been formed from a circular fold of the fore-intestine, disappears by the smoothing out of this folded structure. The sucking stomach also similarly degenerates by withdrawing gradually into the œsophagus, so that instead of the original diverticulum there remains only an enlargement of the œsophageal cavity. At the same time this part of the canal is attacked and destroyed by phagocytes, while the destroyed portions become replaced by the gradually extending imaginal parts of the wall. The imaginal ring of the fore-intestine (Fig. 635, _im_), which, according to Kowalevsky, is concerned in the formation of a great part of the definitive œsophagus, becomes closed at its hinder end so that the communication with the mid-intestine appears to be interrupted.

The hind-intestine of the imago is rebuilt in exactly the same manner. Here also the imaginal ring widens and forms a tube, which while it grows around the openings into the urinary tubes, closes itself against the mid-intestine, while behind it remains in connection with the larval hind-intestine. In a similar way the larval hind-intestine is attacked by the growth from behind of an imaginal ring, which proceeds from imaginal buds near the anus, until finally, when the entire larval hind-intestine is reduced to granule-balls, the two imaginal sections of the tube are brought into contact with each other. (Kowalevsky in Korschelt and Heider.)

The larval salivary glands (Fig. 633, _sp_) are completely destroyed by phagocytes. Then succeeds the new formation of these glands from imaginal buds, which, according to Kowalevsky, form rings situated at their anterior ends.

The nature of the transformation undergone by the urinary tubes is not yet well ascertained. According to Van Rees, there is in this case perhaps a regeneration of the larval cells by division, but on the other side there may be a histolysis of these elements.

The above-described method of transformation of the digestive canal seems, according to Korschelt and Heider, to be very common among the holometabolic insects. It has not only been observed in the Diptera, but also in the Lepidoptera (Kowalevsky, Frenzel), Coleoptera (Ganin), and Hymenoptera (Ganin). The stripping off of the epithelium of the mid-intestine was found by Kowalevsky to occur also in Corethra, Culex, and Chironomus.

=The tracheal system.=—As we have seen (p. 448), the tracheal system of caterpillars just before pupation undergoes disintegration, accompanied by a reformation of the peritoneal membrane and tænidia. The larval ectotrachea undergoes histolysis, that of the imago being meanwhile formed; the larval tænidia also break up, dissolve, and are replaced by new tænidia which arise from the nuclei of the peritoneal membrane. That the tracheal system in the Muscidæ during metamorphosis undergoes a transformation is shown, as Korschelt and Heider claim, by the entirely different shape of the system in the maggot, the pupa, and the fly. The air is admitted to the tracheal system of the maggot, not by lateral openings, but through the two stigmata at the end of the body. On the other hand, the pupa breathes by prothoracic spiracles, while the fly has six pairs of lateral stigmata of the normal structure. There may be in the larva and pupa vestigial closed stigmata, as there are in the thorax of caterpillars, with tracheal branches leading to where were once functional stigmata. These stigmatal branches, as well as some other portions of the tracheal system already observed by Weismann, seem, according to Van Rees, to function as imaginal buds for the regeneration of the tracheal matrix, while frequently also a regeneration of this epithelium, by a simple repeated division of cells, may be recognized. The disintegration of the tracheal system is accomplished by means of phagocytes in the manner already described.

=The nervous system.=—The central nervous system passes directly from the larval into the imaginal stage, since it must continue to exercise most of its functions throughout metamorphosis, though it undergoes important changes of form and position. At the same time, certain histological transformations occur which may be regarded as a histolysis. Such is the destruction and rebuilding in the interior of the organs, which, however, preserve their continuity. Every case of destruction of tissues in the pupa has come to be regarded as a histolysis.

The problem of the transformation of the peripheral nervous system is not yet well understood. Although during the destruction of the larval muscles the motor nerves also in part degenerate, in the case of the nerves distributed to the appendages the conditions are different, as these may be recognized in the larva in the form of the nerve-threads which place the imaginal buds in connection with the central nervous system. These threads, according to Van Rees, pass from the larva into the pupa and imago, so that with the farther development of the rudiments of the extremities, only the distal part of the nerves belonging to them are to be regarded as new formations. (Korschelt and Heider.)

=The fat-body.=—The larval fat-body is also destroyed through the activity of the leucocytes in the same way as the other tissues. The reformation of the fat-body seems to begin in the mesoderm of the imaginal buds. Possibly, also, the masses or collections of embryonic cells which are regarded by Schaeffer as “blood-forming cells,” may serve to regenerate the fat-body. At all events, they have been derived from the mesodermal tissues. Though Wielowiejsky saw the fat-body of Corethra arising from a cell-layer situated under the hypodermis, yet it is not necessary to regard this observation as favorable to the view of Schaeffer that in Musca the larval fat-body is derived in part from the hypodermis, and in part from the tracheal matrix, thus from the ectodermal tissues. (Korschelt and Heider.)

=Definitive fate of the leucocytes.=—We have seen that the formation of the organs of the imago originates in the imaginal buds, in all cases where these do not pass directly from the larva into the pupa. The leucocytes, whose numbers in the pupa are greatly increased, take no direct part in the formation of the tissues. Their importance seems to lie in this, that they destroy those larval organs doomed to destruction, the parts of which they take in and digest, and possibly, by their powers of locomotion, convey particles of food to the developing organs.

What, on the other hand, is the fate of the leucocytes after the developmental processes in the pupa have ceased? There can be no doubt that a part of the so-called granule-cells are again transformed into normal blood-corpuscles. Another, and, as it seems, more considerable, share suffer degeneration. Finally, the leucocytes themselves serve as nourishment for the newly formed tissues. Of interest in this direction is the observation of Van Rees, that numerous leucocytes finally pass into the newly-formed hypodermis and then degenerate in crevices between the hypodermis-cells. (Korschelt and Heider.)

It has been suggested by Van Rees that the phagocytes attack all the larval organs indiscriminately, but that the active metabolism of the imaginal buds preserves them from these attacks. He also thinks that Kowalevsky is probably right in supposing that the buds render themselves immune by some poisonous secretion.

Pratt, however, thinks that the supposition of a protecting or poisonous secretion is scarcely necessary to account for the phenomenon, and suggests that the larval tissues are a prey to the phagocytes, because at the end of larval life they become functionless and inactive, so as to become an easy prey to phagocytes or disintegrating influences of any sort. On the other hand, the imaginal buds “in which there is an exceedingly active metabolism, and _all the larval organs which remain functional during the metamorphosis_ are immune from the attacks of the phagocytes. The heart in the muscids continues to beat, as Künckel d’Herculais has observed, during the entire period of the metamorphosis, with the exception of a day or two in the latter half of it. The nervous system must continue functional during the entire time. The three pairs of thoracic muscles which pass intact from the larva to the imago are probably employed in respiration during the metamorphosis. The reproductive glands are, like the imaginal disks, rapidly growing organs.” He adds that among the other holometabolic insects many or most of the larval organs remain functional during metamorphosis, hence there is but little histolysis. “But the larval intestine would always necessarily become unfunctional, and, as we have seen, Kowalevsky is of the opinion that the larval mid-gut in all holometabolic insects contains imaginal disks, and undergoes degeneration during the metamorphosis.”

=The post-embryonic changes and imaginal buds in the Pupipara (Melophagus).=—The sheep-tick (Melophagus) is still more modified than the Muscidæ; the larva is apodous and acephalous, but, as Pratt observes, much less highly specialized than those of muscids, and in respect to the position of the thoracic buds it closely resembles Corethra. They lie just beneath the hypodermis in two very regular rows, and not in the centre of the body, as in Musca (Figs. 628, 636, _C_). While, however, in Corethra all the thoracic buds are of larval origin, arising after the last larval moult, in Melophagus, on the other hand, each of these buds, except the dorsal prothoracic, arises in the embryo, as is also the case in Musca.

In the cephalic buds the conditions are similar to those in Musca, but still more complicated. Instead of a single pair of head-buds, there are two pairs, one dorsal and one ventral. “The dorsal pair corresponds to the muscidian head-disks in every respect; they are destined to form the dorsal and lateral portions of the imaginal head, together with the compound eyes. The ventral head-disks have no counterpart in Musca. The fate of these disks or buds is to form the ventral portion of the head, the paired projections forming the rudiments of the proboscis.

“The formation of the head-vesicle proceeds in a way similar to that in Musca. The ventral disk fuses early at its lateral edges with the dorsal pair. The communications between both ventral and dorsal disks and the pharynx rapidly widen (in the old larva they have already become very large), and soon the disks and pharynx form together a single vesicle, which is the head-vesicle.” The imaginal buds of the abdomen Pratt finds to be exactly as in the corresponding ones of Musca.

In the embryo of Melophagus the cephalic and thoracic imaginal buds first appear as local thickenings, followed by the invagination of the ectoderm; the cephalic buds first appear very early in the ontogeny of the insect (Fig. 636, _C_), just as the germs of the digestive canal, nervous system, and tracheæ are appearing. The single median thickening (_v_) is destined to form the ventral cephalic bud, while the pair of thickenings behind (_d_) become the dorsal buds, those homologous with the cephalic buds of Musca.

The thoracic buds, which arise as hypodermic thickenings, do not appear until late in embryonic life, until the time of the involution of the head.

Pratt did not observe in the embryo the buds of the internal organs and of the abdominal hypodermis, and thinks it probable that they appear first in the larva.

_c._ General summary

We have seen that in Coleoptera, Lepidoptera, Diptera, and Hymenoptera, and with little doubt in all the holometabolous insects, the parts of the imago originate in single formative cellular masses (imaginal buds) already present in the larva, and often even in the later embryonic stages. There are such imaginal buds for each part of the body,—for the appendages of the head, for the legs and wings, for the ovipositor, and probably for the cercopods, for the hypodermis of the abdomen, and for the different sections of the digestive canal. We have seen, as Korschelt and Heider state, that the formation of the mesodermal organs of the imago (muscles, connective tissue, fat-body) begins in the mesodermal part of the imaginal buds, whose first origin is still obscure. Simultaneously with the formation of the imaginal organs, there goes on under the influence of the leucocytes the destruction of the larval organs. Both processes (destruction and regeneration) therefore go on hand in hand, so that the continuity of the organs in question in most cases remains perfect, inasmuch as the complete destruction only ensues after the formation of the final organs. The only exceptions are most of the muscles of the larva, which are destroyed at a very early period.

Moreover, it is evident that the sharp division into larval, pupal, and imaginal stages only applies to the external surface of the body, since they follow one another after successive moults. The processes of the internal development, on the other hand, form an entirely continuous series of transformations between which is no sharp line of demarcation. Yet as a whole the form of the larva, pupa, and imago are kept distinct in adaptation to their separate environments and habits.

Finally, as Pratt very truly remarks, the epigenetic period in insects, when new organs are forming, does not end with the birth of the larva from the egg, but extends through the larval, and even through the pupal period. “The principal significance of the pupal period and the metamorphosis is that it is the time when the larval characters which were adapted for use during a period of free life in the midst of the development, and which would be valueless to the imago, are corrected or abandoned.”

HYPERMETAMORPHISM

When an insect passes through more than the three normal stages of metamorphosis, _i.e._ the larval, pupal, and imaginal, it is said to undergo a _hypermetamorphosis_. The best-known examples are the supernumerary stages of Meloë, Stylops, etc.

As has already been observed, Schmidt has shown that in the male of the Coccidæ, there is a true hypermetamorphosis, as shown by Fig. 637. In _Aspidiotus nerii_ there are five stages, there being two larval (1, 2) and two pupal stages (3, 4, 7). Stage 3 (Fig. 637, 2) may be compared with the pro-pupa stage of Riley (Fig. 581).

We have already, on page 602, described the hypermetamorphosis of the neuropterous insect Mantispa (Fig. 638).

In Meloë the freshly hatched larva, or “triungulin” (Fig. 639, _a_), is an active Campodea-like larva, which runs about and climbs up flowers, from which it creeps upon the bodies of bees, such as Anthophora and Andrena, who carry it to their cells, wherein their eggs are situated. The triungulin feeds upon and destroys the eggs of its hostess. Meanwhile its inactive life in the bee’s cell reacts upon the organism; after moulting, the-second larval form (Fig. 640, _b_) is attained, and now the body is thick, cylindrical, soft, and fleshy, and it resembles a lamellicorn larva, with three pairs of rather long thoracic legs. This is Riley’s carabidoid stage. This second larva feeds upon the honey stored up for the young or larval bees. After another moult, there is another entire change in the body; it is motionless, the head is mask-like without movable appendages, and the feet are represented by six tubercles. This is called the semipupa or pseudo-pupal stage. This form moults, and changes to a third larval form (_c_), when apparently, as the result of its rich, concentrated food, it is overgrown, thick-bodied, without legs, and resembles a larval bee.

After thus passing through three larval stages, each remarkably different in structure and in the manner of taking food, it transforms into a pupa of the ordinary coleopterous shape (_d_).

The history of Sitaris, as worked out by Fabre and more recently by Valery-Mayet, is a similar story of two strikingly different adaptational larval forms succeeding the triungulin or primitive larval stage. The first larva (Fig. 641, _a_) is in general like that of Meloë, the second (_b_) is thick, oval, fleshy, soft-bodied, and with minute legs, evidently of no use, the larva feeding on the honey stored by its host. The pseudo-pupal stage is still more maggot-like than in the corresponding stage of Meloë, and the third larva (_f_) is thick-bodied, with short thoracic legs.

In the complicated life-history of another cantharid, _Epicauta vittata_, as worked out by Riley (Fig. 642), we have the same acquisition of new habits and forms after the first larval stage, which evidently were at the outset the result of an adaptation to a change of food and surroundings. The female Epicauta lays its eggs in the same warm, sunny situation as that chosen by locusts (Caloptenus) for depositing their eggs. On hatching, the active minute carnivorous triungulin, ever on the search for eggs, on happening upon a locust egg gnaws into it, and then sucks the contents. A second egg is attacked and its contents exhausted, when, owing to its comparatively inactive habits and rich nourishing food after a period of inactivity and rest, the skin splits along its back, and at about the eighth day from beginning to take food the second larva appears, with much smaller and shorter legs, a much smaller head, and with reduced mouth-parts. This is the carabidoid stage of Riley. After feeding for about a week in the egg a second moult occurs, and the change of form is slight, though the mouth-parts and legs are still more rudimentary, and the body assumes “the clumsy aspect of the typical lamellicorn larva.” This Riley denominates the scarabæidoid stage of the second larva.

After six or seven days there is another transformation, the skin being cast, and the insect passes into another stage, “the ultimate stage of the second larva.” The larva, immersed in its rich nutritious food, grows rapidly, and after about a week leaves the now addled and decaying locust eggs, and burrows into the clear sand, where it lies on its side in a smooth cell or cavity, and where it undergoes an incomplete ecdysis, the skin not being completely shed, and assumes the semipupa stage, or coarctate larval stage of Riley.

In the spring the partly loose skin is rent on the top of the head and thorax, and then crawls out of it the “third larva,” which only differs from the ultimate stage of the second larva “in the somewhat reduced size and greater whiteness.” The insect in this stage is said to be rather active, and burrows about in the ground, but food is not essential, and in a few days it transforms into the true pupa state.

These habits and the corresponding hypermetamorphosis are probably common to all the Meloidæ, though the life-history of the other species has yet to be traced.

In the genus Hornia described by Riley, the wings of the imago are more reduced than in any other of the family, both sexes having the elytra as rudimentary as in the European female glow-worm (_Lampyris noctiluca_). These, with the simple tarsal claws and the enlarged heavy abdomen, as Riley remarks, “show it to be a degradational form.”

Its host is Anthophora, and the beetle itself lives permanently in the sealed cells of the bee, and Riley thinks it is subterranean, seldom if ever leaving the bee gallery. The triungulin is unknown, but the ultimate stage of the second larva, as well as the coarctate larva, is like those of the family in general, the final transformations taking place within the two unrent skins, in this respect the insect (Fig. 643) approaching Sitaris.

It appears, then, that as the result of its semi-parasitic mode of life the Campodea-form or triungulin larva of these insects, which has free-biting mouth-parts like the larvæ of Carabidæ and other carnivorous beetles, instead of continuing to lead an active life and feeding on other insects; living or dead, and then like other beetles directly transforming into the normal pupa, moults as many as five times, there being six distinct stages before the true pupa stage is entered upon. So that there are in all eight stages including the imaginal or last stage.

One cannot avoid drawing the very obvious conclusion that the five extra stages constituting this hypermetamorphosis, as it is so well styled, are structural episodes, so to speak, due to the peculiar parasitic mode of life, and were evidently in adaptation to the remarkable changes of environment, so unlike those to which the members of other families of Coleoptera, the Stylopidæ excepted, have been subjected. The fat overgrown body and the atrophied limbs and mouth-parts are with little doubt due to the abundant supply of rich food, the protoplasm of the egg of its host, in which the insect during the feeding time of its life is immersed. Since it is well known that parthenogenesis is due to over, or at least to abundant nutrition, or to a generous diet and favoring temperature, there is little reason to doubt that the greatly altered and abnormally fat or bloated body of the insect in these supernumerary stages is the result of a continuous supply of rich pabulum, which the insect can imbibe with little or no effort.

The life-history of the Stylopidæ is after the same general fashion, though we do not as yet know many of the most important details. The females are viviparous, the young hatching within the body of the parent, as we once found as many as 300 of the very minute triungulin larvæ issuing in every direction from the body of what we have regarded as the female of _Stylops childreni_ in a stylopized Andrena caught in the last of April. The larvæ differ notably from those of the Meloidæ in the feet being bulbous and without claws, yet it is in general Campodea-like and in essential features a triungulin (Fig. 644). The intestine ends in a blind sac, as in the larvæ of bees, and this would indicate that its food is honey. The complete life-history of no Stylopid is completely known. It is probable that, hatched in June from eggs fertilized in April, the larvæ crawl upon the bodies of bees and wasps; finally, after a series of larval stages as yet unknown,[119] penetrating within the abdomen of its host before the latter hibernates, and living there through the winter. The females, owing to their parasitic life, retain the larval form, while the free males are winged, not leading in the adult stage a parasitic life, though passing their larval and pupal stages in the body of their host, and are so unlike ordinary beetles as to be referred by good authorities to a distinct order (Strepsiptera).

The triungulin stage of these insects corresponds in general to the form of the larval Staphylinidæ and allied families, such as the Tenebrionidæ, which are active in their habits, running about and obtaining their food in a haphazard way, often necessarily suffering long fasts. In the external-feeding, less active coleopterous larvæ, like the phytophagous species, which have an uninterrupted supply of nutritious food, we see that the body is thick and fleshy. So also in the larvæ of the Scarabæidæ, Ptinidæ, and the wood-boring groups. In internal feeders, like the larval weevils and Scolytidæ, which live nearly motionless in seeds, fruits, and the sap-wood of plants and trees, with a constant supply of nourishing, often rich food, the eruciform body is soft, thick, and more or less oval-cylindrical. So it is with the larvæ of Hymenoptera, especially in the parasitic forms, and in the ants, wasps, and bees, which are nearly if not quite motionless, at least not walking about after their food.

Now the change from the active triungulin stage to the series of secondary, nearly legless, sedentary, inactive stages is plainly enough due to the change of station and to the change of food. From being an independent, active, roving triungulin, the young insect becomes a lodger or boarder, fed at the expense of its host, and the lack of bodily exertion, coupled with the presence of more liquid food than is actually needed for its bare existence, at once induces rotundity of body and a loss of power in the limbs, followed by their partial or total atrophy.

That this process of degeneration may even occur in one and the same stage of larval existence is very well illustrated by what we know of the life-history of the wasp-parasite of Europe, _Rhipiphorus paradoxus_. Thanks to the very careful and patient observations of Dr. T. A. Chapman, we have a nearly complete life-history of this beetle, the representative of a family in many respects connecting the Meloidæ and Stylopidæ.[120] Where Rhipiphorus lays her eggs is unknown. Dr. Chapman, however, found a solitary specimen of the young larva in the triungulin stage. He describes it as “a little black hexapod, about 1⁄50 inch (.5 mm.) in length, and 1⁄120 inch in breadth, broadest about the fourth segment, and tapering to a point at the tail; a triangular head with a pair of three-jointed antennæ nearly as long as the width of the head, with legs very like those of Meloë larvæ; the tibiæ ending in two or three claws, which are supported and even obscured by a large transparent pulvillus or sucker of about twice their length; this was marked by faint striæ radiating from the extremity of the tibiæ, giving it much the aspect of a lobe of a fly’s proboscis. Each abdominal segment had a very short lateral spine pointing backwards; the last segment terminated by a large double sucker similar to those of the legs; and the little animal frequently stood up on this, and pawed the air with its feet, as if in search of some fresh object to lay hold of.”

This almost microscopic larva finds a wasp grub and bores into its body, probably entering at a point near the back of the first or second segment behind the head. Dr. Chapman succeeded in finding the larva of the beetle within that of the wasp, before the latter had spun up. “Assuming that the wasp larva lives six days in its last skin before spinning up, I should guess that the youngest of these had still two or three days’ feeding to do. The Rhipiphorus larvæ were but a little way beneath the skin of the back, about the fourth and fifth segments [counting the head as the first], and indifferently on either side. The smallest of these was 1⁄16 inch in length, and, except its smaller size, was precisely like the larger ones I am about to refer to, having the same head, legs, plates, etc. These were of the same size as those of the larger larvæ, the difference in size of the latter being due to the expansion of the intermediate colorless integument.”

After the wasp grub has spun the silken covering of its cell the larva of Rhipiphorus may still be detected in some of them, being rendered visible by its black legs and dark dorsal and ventral plates. “On extracting this larva, it bears a general resemblance in size and outline to the youngest larva of Rhipiphorus that I had found feeding externally on the wasp grub, but with the very notable exception of the already mentioned black marks. These are, in fact, a corneous head, six-jointed legs, and a dorsal and ventral series of plates. I immediately recognized the head and legs as identical with those of the little black mite already described, but presenting a ludicrous appearance in being widely separated from each other by the white skin of the larva. I have no doubt that the dorsal and ventral series of black marks are the corresponding plates of the mite-like larva floated away from each other by the expansion of the intervening membrane. By measurement also they agree exactly in size, although the larva extracted from the wasp grub is ten times the length and six times the width of the little Meloë-like larva. In length it is ⅙ inch (4.5 mm.), and 1⁄28 inch in breadth.”

The remarkable changes thus described in the larva of this beetle after it has begun its parasitic life within the body of its host are especially noteworthy because the great increase in size and difference in shape, as well as in habits, all take place before the insect has moulted. The rapid development in size, and consequent distension of the body and the separation of the sclerites of the segments behind the head, are paralleled, as Chapman says, by the greatly swollen abdominal region of the body in _Sarcopsylla penetrans_ and in the female of the Termitidæ. In those insects this distension is due to the enlargement of the ovaries and of the eggs contained within them, but in the Rhipiphorus it is due to the comparative inactivity of the larva, and to its being gorged with an unending supply of rich food, the blood and fat of its host. It follows, then, that if a sedentary life and over, or at least abundant, nutrition will have this effect within the short period covered by the single first larval stage of the Rhipiphorus, it is reasonable to infer that the hypermetamorphosis is also due to the same factors.

Chapman then goes on to say that finally, within six hours of the time of spinning up of the wasp grub, the Rhipiphorus larva at the end of Stage 1., which is “usually in motion, and for its situation might be called tolerably active, is seen to lay hold of the interior of the skin with its anterior legs, and keeps biting and scratching with its strong and sharp jaws until it is able to thrust through its head, when, in less than a quarter of an hour, it completely emerges by a vermiform movement; and at the same time it casts a skin, together with the black head, legs, plates, etc.”

The larva, now in its second stage, passes forward and seizes hold of the upper or lateral aspect of the prothoracic segment of the wasp grub. On emerging it becomes shorter and thicker, “and very soon loses length by that curving forward of its head which is so marked in the full-grown larva, and which does not exist before its emergence.” The larva is now found “lying like a collar immediately under the head of the wasp grub, and is attached to it by the head, though not very firmly.” At this stage the feeding of the young Rhipiphorus is rather sucking than eating.

When about 6 mm. in length it moults a second time, and the full-grown larva closely though superficially resembles a Crabro or Pemphredon larva, the small head being bent over forwards. By the time it is ready to pupate it has wholly eaten the wasp larva, and the temperature of the cell being high, a larva 5 mm. long grows large enough in two days to fill the top of the cell of its host, and the larva is ready to pupate about a week after hatching, so that its development is very rapid. The beetles themselves do not live in the cells. Chapman thinks they hibernate, and that the eggs are laid in the spring or summer.

We thus have in this insect three larval stages, the triungulin, and two later stages, the great differences between the first and the last two being apparently due to their parasitic mode of life, the larva spending its second stage within its host, involving an existence in a cell with a high temperature, an uninterrupted supply of rich, stimulating food, and a comparatively sedentary mode of life compared with that of the triungulin at the beginning of its existence. It is quite obvious that the hypermetamorphosis is primarily due to a great change in its surroundings, _i.e._ the parasitic mode of life of the beetle, habits of very rare occurrence in the Coleoptera, numerous in species as they are.

In this connection attention may be drawn to a supernumerary larval stage observed by Riley in the pea- and bean-weevils (Figs. 646 and 647). The larva on hatching has long slender legs, though differing from those of an ordinary coleopterous larva in having but three joints (_j_, _g_, _h_). This stage is very short, and the legs temporary, as, after entering the bean or pea, it casts its skin, losing its legs, and assuming the vermiform shape of the second larval stage. In this case the change from a pedate to an apodous larva is plainly enough due to the change from an external feeder, like a chrysomelid larva, to a larva leading a boring, internal, almost quiescent life.

Certain ichneumons also appear to have two distinct larval stages, as Ratzeburg inferred that in Anomalon there are four larval stages (Fig. 648).

In another ichneumon, Klapálek detected what he calls the “sub-nymph.” The insect pupates within the case of a caddis-fly, Silo (Fig. 649).

In the Proctotrypidæ there is also a hypermetamorphosis, though the remarkable precocious stages they pass through are rather embryonic than larval.

In a species of Platygaster which is parasitic in the larva of Cecidomia, the first larva (Cyclops stage) is of a remarkable shape, not like an insect, but rudely resembling a parasitic Copepod crustacean. In this condition it clings to the inside of its host by means of its hook-like jaws, moving about, as Ganin says, like a Cestodes embryo with its well-known six hooks. In this stage it has no nervous, vascular, or respiratory system, and the digestive canal is a blind one (Fig. 651).

After moulting, the insect entirely changes its form; it is thick oval-cylindrical, nearly motionless, with no appendages, but with a digestive canal and a nervous and vascular system (Fig. 652).

After a second moult the third and last larval stage is attained, and the insect is of the ordinary appearance of ichneumon larvæ.

Not less striking is the life-history of Polynema, which lays its eggs in those of a small dragon-fly (_Agrion virgo_). The first larval stage is most remarkable. It hatches as a microscopic immovable being, entirely unlike any insect, with scarcely a trace of organization, being merely a flask-shaped sac of cells. After remaining in this state five or six days it moults.

The second stage, or Histriobdella-like form, as Ganin names it, is more like that leech-like worm than an insect.

The third larval form is very bizarre, though more as in insects, having rudimentary antennæ, mouth-parts, legs, and ovipositor. In this condition it lives from six to seven days before pupating (Fig. 653).

The strange history of another egg-parasite (Ophioneurus) agrees in some respects with that of the foregoing forms. It is when hatched of an oval shape, with scarcely any organs, and differs from the genera already mentioned in remaining within its egg-membrane, and not assuming their strange shapes. From the cylindrical sac-like non-segmented larva resembling the second larva of Platygaster it passes directly into the pupa state.

A fourth form, Teleas (Fig. 654, _A-D_), is an egg-parasite of Gerris, and in America one species oviposits in the eggs of Œcanthus.

The spindle-shaped larva in its first stage roughly resembles a trochosphere of a worm rather than the larva of an insect so high in the scale as a Hymenopter. It is active, but after moulting the second larva is oval, still without segments. Dr. Ayers gives a profusion of details and figures of the first and second stages of our Teleas, the second strongly resembling the Cyclops stage of Ganin. He describes three stages, and though he did not complete the life-history of the insect, he thinks it changes to an ovoid flattened form which succeeds the Cyclops stage in other Pteromalidæ, and that there are at least four ecdyses.

It is difficult to account for these strange larval forms, unless we suppose that the embryos, by their rich, abundant food, have undergone a premature development, the growth of the body-walls being greatly accelerated, the insects so to speak having been, under the stimulus of over-nutrition and their unusual environment, and perhaps also the high temperature of the egg, hurried into vermian existence on a plane scarcely higher than that of an active ciliated gastrula.

Further observations, difficult though they will be, are needed to enable us to account for the singular prematurity of the embryo of these parasites. That these stages are reversional and a direct inheritance from the vermian ancestors of these insects is not probable, but the forms are evidently the result of adaptation in response to a series of stimuli whose nature is in part appreciable but mostly unknown.

It may be noted, however, that the appearance of a primitive band in the second larval stage suggests the origin of these forms, as well as that of insects in general, from a Peripatus-like, and again from an earlier leech-like Annelid ancestor. Hence the first larval or Cyclops stage is due to a precocious development caused by the unusual environment, and is simply adaptational, and not of phylogenetic significance.

SUMMARY OF THE FACTS AND SUGGESTIONS AS TO THE CAUSES OF METAMORPHISM

An explanation of the causes of metamorphosis is one of the most difficult undertakings in biology, and the phenomenon has been considered as one of the chief difficulties in the way of the acceptance of the theory of descent.

A review, however, of the facts of hypermetamorphism, particularly the life-history of Mantispa, throws much light on the subject, since it is very probable that the supernumerary stages and marked changes of form characterizing them are due to changes of environment, of habits, and of food, causes which have exerted such a profound influence on organic beings throughout all time. Besides these, as the result of changes in the environment and nature of the food, we have the results brought about by the use or disuse of structures brought into existence by the action of stimuli from without, the class of insects abounding in examples of temporary structures which perform a certain function, and then disappear.

Again, if the origin of a hypermetamorphosis can thus be explained, it follows that normal metamorphosis is most probably due to changes of habitat, of seasons, of food, and to accelerated growth resulting from the approach of sexual maturity.

The following facts and conclusions appear to be well established:—

1. The apterous insects (Synaptera) are ametabolous, only the winged insects undergoing a metamorphosis.

2. The complete metamorphosis was not inherited from the primitive ancestor of all insects, but acquired at a later period (F. Müller). The eruciform type is a secondary, adaptive form, derived from the earlier, campodeoid type of larva.

3. The earliest, most primitive pterygote insects passed through only a slight metamorphosis. In other words, as soon as the wings were evolved and insects became adapted to live or take refuge in a new medium, the air, at the approach of the period of adult life, with the ripening or perfection of the reproductive organs, a metamorphosis began to take place, and the number of species greatly multiplied. On the other hand, the Arachnida and Myriopoda, in which as a rule there is no metamorphosis, being confined to a creeping life, with no change of medium, remained poor in number of species.

4. At first the nymphs mainly differed from the adults in lacking wings, though having the same habits; in holometabolous insects, the larva became adapted to entirely different habits and environments, so that in Hymenoptera, and especially Diptera, the larva became remarkably unlike the imago.

5. Until the Mesozoic age, or late in the Carboniferous period, there were, so far as we now know, only ametabolous and heterometabolous insects, and these orders (Orthoptera, Dermaptera, Hemiptera, Plectoptera, Odonata, and Neuroptera) were not numerically rich in genera and species, while since early Mesozoic times geological extinction has reduced their numbers.

6. During the Mesozoic age, and since then, the number of species, genera, families, and orders has greatly increased, and insects have become more and more holometabolous. The orders of Coleoptera, Lepidoptera, Hymenoptera, and Diptera are many fold greater in number of species and variety of form than the heterometabolous orders.

The rapid increase in the number and variety of types of insects evidently is correlated with the profound geological changes which took place at the end of the Paleozoic age, involving the appearance of larger continental masses, or a greater land area, thus opening new regions for settlement. Also the origin of flowering plants at about this time undoubtedly had much to do with the genesis of new adaptive structures, such as the changes in the mouth-parts and wings.

7. The process of metamorphosis, at least in the subtropical, temperate, and polar regions, is largely dependent on the change from summer to winter, and, in the tropics, from the rainy to the dry season.

As regards the organization of larval (nepionic) as compared with imaginal forms, the nymphs and larvæ of insects are, with the exception of many Diptera, nearly as perfectly developed as the adult. In this respect the immature insect differs fundamentally from the larvæ of certain worms (for example, the pilidium of Nemerteans) and from the pluteus and brachiolaria stages of echinoderms, which possess only digestive and water-vascular organs.

Insect nymphs and larvæ also differ from the nauplius and zoëa of Crustacea in having at birth all the most important systems of organs (digestive, circulatory, respiratory, nervous, muscular, with sometimes a nearly perfected reproductive system) of the imago, also the same number of cephalic, thoracic, and abdominal segments and appendages. Metamorphism in insects involves (except in the Diptera) rather modifications in the form and functions of organs and appendages already present than the formation of new ones. In larval Crustacea, the thoracic and abdominal appendages do not arise until some time after hatching from the egg.

8. While cases of the suppression or abbreviation of larval characters and direct development are not uncommon in echinoderms and crustaceans, in insects this phenomenon occurs only so far as yet known in the Diptera. In these insects the polypody in the embryo is outgrown, or lost, the embryos and larvæ not having even the temporary rudiments of abdominal appendages. The campodeoid characters also are entirely suppressed, dropped, or lost in the more specialized holometabolous orders, Lepidoptera, Hymenoptera, and Diptera, though retained in the more primitive and generalized Coleoptera. (This proves that the Coleoptera are lower or more primitive and generalized than the other orders mentioned.) This abbreviation or loss of organs is, as Hyatt and Arms claim, due to the prepotency of acquired characters in phylogeny, and are also the result of homochronous heredity.

“The Insecta of the more specialized orders, x.-xvi., afford, next to some parasites, the most notable examples of this mode of evolution. Their larval or nepionic, and pupal or neanic, stages are prolonged at the expense of the ephebic, winged stage, and the reasons for this prolongation are found in the great number of new features introduced into these stages of development in these orders as contrasted with those of the more primitive, and, in large part, more ancient orders, i.-ix. The law of tachygenesis has been at work here, as in the former cases alluded to above, and it is shown in the encroachments of the adaptive characteristics of the caterpillar, grub, and maggot upon the inherited characteristics of the Thysanuran stage, which loses its ancestral characteristics, until in most cases they are either obsolete or recognizable with difficulty.” (Hyatt and Arms, Natural Science, 1896, p. 400.)

9. In the holometabolous insects there is a resting, quiescent stage during the pupal period, when the insect takes no food. In this respect the more specialized insects differ from other metamorphic animals. The larva has an abundant supply of fat lasting through pupal life, while in the quiescent pupa, respiration and circulation is much lessened, the animal being as a rule motionless. This resting stage is also necessary for the histolysis and formation of the adult body from the imaginal buds present in the larva.

10. The hypermetamorphosis of Mantispa, Meloë, Stylops, etc., indicate very plainly that the eruciform type of larva is derived from the campodeoid, since one and the same insect passes through these stages before reaching sexual maturity.

11. As observed by Miall, the larva of insects differs from that of other invertebrate animals in being larger than the adult.

12. The metamorphoses of insects are in some important respects paralleled by those of the Amphibia. The case of pædogenesis of Chironomus affords a parallel with that of the Siredon, or larva of Amblystoma. Also the organs and appendages of the insects, such as caterpillars, are present, just as the skeleton and other organs of the tadpole are the homologues of those of the adult, although these parts undergo a profound modification, and new structures are added. (See the discussion of this point by Miall, and by Hyatt and Arms.)

=Theoretical conclusions; Causes of metamorphosis.=—It results from a review of the known facts, together with reasonable inductions from such facts, that so far from opposing the theory of descent, the facts of metamorphosis, and particularly of hypermetamorphosis, seem to afford solid foundation for the theory. While natural selection was not the initiative cause, it plays a part as one of several factors; but the fundamental causes are the same as those which have controlled the origin of species and of the larger groups of animals in general. Owing to the struggle for existence, due to overcrowding, the early insects were forced to take to the air, acquiring wings to enable them to avoid the attacks of creeping and running insects. In the end the insects became, owing to this acquisition of wings, and afterwards to the establishment of a complicated metamorphosis, numerically the most successful type of life in existence, the number of species, extinct and living, mounting into the millions.

All aquatic insects are evidently the descendants of terrestrial forms, and the numberless contrivances and temporary larval organs, particularly of dipterous larvæ, are evidently adaptations to the needs of the insect during its aquatic life, and which are cast aside when the creature passes to a different medium. The sudden or tachygenic appearance of temporary structures, such as hatching spines, various setæ, spines, respiratory organs, so characteristic of dipterous larvæ, and of the protective colors and markings of caterpillars, and which are discarded at pupation, or imagination, are evidently due to the action of stimuli from without, to the primary neolamarckian factors, the characters proper to each larval stadium, and to the pupal and imaginal stadia,—characters probably acquired during the lifetime of the individual,—becoming finally fixed by homochronous heredity.

LITERATURE ON POSTEMBRYONIC DEVELOPMENT AND METAMORPHOSES

=Herold, Moritz Johann David.= Entwicklungsgeschichte der Schmetterlinge anatomisch und physiologisch bearbeitet. (Cassel, u. Marburg, 1815. 33 Taf., 4º, pp. 1–118, i-xxxiv.)

=Ratzeburg, F. T. C.= Ueber Entwickelung des fusslosen Hymenopteren-larven, mit besonderer Rücksigt auf die Gattung Formica. (Nova Acta Natur. Curios., xvi, 1832, pp. 145–176.)

=Agassiz, Louis.= The classification of insects from embryological data. (Smithsonian Contr., ii, Washington, 1851, pp. 28, 1 Pl.)

=Ganin, M.= Beiträge zur Erkenntniss der Entwicklungsgeschichte bei den Insecten. (Zeitschr. f. wiss. Zool., xix, 1869, pp. 381–451, 4 Taf.)

—— Ueber die Embryonalhülle der Hymenopteren- und Lepidopteren-embryonen. (Mém. Acad. St. Petersbourg (7), xiv, 1869, pp. 18, 1 Pl.)

—— Materialien zur Kenntniss der post-embryonalen Entwicklungsgeschichte der Insecten. (Russian.) Warschau, 1876. (Abdruck aus den Arbeiten der V. Versammlung russischer Naturf. und Aerzte in Warschau, 1876. Abstract by Hoyer in Jahresber. der Anat. und Phys. von Hoffmann und Schwalbe, v, 1876, and in Zeitschr. f. wiss. Zool., xxviii, 1877, pp. 386–389.)

=Weismann, A.= Die nachembryonale Entwicklung der Musciden nach Beobachtungen an _Musca vomitoria_ und _Sarcophaga carnaria_. (Zeitschr. f. wiss. Zool., xiv, 1864, pp. 101–263, Taf. 8–14.)

—— Die Metamorphose von _Corethra plumicornis_. (Zeitschr. f. wiss. Zool., xvi, 1866, pp. 1–83, 5 Taf.)

=Packard, A. S.= Observations on the development and position of the Hymenoptera, with notes on the morphology of insects. (Proceedings Boston Society of Natural History, 1866, pp. 279–295.)

=Künckel d’Herculais=, J. Recherches sur l’organisation et le développement des Volucelles. Paris, 1875, Pt. I, pp. 208, 12 Pis.; II, 1881. Atlas of 15 Pls.

=Dewitz, H.= Beiträge zur Kenntniss der post-embryonalen Gliedmaassenbildung bei den Insecten. (Zeitschr. f. wiss. Zool., xxx, Suppl., 1878, pp. 78–105, 1 Taf.; Nachtrag, Ibid., pp. 25–28.)

—— Ueber die Flügelbildung bei Phryganiden und Lepidopteren. (Berl. Ent. Zeitschr., xxv, 1881, pp. 53–66, 2 Taf.)

=Lowne, B. Th.= Anatomy, physiology, morphology, and development of the blow-fly. London, Part I, 1880; Part II, 1891.

=Viallanes, H.= Recherches sur l’histologie des Insectes et sur les phénomènes histologiques qui accompagnent le développement post-embryonnaire de ces animaux. (Ann. Sc. Nat. (6), xiv, 1882.)

=Metschnikoff, E.= Untersuchungen über intracelluläre Verdauung bei wirbellosen Thieren. (Arb. a. d. zoolog. Inst. zu Wien., v, 1883.)

—— Untersuchungen über die mesodermalen Phagocyten einiger Wirbelthiere. (Biol. Centralbl., iii, 1883.)

=Wielowiejsky, H. v.= Ueber den Fettkörper von _Corethra plumicornis_ und seine Entwicklung. (Zool. Anzeiger, vi Jahrg., 1883, pp. 318–322.)

=Pancritius, P.= Beiträge zur Kenntnis der Flügelentwicklung bei den Insecten. In.-Diss. Königsberg, 1884.

=Rees, J. van.= Over intra-cellulaire spijsverteering en over de beteekenis der witte bloedlichampjes. (Maandblad voor Natuurwetenschappen, xi Jaarg., 1884, pp. 28.)

—— Over de post-embryonale ontwikkeling von _Musca vomitoria_. (Maandblad voor Natuurwetenschappen, Juli, 1885.)

—— Beiträge zur Kenntniss der inneren Metamorphose von _Musca vomitoria_. (Zool. Jahrb. Abth. f. Anat. u. Ontog., iii, 1888, pp. 1–134, 2 Taf., 14 Figs.)

=Frenzel, J.= Einiges über den Mitteldarm der Insecten, sowie über Epithel-regeneration. (Arch. Micr. Anat., xxvi, 1885.)

=Kowalevsky, A.= Beiträge zur nachembryonalen Entwicklung der Musciden. (Zool. Anzeiger, viii, 1885, pp. 98, 123, 153.)

—— Beiträge zur Kenntniss der nachembryonalen Entwicklung der Musciden. (I. Theil., Zeitschr. f. wiss. Zool., xlv, 1887, pp. 542–594, 5 Taf.)

=Schneider, Ant.= Ueber die Anlage der Geschlechtsorgane und die Metamorphose des Herzens bei den Insecten. (Zool. Beiträge, 1885, i, pp. 140–143, 1 Taf.)

=Rehberg, A.= Ueber die Entwickelung des Insectenflügels (an _Blatta germanica_). (Marienwerder, 1886, pp. 12, 1 Taf.)

=Schäeffer, C.= Beiträge zur Histologie der Insecten. (Spengel’s Zool. Jahrb., iii, Abth. f. Anat., 1889, pp. 611–652, 2 Taf.)

=Hurst, H.= The post-embryonic development of a gnat (Culex). Manchester, 1890, pp. 26, 1 Pl.

=Verson, E.= Der Schmetterlingsflügel und die sog. Imaginalscheiben desselben. (Zool. Anzeiger, xiii, 1890, pp. 116, 117.)

=Bugnion, Edouard.= Recherches sur le développement post-embryonnaire, l’anatomie, et les mœurs de l’_Encyrtus fuscicollis_. (Recueil zool. Suisse, v, 1891, pp. 435–534, 6 Pls.)

=Petersen, Wilhelm.= Die Entwicklung des Schmetterlings nach dem Verlassen der Puppenhülle. (Deutsch. Ent. Zeitschr., 1891, 2 lepid. Hft., pp. 199–214, 5 Figs.)

=Kulagin, Nicolas.= Notice pour servir à l’histoire de développement des hyménoptères parasites. (Congrès international de Zoologie, 2^e Session, à Moscou, 1892, pp. 253–277. Also in Zool. Anzeiger, xv, 1892, pp. 85–87.)

—— On the development of Platygaster. (Journ. of Friends of Nat. Sc. Moscow. Zool., 1890.) (In Russian.)

—— Beiträge zur Kenntniss der Entwicklungsgeschichte von Platygaster. (Zeitschr. f. wiss. Zool., lxiii, 1897, pp. 195–235, 2 Taf.)

=Miall, L. C., and Hammond, A. R.= The development of the head of Chironomus. (Trans. Linn. Soc. London, 2d Ser., v, 1892, pp. 265–279, 4 Pls.)

=Pratt, Henry S.= Beiträge zur Kenntniss der Pupiparen. In.-Diss. Berlin, 1893, pp. 53 (Archiv f. Naturgesch., 1893), 1 Taf.

—— Imaginal discs in insects. (Psyche, viii, 1897, pp. 15–30, 11 Figs.)

=Gonin, J.= Recherches sur la métamorphose des lépidoptères. De la formation des appendices imaginaux dans la chenille du _Pieris brassicæ_. (Bull. Soc. Vaud. sc. nat., xxx, 1894, pp. 1–52, 5 Pls.)

=Heymons, R.= Ueber Flügelbildung bei der Larve von _Tenebrio molitor_. (Sitz. Ber. Gesell. Natf. Freunde. Berlin, Jahrg. 1896, pp. 142–144, 1 Fig.)

Also the writings of Malpighi, Swammerdam, De Geer, Lyonet, Bonnet, Newport, Brauer, Chapman, Fabre, Valery-Mayet, Riley, Chobaut, Nassonow, Miall (Nature, 1895, pp. 152–158), Hyatt and Arms (Natural Science, 1896, pp. 395–403).

END OF PART III.

INDEX

Abantiades, 57.

Abbreviation of larval characters, 707.

Abdomen, 162.

Abdominal appendages, in the embryo, 164; embryonic appendages, 476; jointed appendages, 468.

Acetabulum, 94.

Acid, formic, 358; uric, 352.

Acinose salivary glands, 334.

Acoustic nerve, 290.

Acronycta, 615; hastulifera, 194.

Acrydium, 421.

Actias luna, its cocoon-cutters, 634.

Adelops, 630.

Adhesive hairs, 111, 113; fluid, 113; glands, 360.

Adiscota, 672.

Adminicula, 629.

Adranes cæcus, 57.

Adult insects, tracheal gills of, 476.

Æroscepsis, 265.

Æschna, 53; rectal respiration in nymph of, 463.

Agriotypus, hypermetamorphosis of, 701.

Aileron, 124.

Air-sacs, 456; use of, 457.

Aletia xylina, tongue of, 66.

Aleurodicus, 518.

Aleyrodes, 518.

Alitrunk, 90.

Alluring glands, 391.

Alula, 123, 125.

Ametabola, acquired, 599.

Ametabolia, 596.

Amnion, 533; absence of, 534; cavity, 532; fold, 531; skin, shedding of, 584.

Amphizoa, 461.

Anabolia furcata, buccal organs of, 74.

Anabrus, 49, 73; cuticula of, 187.

Anal glands, 319, 326, 372; operculum, 181; silk glands, 346.

Androconia, 197, 199.

Anisomorpha, 371.

Anisopleura, lateral gills of, 468.

Anobium, 293, 620.

Anomalon, hypermetamorphosis of, 701.

Anophthalmus, brain of, 241; head of, 74; olfactory organs of, 276; salivary glands of, 334; tongue of, 74.

Anoplus, 101.

Ant, cement glands of, 360; organ of hearing in, 291; taste in, 282; phosphorescent, 424; poison sac of, 359; sounds produced by, 294; stingless, 359.

Antefurca, 92.

Antennæ, 57; imaginal buds of, 665; origin of imaginal from larval, 656; use of, 59, 270.

Antennal auditory hairs, 292; lobes, 237; nerves, 650.

Antheræa, 616.

Anthrax, 612.

Anurida, 51; maritima, 537.

Anurophorus, 424.

Anus, 319; absence of, 300, 320; of embryo, 537.

Aphides, changes of color in, 205; honey dew of, 364; wax glands of, 364.

Aphis, 616; reduction of tarsal joints of, 103.

Aphrophora permutata, wings of, 141.

Apis, premandibular segment in embryo of, 52; germ-layers of, 558.

Apneustic type of tracheal system, 459.

Apodemes, 92.

Apodous larvæ, 103.

Appendages, abdominal jointed appendages, 468; abdominal, origin of, 550, 551; abdominal, absence of, 550; cephalic, origin of, 548; of embryo, 548, 551; oral, 549; thoracic, origin of, 550.

Aquatic insects, 459; descent of, from terrestrial, 708; life, adaptations to, 460.

Arachnida, 6.

Arctia, 391.

Arctian larvæ, 615.

Argida, 391.

Armature, 187, 192.

Arolium, 97, 100, 113.

Arthromeres, 30.

Arthropoda, classes of, 3.

Articerus, 57.

Ascalaphus, 616.

Ash, on eversible glands, 377.

Asilus, mouth-parts of, 79.

Aspidiotus, 538, 627; nerii, hypermetamorphosis of male of, 690; nerii, metamorphosis of male of, 640, 690.

Ateuchus sacer, 101.

Attacus, mode of escape from its cocoon, 635.

Attacine moths, 634.

Attelabus, 538.

Auditory organs, 287.

Audouin, on the median segment, 163; on peritreme, 90.

Autolyca, 371.

Auzoux, on the salivary glands of silkworm, 332.

Ayers, on embryonic abdominal appendages, 550; on fecundation of the egg, 505; on hypermetamorphosis of Teleas, 703; on origin of heart, 573.

Bætisca, 467.

Balancers, 124.

Balbiani, on the polar cells of Chironomus, 580.

Ballowitz, on spermatozoa, 497.

Band, germinal, 531; invaginated, 538; overgrown, 538; primitive, 531, 536, 545.

Bapata, 392.

Basilar membrane of eye, 253.

Bee, honey, air-sacs of, 458; breathing of, 456; cement glands of, 360; egg of, 521; flight of, 151; head, 80; moulting of, 611; mouth-parts of, 79; number of moults of, 618; premandibular segment in embryo of, 52; salivary glands of, 334; sanitary conditions observed by its larva, 623; seminal packet of, 500; spermatheca, 506; tongue of, 80, 81; tracheæ of, 458; wax glands of, 364.

Bee’s foot, action of, in climbing, 114; sting, 172.

Bees, twisted hairs of, 189.

Beetles, anal glands of, 372; phosphorescent, 424; tongue of, 73; tracks of, 106; walking, 103.

Benasus griseus, tongue of, 73.

Bladder, urinary, 35.

Blanc, on salivary glands of silkworm, 331, 332; on silk glands of silkworm, 340; on spinning glands of silkworm, 340.

Blaps, 373; gait of, 109; tracks of, 109, 111.

Blastoderm, 526, 529.

Blatta, 43, 69; egg-tubes of, 501; embryology of, 537.

Blattidæ, fœtid glands of, 370.

Blepharocera, 474.

Blochman, on embryology of Musca, 530.

Blood, 407; corpuscles, 407, 419, 574; crystals from, 407; -forming cells, 574, 685; gills, 475; veins of wings, 121; lacunæ, 573; repellent nature of, 374, 407; serum, 407; tissue, 408, 419; vessels in the head, 405.

Blow-fly, duration of embryonic life of, 582; egg of, 521.

Boas, on spiracles of Melolontha larva, 439.

Bobretsky, on embryology of Pieris, 529.

Body, cavity, formation of, 563, 566; central, 232, 237; completion of embryonic, 555; form, development of outer, 668; mushroom, 233; pedunculated, 232, 233; stalked, 232, 233.

Boll, on repellent glands, 371.

Bombus, 219, 618; post-embryonic changes in, 661.

Bombyx mori, 339, 366, 405, 496, 499, 608; embryonic abdominal legs of, 552.

Bordas, on poison glands, 358; on salivary glands of Hymenoptera, 337.

Bot-fly, of horse, 475; of ox, 518.

Bothriothorax, 623.

Brain, 222, 226; development of, 567; histology of, 238; modifications of, in different orders, 240.

Brauer, on Campodea-form larvæ, 600–602; on metamorphosis, 598.

Breathing, mechanism of, 451; rectal, 463.

Brin, 342.

Bristles, 188.

Bruchus, hypermetamorphosis of, 700.

Buccal appendages, 59.

Bucculatrix, 634.

Buckton, on change of color in aphides, 205.

Buds, antennal, 665; buccal, 665; femerotibial, 656; frontal, 676; imaginal, 674; of Encyrtus, 663; Melophagus, 686; ocular, 665; of ovipositor, 665; of wings, 669.

Bugnion, on composition of head of Hymenoptera, 55; on the germs of the sexual glands of Encyrtus, 582; on the imagined buds of ovipositor, 171; on the post-embryonic changes in Hymenoptera, 663.

Burgess, on colors, 203; on hypopharynx, 76; on scales, 195.

Burmeister, on organs of smell, 265.

Bursa copulatrix, 505.

Busgen, on honey dew, 365.

Bütschli, on an under-lip structure in bee, 547; on origin and morphology of the tracheæ, 447; on premandibular segment, 52; on temporary abdominal appendages, 550.

Butterfly, atrophy of tarsi of, 102; olfactory organs of, 274; larval, hibernating, 615.

Caddis-worm, blood-gills of, 475; eversible glands of, 375; pupal mandibles of, 633.

Cæca of mid-intestine, 300, 325, 347, 348; secretion of, 348.

Calcar, 97.

Calcaria, 97.

Calculi in intestine, 325.

Calliphora vomitoria, 618; eggs of, 521.

Callosamia promethea, 192; number of moults of, 616.

Callosune, 202.

Caloptenus, 43.

Calopteryx, 54, 464.

Caltrops, 189.

Calypta, 124.

Calyx of brain, 233.

Campodea, embryology of, 22, 52; ligula of, 721; moulting of, 616; premandibular segment of, 52.

Campodea-form larva, 600.

Campodeoid characters, loss of, in holometabolous insects, 707; larvæ, 600.

Capillary tracheæ, 655.

Carabidoid stage, 692.

Carabus, walking, 107; tracks of, 109.

Cardiac valvule, 312.

Cardioblasts, 572.

Cardo, 63.

Carlet, on the poison apparatus of bees, 357; on walking in beetles, 109; on wax glands, 364.

Carus, on the circulation, 397, 409.

Case-worms, blood gills of, 475; functional salivary glands of, 331; spinning glands of, 337.

Caterpillar, actions before pupation, 644; changes in mouth-parts during metamorphosis, 645; eversible sacs of, 375; excrement of, before pupation, 644; internal changes in, 645; moulting of, 609; number of moults in, 615.

Catocala, 392.

Cauliculus, 233.

Cavity, peripodal, 669.

Cecidomyia, 113; urinary tubes of, 351.

Cells, absorbent, 328; amœboid egg, 529; egg, 502; embryonic, of buds of larval Lepidoptera, 655; genital, 575; setigenous, 191.

Cement glands, 360.

Centrosome, 525.

Ceratopogon, 678.

Cerci, 164, 178.

Cercopoda, 164, 178.

Cerura, 375.

Ceuthophilus, 393.

Chabrier, on use of elytra, 159.

Chalicodoma, 542.

Chambers, egg, 502; yolk, 502.

Chapman, on cremaster, 636; the hypermetamorphosis of Rhipiphorus, 697; on mode of escape from cocoon, 632, 633; on the moulting fluid, moulting of arctians, 615; on value of pupal characters, 628.

Chermes, 361.

Cheshire, on admission of air into bee’s cocoon, 623; on bee’s foot, 114; on bee’s sting, 172; on bee’s tongue, 79, 82.

Chiasma, 231.

Chironomus, 36, 491; formation of the imago in, 671, 678; polar cells of, 580.

Chitin, 29.

Chlænius, brain of, 241.

Cholodkowsky, on homologies of propleg or abdominal leg of caterpillars, 552; on patagia, 89; on testes of Lepidoptera, 496; on urinary tubes, 354.

Chordotonal organs, 289.

Chorion, 520, 534.

Chromatin, 498.

Chrysalis, 625; mode of suspension of, 637.

Chrysopa, 517, 525.

Chun, on the tænidia, 445.

Cicada, shrilling organ of, 295.

Cicada septemdecim, 616; hatching of, 584.

Cimbex, 374.

Circulation, of blood, 409; organs of, 397; peritracheal, 397.

Citheronia, 392.

Claspers, 176, 179.

Claus, on eversible glands, 374.

Clavola, 57.

Climbing, mode of, 116.

Closure, dorsal, of embryo, 556.

Clypeus, 546, 547.

Coarctate Diptera, 620.

Coccidæ, male, 626; metamorphosis of, 641.

Coccinella, moulting of, 611.

Coccinellidæ, 375.

Cockerell, on hatching of mantis, 584.

Cockroach, 455, 456, 487; brain of, 229, 242; cement glands of, 360; chorion of egg of, 521; circulation of blood in wings of, 410; colleterial glands, 506; deposition of eggs of, 519; digestion of, 325; egg-tubes of, 501; fœtid glands of, 370; micropyle of eggs of, 523; mode of hatching, 583; oötheca of, 517; wingless, 598.

Cocoon, admission of air in, 623; breaker, 634; cutter, 634; formation of, 619; mode of escape from, 635; spinning of, 621.

Cœcal appendages of stomach, 300, 325, 347.

Cœcum of colon, 318, 325.

Cœlom-sac, 563, 566, 576.

Coleoptera, embryology of, 537; gustatory organs of, 284; internal changes during metamorphosis of, 641; larval types of, 604, 606; number of moults of, 617; olfactory organs of, 275; phosphorescent, 421; pupa of, 630; salivary glands of, 334; seminal ducts of, 496; sounds produced by, 293; spermatozoa of, 497, 499; tongue of, 73.

Colleterial glands, 506.

Colon, 317; cæcum of, 318.

Color sense, 260.

Colors, 201; dermal, 203; interference, 201, 202; metallic, 204; natural, 203; optical, 201; order of development of, 208.

Comb, tarsal, 97.

Commissure of œsophageal ring, 237.

Conditions of existence, 463.

Cone, crystalline, 250, 251.

Conglobate gland, 487.

Coniopteryx, 620.

Conjunctivus, 61.

Conorhinus, 616.

Cope, on causes of segmentation of body of arthropods, 33.

Copidosoma, 623.

Copris carolina, 61.

Copulation, signs of, 507.

Copulatory pouch, 505.

Cord, stigmatic, 460; supraspinal, 240.

Corethra, 433, 460, 618; auditory organs of, 291; formation of the imago in, 668, 678; plumicornis, wing-germs of, 129; tracheoles of, 133.

Corixa, eggs of, 538.

Cornea, 250.

Corneal lens, 250.

Corydalus, 46, 48, 59, 70, 460, 468.

Corydalus cornutus, hatching spine of, 585.

Coste, on pigments, 206.

Cotylosoma, 478.

Coxa, 95; origin of imaginal from larval, 656.

Coxal glands, 369; sacs, 475; of myriopods, 14.

Cremaster, 636; absence of, 636; mode of formation of, in butterflies, 637.

Cremastral hook-spine, 638.

Cricket, 487; anal glands of, 372.

Crop, 303, 324.

Crustacea, 4.

Cucujo, 426.

Cuénot, on blood, 374, 408; corrosive glands, 574; digestion, 329; leucocytes, 421; phagocytes, 421, 422.

Cuilleron, 124.

Culex, 461, 465, 474, 599; formation of the imago in, 668, 678; mouth-parts of, 78; phosphorescent, 424; sense of hearing in, 292; urinary tubes of, 350.

Cup, spermatophore, 499.

Cuticula, 187, 203; new layer of, formation of, 612.

Cyclops stage of Proctotrypid parasites, 701.

Cyphon, 472.

Cytoplasm, 525.

Dahl, on constancy of number of six legs, 100; on motion of insects on smooth, 100, 111, 113, 116.

Danais, 197, 381; archippus, hypopharynx of, 75; plexippus, wings of, 137.

Dandolo, on amount of food eaten by the silkworm, 608.

Datana, 634.

Datana ministra, moulting of, 611; section of larva, 131; setæ of, 188; tænidia of, 444, 448.

Death-watch, 293.

Deltochilum gibbosum, 101.

De Moor, on tracks of insects, 106, 109.

Dermal glands, 365.

Dermaptera, wingless, 598.

Deutocerebrum, 231, 237.

Development, direct, 598.

Dewitz’s discovery of imaginal bud-stalks, 673; locomotion of insects on smooth surfaces, 111; on movement of leucocytes independent of the circulation, 413; openings of glandular hairs, 192; ovipositor, 168, 170, 171; stigmata of odonate nymphs, 439; open tracheal system, 460, 464; wing-buds, 127, 142.

Diapheromera, 371, 616; eggs of, 517, 520.

Diaphragm, pericardial, 412.

Didonis, 381, 391.

Digestion, 324.

Digestive canal of imago in the fly, appendages of, 297, 302, 331; formation of, 681; histology of, 320; length of, not a criterion of its habits, 301; primary regions, 299, 302.

Dimmock, on hypopharynx, 71; on labrum epipharynx, 44; on pseudo-trachea, 446.

Diplopoda, 12.

Diptera, coarctate, 620; cyclorhapha, 621; development of imago of, 666; food reservoir of, 305; germs of genital glands, 580; hypopharynx of, 78; larval types of, 607; mouth-parts of, 78; olfactory organs of, 273; origin of legs of imago in, 654; orthoraphous, 621, 626; post-embryonic changes in, 666; salivary glands of, 333.

Dipterous embryo, suppression of polypody in, 707.

Direct development, 598.

Discota, 672.

Division nuclei, 530.

Donacia, 620.

Dorsal organ, 535.

Doryphora, 50; embryology of, 544; hatching spine of, 586.

Dragon-fly, 53; muscles of flight of, 157; number of moults of, 616.

Draught power, 218.

Drosophila, 518.

Dryocampa, 187.

Dubois on the cucujo, 424, 426.

Ducts, ejaculatory, 496; seminal, 496.

Dufour, gland of, 358.

Dujardinia, 34.

Dyar on the number of moults, 615.

Dyticus, 461; foot of male, 93, 99, 114; larva, poisonous saliva of, 359; mode of swimming of, 116; tænidia of, 445; trail curves of, 108.

Eacles, 616.

Ears, 288.

Eaton, on nymph stage, 594; of rectal respiration in nymphs of ephemerids, 465.

Ecdysis, 609, 611.

Ectoderm, 534; formation of, 558.

Ectotrachea, 432, 448, 684.

Egg, 515; burster, 585; capsule, 517, 520; cells, 504; chambers, 502; fertilization of, 525; germs, 502; guide, 183; internal structure of, 524; markings of, 521; maturation of, 525; mode of deposition of, 518; number laid, 515; ovarian, 501; ripe, 520; sacs, 361, 517, 520; shell, 520; smaller in holometabolous insects, 515; tubes, 501; vitality of, 520.

Ejaculatory ducts, double openings of, 486; origin of, 578.

Elater of Collembola, 551.

Electricity, influence of, on action of heart, 412.

Eleodes, 372.

Elliott on color sense, 261.

Elmis, 462, 473.

Elytra, 124; glands in, 125.

Embidæ, 620.

Embryo, 531; revolution of, 540.

Embryology of insects, 515.

Embryonic life, length of, 582.

Embryonic membranes, involution of, 556.

Embryonic, post-, changes, 650.

Emery, on homologies of the tracheæ, 443; on the firefly, 424, 427.

Empodium, 97, 111, 116.

Empretia stimulea, 192.

Eucyrtus, 55, 171; post-embryonic changes in, 663.

Endochorion, 520.

Endoderm, 534, 561.

Endomesoderm, 542.

Endosternite, 94.

Endotrachea, 432, 448.

Entomoline, 29.

Entothorax, 92.

Environment, 463.

Ephemera, circulation in, 409; double sexual openings of, 489; nymph of, lacinia of, 61; thorax of, 91.

Ephemerella, 466.

Ephemeridæ, 459, 460; double genital openings of, 492; gills of, 460; rectal respiration in, 464.

Ephydra, 36, 553.

Epicauta, life-history of, 692.

Epilabrum, 13.

Epimerum, 89.

Epiopticon, 231, 253.

Epipharynx, 43, 54.

Epipleurum, 124.

Episternum, 88.

Erichson on sense of smell, 266.

Eriocephala calthella, mandibles of, 62; maxillæ of, 68.

Eristalis, 189, 461.

Eruciform type of larva, 602, 605, 705.

Escherich, on male genital organs of beetles, 495.

Euphæa, lateral gills of, 468, 477.

Euphoria inda, moulting of, 611.

Eupolus, 113.

Eupsalis minuta, 103.

Eversible sacs, 475.

Excretion, defined, 327; process of, 328.

Excretory system, 348.

Exner, on vision, 258.

Exochorion, 520, 534.

Exuvia, 609.

Exuvium, 609.

Eye, 249; acone, 251; buds, 665; compound, 250; embryonic development of, 567; eucone, 251, 252; facetted, origin of, 255; glazed, 629; pseudocone, 251, 252; simple, 249.

Fabre, on life-history of Sitaris, 69.

Facet, 250.

Facets, number of, 249.

Facetted eye, origin of, 255.

Faivre, on function of brain, 244.

Fat, amount of, in caterpillar, 644.

Fat-body, 419; concretions in, 420; destruction and reformation of, in muscids during metamorphosis, 685; origin of, 574.

Feet, post-embryonic development of, 653.

Female reproductive organs, 485, 500; origin of, 575.

Femur, 96; formation of, in imago of Lepidoptera, 655.

Fernald, on rectal glands of Passalus, 318.

Fertilization of the egg, 525.

Filator, 340.

Filippi’s glands, 345.

Firefly, 426.

Fischer, on color of butterflies, 200.

Flagellum, 57.

Flea, 438; hatching spine of, 586; hypopharynx of, 77; number of moults of, 617.

Flies, syrphid, 189.

Flight, 148, 219; theory of, 150.

Fluid, exuvial, 612; moulting, 612; softening fluid of moths in escaping from the cocoon, 635.

Fly, blow, hatching of, 585; development of imago in, 666; horse, mouth-parts of, 79; thorax of, 91; house, length of embryonic life of, 582; thorax of, 88; number of moults of, 618; meat, hatching of, 585; tenent hairs of, 111.

Fold, amnion, 531.

Folds, gastro-ileal, 317; giving rise to head of fly, 671.

Folsom, on lateral gills of Euphæa, 468.

Food-reservoir, 305.

Foot, of beetle, 111; of fly, 111.

Footprints of beetles, 106.

Fore-stomach, 306.

Forel, on gustatory organs, 281; on honey-dew, 365; on vision, 256.

Forficula, 454, 491; fœtid glands of, 369; hatching spine of, 585.

Formic acid, 358.

Frenulum, 122.

Fulgora, 424.

Funiculus, 460.

Funnel, 313.

Galea, 63, 64.

Galeruca, 113.

Ganglia, function of, 244; fusion of, 225; optic, 231, 232; primitive number of, 567.

Ganglion frontale, 569.

Ganglion opticum, origin of, 567.

Ganin, on abdominal imaginal buds, 170; on hypermetamorphosis of ichneumon parasites, 701.

Gastro-ileal folds, 317.

Gastropacha, 552; flattened hairs of, 194.

Gastrophilus equi, 475.

Gastrula, 558; stage, 535.

Gegenbauer, on homology of wings with gills, 142.

Gehuchten, on histology of muscles, 217; on the histology of mid-intestine, 316; on the pyloric valvule, 315; on secretion, mechanism of, 326.

Gena, 46.

Genital armature, male, 176; cells, 575; claspers, 176.

Germarium, 501.

Germ-layers, formation of, 558.

Giard on urinary tubes, 351.

Gills, blood, 475; in embryo insects, 476; tracheal, 459; adult insects, 476; rectal, 463.

Gilson, on anal glands, 373; on spinning-glands, 340.

Gissler on anal glands, 372.

Gizzard, 311, 324.

Glands, acid, 358; adhesive, 360; alkaline, 358; alluring, 391; of androconia, 199; anal, 319, 372; accessory of vasa deferentia, 497; cement, 360; colleterial, 506; conglobate, 487; corrosive, 374; coxal, 369, 383; dermal, 365; defensive, 368; eversible, 368, 382; Filippi’s, 346; fœtid, 369; mucous, 497; mushroom, 497; odoriferous, 381; repugnatorial, 368; salivary, 331, 570; setiparous, 444; sexual, origin of male, 579; unicellular, 366; wax, 361.

Glandulæ mucosæ, 497.

Gnathal segments of embryo, 556.

Gonapophyses, 167, 168.

Gonin, on moulting fluid, 613, 614; on the post-embryonic changes of Pieris, 651; on process of pupation, 659; on tracheæ of wings, 145; on the wing-germs, 131.

Gorgeret, 170.

Gottsche on vision, 257.

Graber, on abdominal legs of caterpillars, 552; on blood, 408; on blood-gills of embryo, 476; on climbing, 116; on development of wings, 138; on flight, 153; on folding of wings, 155; on foot-tracks of beetles, 109; on heart, 398, 400, 402; on mechanics of segmented body and limbs of insects, 31, 38; on mechanics of walking, 103; on organs of hearing, 290; on organs of smell, 267; on premandibular segment, 52; on respiration, 454; on successive appearance of embryonic segments, 546; on swimming, 116.

Grassi, on premandibular segment in Apis, 52; on Scolopendrella, 20.

Grège, 340.

Grès, 340.

Griffiths, on pigments, 207.

Grobben on heart, 399.

Gromphas, 101.

Gross, on color sense, 261.

Gryllotalpa, 572; maxilla of, 64.

Guide, egg, 183.

Gula, 46, 68.

Gulo-mental region, 46.

Gummy layer of silk, 340.

Gyrinus, 471, 620.

Haase, on coxal sacs, 14; on eversible glands, 371; on the formation of the copulatory pouch, 505; on Scolopendrella, 21, 24; on the homology of the ovipositor, 171.

Hadenœcus, 44, 392.

Hagen, on colors, 201; on gills of Perlidæ, 477; on lateral gills of Euphæa nymph, 468, 477; on vestigial gills in other odonate nymphs, 469.

Hair-fields, 197.

Hair-forming cells, 188.

Hair-scales, 197; tactile, 193.

Hairs, 188; adhesive, 111, 113; development of, 193; gathering, 45; glandular, 190, 192; nettling, 191; plumose, 189; tenent, 99, 190; in tracheæ, 451; twisted, 189.

Halteres, 124, 629.

Hammond, see Miall.

Hampson, on scent-glands, 391.

Haplopus, 521.

Harpes, 180.

Harpiphorus, 618.

Harpyia, 375.

Hatching, process of, 583; spine, 585.

Hauser, on organs of smell, 267, 269, 279.

Head, 42; blood-vessels in, 405; completion of head of embryo, 548; formation of, in aculeate Hymenoptera, 57; lobes, 544; number of segments in, 50, 54; of Musca, post-embryonic development of, 675; post-embryonic development of appendages of, 653.

Hearing, organs of, 287.

Heart, 397; beat, 411; free from histolysis, 667; origin of, 572, 577; pericardial diaphragm, 402; propulsatory apparatus, 401; supraspinal vessel, 403.

Heathcote, on double segments of Diplopods, 14.

Heider, on embryology of Hydrophilus, 530.

Heinemann, on the firefly, 426.

Helcodermatous spines, 612.

Helecomitus, 616.

Helichus, 474.

Heliconidæ, 379.

Helm, on spinning-glands, 339.

Hemelytra, 124.

Hemerobiidæ, hatching spine of, 585.

Hemimetabola, 598.

Hemiptera, cardo of, 69; fœtid glands of, 372; lacinia of, 74; palps of, 68; salivary glands of, 333; stipes of, 69.

Hepialus, 392, 495.

Heptagenia, 467; lingua of, 73.

Heredity, bearer of, 498; homochronous, 708.

Heremetabola, 597.

Herold, on the metamorphosis of the butterfly, 642; on wing-germs, 128.

Heterochrony, 542.

Heterometabola, 597.

Heymons, on homologies of the labrum, 43; on nature of elytra, 126; on origin of fat-body, 575; on paired sexual openings, 493; premandibular segment, 52; on the primitive segments of Phyllodromia, 563; on reproductive glands, 575; tentorium, 50.

Hicks, on auditory organs, 293; on olfactory organs, 266.

Histogenesis, 650.

Histolysis, 643, 650, 678, 680, 685, 688.

Hoffbauer, on the structure of elytra, 125.

Holmgren, on tracheal end-cells, 437.

Holometabola, 598.

Holometabolous insects, 595.

Holopneustic type of tracheal system, 459.

Holoptic head, 98.

Homalotylus, 623.

Homoptera, number of moults of, 616.

Homotenous insects, 597.

Honey-dew, 364; deterrent use of, 365; sac, 309; stomach, 309.

Hopkins, on pigments, 207.

Hornia, hypermetamorphosis of, 693.

Horns, 187.

Horn, on loss of tarsi, 101; on Platypsylla, 62.

House-fly, thorax of, 88.

Hum of bee, 295.

Hunter, John, on the air-sacs, 456.

Hurst, on the formation of the imago in Culex, 670.

Hybocampa, 635.

Hydrobius, 471.

Hydrophilus, 374, 432; embryology of, 536, 537, 542, 546, 558, 575.

Hydropsyche, blood-gills of, 475; gills of, at all stages, 469.

Hydroüs, 49, 50.

Hylotoma, 52.

Hymenoptera, composition of the head in, 55; mouth-parts of, 79, 81; olfactory organs of, 277; open stigmata of, 462; poison glands of, 357; post-embryonic changes in, 661; salivary glands of, 334.

Hyperchiria io, 187, 378; poisonous spines of, 192.

Hypermetamorphosis, described, 688; causes of, 693.

Hypodactyle, 73.

Hypoderma, 518.

Hypodermis, 188, 612; origin of an imago, 678.

Hypopharynx, 13, 54, 68, 70.

Hypoptère, 89.

Icerya, 616, 626.

Ichneumon, 622; hypermetamorphosis of, 701; poison glands of, 359.

Ileum, 317.

Imaginal buds, 653; disks, 653.

Imago, formation of, in Chironomus, 671; Corethra, 668; Culex, 668; Hymenoptera, 661; Lepidoptera, 641; Melophagus, 686; Musca, 673; of fly, development of internal organs of, 678; Simulium, 668.

Incasement theory, 641.

Indusium, 534.

Infra-anal lobe, 183.

Infraœsophageal ganglion, 227.

Ingluvies, 303.

Insecta, diagnostic characters of, 26.

Insects, ancestry of, 17; number of species of, 1; relation of, to other Arthropoda, 2; Symphyla, 18.

Intestine, fore, embryonic development of, 547; formation, imaginal, 682; hind, 316, 547; histology of, 316; large, 316; mid, 314; origin of, 569.

Invaginations of the imaginal buds, 678.

Involution of the embryonic membranes, 556.

Isotoma, 534.

Jackson, on the structure of the cremaster and pupa, 639.

Janet on muscular fibres, 216.

Japyx, 486.

Jolia, 466.

Jugum, 123.

Julus, larva of, 14.

Katydid, 616.

Kellogg, on Androconia, 199; on spinules, 197; on striæ of scales, 195, 198; on use of scales, 195.

Kennel, on origin of tracheæ of Peripatus, 443.

Kenyon, on double segments of Diplopods, 14; on mushroom bodies, 234.

Kettelhoit, on specific characters of, 195.

Kingsley, on classification of Myriopoda, 12.

Kirbach, on salivary duct, 336.

Kirby and Spence, views of, on metamorphosis, 642.

Klapálek, on gills of case-worms, 467; on sub-nymph of Agriotypus, 701.

Klemensiewiez, on eversible glands, 377.

Kolbe, on atrophy of tarsi, 101; on blood, 408; on flight of Agrioninæ, 159; on tracheæ, 435; on tracheal gills of Perla, 477; on embryology of the mole-cricket, 529, 572.

Korschelt, on the egg-tubes, 502; on egg-genesis, 504.

Korschelt and Heider, on the embryology of insects, 531, 535, 538, 554, 559, 570, 579; on formation of the imago of Corethra, 668; on position of genital glands in myriopods, 15; on stem form of myriopods, 17.

Koulaguine, on dorsal opening of urinary tubes, 355.

Kowalevsky, on embryonic abdominal appendages, 550; on embryonic membranes, 562; on origin of blood corpuscles, 574; experiments on feeding maggots with lacmus, 326; on fat-body, 420; on embryo origin of fat-body, 574; on the mesoderm of the rudiments of the appendages, 675; on openings of heart, 400; on pericardial cells, 420; on phagocytes, 421, 422, 655; phagocytosis, 686.

Kraepelin, on homologies of the ovipositor, 168; on organs of smell, 267; on taste, 282.

Krancher, on the stigmata, 438.

Krauss, on eversible glands, 371.

Krawkow, on chitin, 29.

Krukenberg, on colors, 202, 205, 206.

Künckel d’Herculais, on beating of heart throughout the post-embryonic changes, 686; on the origin of the imaginal from the larval legs, 654.

Kupffer, on fine tracheæ, 435.

Labella, 13, 446.

Labial palpi, imaginal buds of, 658.

Labidura, 491.

Labium, 68, 549.

Labrum, 42, 79; epipharynx, 43, 79; origin of, 546, 547; homologies of, 546, 547.

Lacaze-Duthiers on the ovipositor, 167, 169.

Lace-winged fly, 517.

Lachnosterna, nervous system of, 225.

Lachnus, 372.

Lacinia, 63; of Eriocephala, 67; mandible of Copris, 61; Ephemera nymph, 61; mandible of Passalus, 61; mandible of Phanæus, 61; mandible of Staphylinus, 61.

Lady-birds, 375; bug, 375.

Lagoa crispata, 191, 378.

Lamarckian factors, 708.

Lamina supra-analis, 181.

Lampyris, 425, 451.

Landois, on pigment, 207; on sense of smell, 267; on origin of the tracheæ and veins of the wings, 145; on wings as respiratory organs, 461.

Lang, on metamorphosis of seventeen-year Cicada, 698; on origin of coxal from setiparous glands, 444; on Peripatus, 9; on relation of myriopods to insects, 17; on segmented structure of arthropods, 32; on respiratory system, 430.

Langley, on the light of the firefly, 426.

Larva, defined, 599; Campodea-form, or campodeoid, 600; growth of, 608; voracity of, 608; eruciform, 705.

Larvæ, apodous, 103, 653.

Larval insects, tracheal gills of, 466; stage, 593.

Latreille, on the median segment, 163; metamorphism, 597; on the term pupa, 625.

Latzel, on coxal sacs, 14.

Layer, superficial protoplasmic, of egg, 524, 526; germ, formation of, 558.

Leach, on ametabola, etc., 596.

Leaping power, 219.

Legs, abdominal, of lepidopterous larvæ, and larval saw-flies, are they true legs?, 552; atrophy of, 102; mechanism of, 104; movements of, 105; muscles of, 215; post-embryonic development of, 653, 654; pulsatile organs in, 405.

Lehrman, on organs of smell, 265.

Leidy, on fœtid glands, 373.

Lens, crystalline, 250, 251.

Lendenfeld on flight, 149, 151, 159.

Léon, on labial palpi of Hemiptera, 68; tongue of Hemiptera, 73.

Lepidoptera, embryology of, 537; eversible sacs of, 375; maxillæ of, 65, 67; number of moults in, 616, 617; origin of legs of imago, 654; paired oviducts, 492; pupæ of, 628; testes of, 493, 495.

Lepisma, 52; double sexual openings in, 486.

Leptiform larvæ, 600.

Leptis, thorax of, 91.

Leucarctia, 391.

Leucine, 352.

Leucocytes, 407, 421, 650, 678, 680, 685; size of, 407.

Leucopis, 113, 517.

Leydig, on colors, 202, 204; on nerve-end apparatus in the wing, 153; on organs of smell, 266; on tracheæ, 432.

Libellula, 463.

Life, embryonic, length of, 582.

Light of the firefly, 426; its use, 428.

Ligula, 68.

Limacodes scapha, 606.

Limbs, homologies of, 39; mechanical origin of, 34, 35; lost, reproduction of, 619; result of disuse of, 101.

Limnephilus pudicus, 46; maxilla of, 65.

Limulus, 5.

Lina, 374, 545, 546.

Lingua, 68, 70.

Lip, under, 68.

Lipochrome, 206.

Liponeura, 475.

Lithocolletis, 618; its cocoon-cutter, 634.

Litognatha nubilifasciata, 102.

Lobe, axillary, 124; infra-anal, 183.

Lobes, œsophageal, 237; antennal, 237; head, 544; procephalic, 544, 548; procerebral, 232.

Lobulus, 124.

Locomotion, 103; on smooth surfaces, 111.

Locust, air-sacs of, 424, 456; brain of, 231; cæcal appendages of, 347; digestive canal of, 298; ear of, 288; head-segments of, 546; mode of breathing, 451; hatching, 583; moulting of, 609; nervous system of, 223; number of stages of, 595; number of moults of, 616; olfactory organs of, 272; oviposition of, 520; rectal glands of, 318; reproductive organs of, 488, 489.

Locusta viridissima, rectal glands of, 318.

Locy, on pulsatile organs in legs of Nepidæ, 405.

Lonchodes, 521.

Loop of wing, 122.

Lophyrus, 59.

Lora, 68.

Lubbock, on color sense, 261; on vision, 256–258; on distribution of tracheæ, 433.

Lucanus, 59, 620; thorax of, 94; dama, nervous system of, 225.

Lucas, on segmental arrangement of salivary glands, 337.

Luciola, 424, 427, 451.

Luna moth, its mode of escape from its cocoon, 635.

Lutz, on blood, 275.

Lycæna, 381.

Lymph, 206.

Lyonet, on the imaginal buds, 656; on wing-germs, 128.

Machilis, 164, 223, 369, 476; hypopharynx, 72.

MacLeod, on the tænidia, 445.

Macloskie, on the tænidia, 445.

Macrotoma, 616.

Macrurocampa, 375.

Maggots, 607; rat-tailed, 461, 474.

Malachius, 374.

Male reproductive organs, 485, 494; origin of, 579.

Malpighi, on germs of wings, 128; on the heart, 397; on the metamorphosis of silk moth, 641; on urinary vessels, 350.

Malpighian tubes, 316, 348.

Mandibles, 59; composite structure of, 60, 61; lacinia of, 60, 61; vestigial, 62.

Manometabola, 597.

Mantidæ, coxal glands of, 372.

Mantis, oötheca of, 517.

Mantis religiosa, embryo of, 584; hatching of, 584; number of moults of, 584, 616.

Mantispa, 46, 48, 68, 95, 97; hypermetamorphosis of, 602, 690, 705; life-history of, 602.

Marchal on the function of the fat-body, 420.

Marey, on motion of insects, 111.

Marey’s views on flight, 148, 151.

Marshall, on the way Microgaster spins its cocoon, 622.

Mason, Wood, on jointed structure of mandibles, 60; on Scolopendrella, 19, 22.

Mastopoda pteridis, 103.

Maxillæ, first, 62; second, 68; imaginal buds of, 658.

Mayer, A. G., on the development of the wings of Pieris and Danais, 136; on formation of scales, 196; on homologies of tracheæ, 443; pigments, 206–208.

Mayer, Carl, on scales, 197.

May-fly, lingua of nymph of, 73; number of moults of, 616; thorax of, 91.

Mechanics of walking, 103.

Mechanism of motion, 32; of limbs, 35; of secretion, 326.

Meconium, 611.

Mecoptera, maxillæ of, 65; number of moults of, 616.

Median segment, 90, 163.

Medifurca, 92.

Megalopyge, 191, 378.

Meinert, on buccal organs of myriopods, 13; on coxal sacs, 14; on elytra, 125; on hypopharynx, 78; on organs of taste, 281.

Melanoplus, 43, 72, 86, 456; gastro-ileal folds of, 317; hatching of, 583; number of stages of, 595; rectal glands of, 318; tongue of, 72.

Meldola, on yellow pigment, 206.

Melipona, 359.

Meloë, 110, 374; hypermetamorphosis of, 690; lacinia of mandibles of, 62; number of moults of, 617; small eggs of, 524.

Melolontha, 213, 438, 455, 456, 458, 498, 549, 550, 551, 562.

Melophagus, 507; post-embryonic changes in, 686.

Membrane, peritrophic, 313; retaining, of pupæ, 638; serous, 532; vitelline, 520, 534.

Membranes, embryonic, 531; formation of, 532; embryonic, involution of, 556.

Menge on Scolopendrella, 18.

Mentum, 54, 68.

Merostomata, 5.

Mesoderm, 534, 561, 563; cells, 576.

Mesothorax, 86.

Metabolia, 596.

Metabolous insects, 595.

Metameric structure, 33.

Metamorphoses of insects, 593; stadia of, 594; stages, 594.

Metamorphosis, causes of, 607, 705, 708; significance of, 688.

Metapneustic type of tracheal system, 461.

Metathorax, 87.

Metschnikoff, on embryology of myriopods, 13, 16; on the germs of the genital glands, 580.

Miall and Denny, on the blood, 407; on cement glands, 361; on chitin, 29; on digestive canal of cockroach, 316, 317; on heart, 398, 402; on labium, 53; on lingua, 72; on reproductive organs of the cockroach, 487; on respiration, 452; on salivary glands, 331; on tænidia, 447; on the tentorium, 49; on urinary tubes and products, 353.

Miall and Hammond, on the differences between the pupa of Lepidoptera and Diptera, 629; on the formation of the imago of Chironomus, 671.

Microcentrum, 616.

Microgaster, mode of spinning its cocoon, 622.

Micropteryx, 621, 626; escape from its cocoon, 633; hypopharynx of, 76; labium of, 76; pupal jaws of, 633.

Micropyle, 522; use of, 524.

Mid-intestine, 314; origin of, 569.

Minchin, on eversible glands, 370.

Minot, on cæca of stomach, 347; on colors, 203; on the cuticula, 187; on digestive canal, 318, 320, 321; on distribution of tracheæ, 433; on gastro-ileal folds, 317; on rectal glands, 318; on tænidia, 445.

Molar, 61.

Mole-cricket, 102, 527, 543, 572, 574; digestive canal of, 350; urinary tubes of, 350.

Mosaic theory of vision, 257.

Moseley, on circulation of blood, 410; on composition of chitin, 30.

Mosquito, 461, 464, 465; poison gland of, 359; sense of hearing of, 292.

Moulting, process of, 609; hairs and spines, 612.

Moults, number of, 615.

Mouth, 302; -appendages, buds of, 665; of embryo, 537.

Müller, F., on blood-gills, 475; on larvæ of Psychodes and of Blepharocera, 474; on non-inheritance of the complete metamorphosis, 595.

Müller, J., on alluring glands, 391; on heart, 399; on sense and organs of smell, 265; on the development of wings, 138, 143; on vision, 255.

Müller, W., on gills of Paraponyx, 470.

Müller’s, J., thread, 577.

Mumia, 625.

Musca, 88, 111; embryology of, 530, 536, 563; wing-germs of, 133.

Muscidæ, appendages of imago, development of, 674; post-embryonic changes in, 666, 673, 678, 681.

Muscles, 31; of caterpillars, 213; of cockchafer, 213; of Cossus, 211; destruction of, during metamorphosis, 680; of flight, 149; of Pygæra, 211; respiratory, 454; structure of, 215.

Muscular fibres, 214; power, 217, 219; system, 211.

Musculature, mode of origin of, 574.

Mushroom bodies, 233.

Mutilations, inheritance of, 102.

Myriopoda, 11.

Myrmeleon, maxilla of, 64; palpifer of, 69.

Mystacides, scent scales of, 199.

Nagel, on saliva of larval Dyticus, 324.

Nassonow, on double sexual openings, 486.

Necrophorus, tracks of, 109.

Nematois, 496.

Nematus, 53, 54, 618.

Nemognatha, epipharynx of, 285; maxillæ of, 64, 65; organs of taste in, 285.

Nemoptera, larva of, 42.

Nemoura, 468.

Neolamarckian factors, 708.

Neolepidoptera, 628.

Nepa, 523.

Nephridia, 348.

Nepionic stage or form, 706.

Nepticula, 606.

Nerve-centre, 222.

Nerve-centres, functions of, 243; antennal, 650.

Nerves, motor, 222; motor, degeneration of, during metamorphosis, 684; optic, 650; peripheral, transformation of, 684; sensory, 222; stomogastric, 238; sympathetic, 238; visceral, 238.

Nervous system, 222; formation of, 566; free from histolysis during pupation, 667, 684; origin of, 554, 566; primitive rolls or strips, 566; slight changes in, during metamorphosis, 684.

Neuroblasts, 567.

Neuromeres, 227, 231.

Neuroptera, 632; lingua of, 69.

Newman, on the median segment, 163.

Newport, on changes in nervous system of Sphinx during metamorphosis, 648; on circulation, 409; on heart, 399; on larval Julus, 13; on the median segment, 163; on muscular power, 94; on muscles of Sphinx, 213; on number of segments of head, 50; on occiput, 48; on the process of moulting in Sphinx, 610, 611; Scolopendrella, 18; on sense of smell, 265; on tentorium, 49; on tracheal gills of Pteronarcys, 476.

Nola, 618.

Nucleus, division, 530; sperm, 525.

Nusbaum, on origin of efferent sexual passages, 578.

Nymphalid pupæ, 631.

Nymph, 706; stage, 593, 600; sub-, 701.

Occipital foramen, 46.

Occiput, 48, 53.

Ocellus, 249; development of, 567.

Ockler, on feet of insects, 115.

Odonata, 53; embryology of, 540; labium of, its mode of origin, 549; lateral gills of, 468; lingua of, 73; number of moults of, 616; nymphs, 460, 463.

Odors, 368.

Œcanthus, 476; embryology of, 541, 544, 549, 551, 573.

Œceticus, 634.

Œnocytes, 423.

Œsophageal valve, 311; valvule, 312.

Œsophagus, 303.

Œstridæ, 618.

Oken, on homology of maxillæ with legs, 39.

Olfactory organs, 264.

Oligonephria, 354.

Oligoneuria, 466.

Ommateum, 250.

Onychium, 97.

Oölemma, 520.

Oötheca, 517.

Operculum, 181.

Optic nerves, 650.

Optic tract, 253.

Opticon, 253.

Oral appendages, 549.

Orchelimum, 534.

Organ, dorsal, 535, 556.

Organs, sensory, 249; of smell, 264.

Orgyia, 377, 618; poisonous hairs of, 192.

Orgyia antiqua, number of moults of, 618.

Orthoptera, fœtid glands of, 369; number of moults of, 616; phagocytes in, 421; salivary glands of, 331; tongue of, 70.

Orya, 424.

Osmeterium, 377.

Osten Sacken, on holoptic heads of Diptera, and on running flies, 98.

Ostia, 397, 400.

Otiocerus, 58.

Oudemans, on relation of myriopods to insects, 17.

Oustalet, on rectal respiration, 463.

Ovarian tubes, 501; formation of, 578.

Ovaries, 500, 502; formation of, 578; groups of, 502.

Oviduct, 500, 503; double openings of, 486; origin of, 578, 579.

Oviducts, segmental arrangement of, 486.

Oviposition, 518, 520.

Ovipositor, 167; germinal buds of, 665; imaginal buds of, 666; origin of, 551.

Ovum, 515, 521, 524.

Packard, A. A., on muscular power of insects, 219; use of air-sacs, 457.

Pad, peripodal, 653.

Pædogenesis, 708.

Paleacrita vernata, maxilla of, 66.

Paleolepidoptera, 628.

Palmén, on double sexual openings, 490, 492; on tracheal gills, 459, 466; on the tentorium, 50.

Palmula, 97.

Palpifer, 63, 68.

Palpus, first maxillary, 63, 64, 67; second, 68; in Hemiptera, 68.

Palpi, labial, imaginal buds of, 658.

Pancritius, on wing-germs, 130, 143.

Paniscus, 517.

Panorpa, abdomen of, 162; maxilla of, 65; number of moults of, 616.

Panorpidæ, 602.

Papilio, 377.

Paraglossa, 54, 68.

Parapodial glands, 444.

Paraptera, 89.

Parnassius, 381.

Paronychium, 97.

Paraponyx, 470.

Passalus, 61; rectal glands of, 318.

Pasteur, on the spectrum of the light of the firefly, 426.

Patagia, 89.

Patten, on embryonic abdominal appendages, 550; on the homologies of the tracheæ, 444; salivary glands, 337.

Patula, 391.

Pauropus, 18.

Paussus, 57.

Pawlowa, on blood-vessels in the head, 405.

Pedicel, 57.

Pediculina, embryology of, 541.

Pelidnota, moulting of, 611.

Pellicle, subimaginal, 613.

Pelobius, 461, 475.

Penis, 180; velum of, 181.

Pericardial cells, 405; diaphragm, 402; septum, 574.

Periopticon, 231, 253.

Peripatus, 9, 580; nephridia of, 349; trachea of, 443.

Peripodal cavity, 669; membrane, 669; sac, 653.

Periplaneta, 72, 370; egg-tubes of, 501; tongue of, 72.

Peritoneal membrane, 444.

Peritracheal circulation, 397; membrane, 444.

Peritreme, 90.

Peritrophic membrane, 313.

Perlidæ, 69, 468, 491, 493.

Perris, on organs of smell, 266.

Phagocytes, 421, 650, 655, 680, 683, 685.

Phagocytosis, 421.

Phanæus, 61; reduced tarsi of, 101.

Phanæus pegasus, 188.

Phaneroptera, 44.

Pharynx, 302.

Phasmidæ, eggs of, 521.

Phosphorescence, 424; physiology of, 426.

Photogenic organ of beetles, 424.

Phragma, 93.

Phryganea, 455.

Phyllium, 521.

Phyllocnistis, 606.

Phyllodromia, 506; embryology of, 541, 563, 583; eversible glands of, 370; fat-body, origin of, 575; micropyle of eggs of, 523; pleuropodia of, 551; origin of sexual organs of, 576, 578–580; mode of hatching, 583; oötheca of, 519.

Phytonomus, 617.

Phytophagous larvæ, 606.

Pictet, on blood gills, 475.

Pieris, 39, 614; embryology of, 546; green pigment of, 206; post-embryonic changes of, 651; wing-germs of, 133.

Pigment, 183, 203; chemical nature of, 206; of eye, 253; physical nature of, 206.

Pits, olfactory, 271.

Planta, 638.

Plantula, 97.

Plate, extensor, of foot, 116; pressure, 116.

Plateau, on digestion, 324; on functions of ganglia, 244; on muscular power, 217; on respiration, 453; on vision, 256, 259.

Platycrania, 521.

Platygaster, hypermetamorphosis of, 701; sexual cells of, 581.

Platypsyllus, 62.

Platysamia, 460.

Platyzosteria, 371.

Plectoptera, 459, 466.

Pleuropodia, 476, 551, 583.

Pleurum, 87.

Plumules, 198.

Pocock, on classification of myriopoda, 12, 21.

Poduridæ, 574.

Poisonous spines, 191, 199.

Poisons, effect of, on pulsations, 412; on hairs, 191, 199.

Poison apparatus, 357; glands, 357; nature of, 357.

Polar cells, 580.

Polymitarcys, 467.

Polymorphous insects, 597.

Polynema, hypermetamorphosis of, 702.

Polynephria, 354.

Polyphemus silkworm, 621.

Polypodous ancestor of insects, 22; embryos, 550.

Polypody, 550; suppression of in dipterous embryos, 707.

Pore-canals, 188.

Porthesia, 529.

Postfurca, 92.

Post-gula, 54, 68.

Post-retinal fibres, 231, 232.

Postscutellum, 87.

Pouch, copulatory, 505.

Poujade, on flight, 159.

Poulton, on the differences between the limbs of the pupa and imago, 628.

Præscutum, 87.

Pratt, on absence of polypodous embryo in Muscidæ, 55; on the dorsal position of the stomodæum of Diptera, 537; on epigenetic period, 688; on fate of the leucocytes, 685; post-embryonic changes of Melophagus, 686; significance of metamorphosis, 688; on wing-buds, 127.

Preantennal appendages, 548.

Premandibular segment, 51, 52, 549.

Press of spinning apparatus, 341.

Primitive band, 531; streak, 531.

Prionocyphon, 472.

Prisopus, 69, 477.

Proboscis, 446.

Procephalic lobes, 544.

Proctodæum, 537, 547, 569.

Prodoxus, 606.

Progoneate myriopods, 21.

Pronotum, 87.

Pronucleus, 526.

Propodeum, 163.

Propupa, 627.

Prosopistoma, 467.

Prostheca, 61.

Prothorax, 86.

Protocerebrum, 231, 232; its representative in annelids, 227.

Proventricular valvule, 313.

Proventriculus, 306; “beak” of, 312; formation of, in imago, 682; use of, 311, 324, 325.

Prussic acid, 374.

Psephenus, 462, 473.

Pseudoglomeris, 598.

Pseudonychium, 97.

Pseudo-tracheæ, 446.

Psocus, 616.

Psychodes, 474.

Psylla, 361, 518; glandular hairs of, 192; nymph of, 163.

Pteronarcys, 468, 476.

Pterygodes, 89.

Pterygota, 27, 595.

Pulex, number of moults of, 617.

Pulex canis, hatching spine of, 586; hypopharynx of, 77.

Pulling power, 218.

Pulse, 411.

Pulvillus, 97, 114, 116.

Pump, pharyngeal, 302; adaptation of, to its surroundings, 631; armature of, 631; structure, 632.

Pupa, coarctate, 621, 626; libera, 626; mode of escape of, from its cocoon, 632; mode of suspension, 637; nymphalid, 631; obtecta, 626; spines of, 629; state defined, 625.

Pupa, semi, 691.

Pupal, pseudo-, stage, 691; sustainers, 638.

Puparium, 621.

Pupation, mechanism of, 661; process of, 659.

Pupipara, post-embryonic changes in, 686.

Pushing power, 218.

Pygidium, 163.

Pyloric valvule, 315.

Pyrophorus, 424.

Pyrrarctia, 617.

Pyrrhocoris, 372.

Quiescent pupal life, 598.

Ranatra, 523.

Raphidia, 631.

Raschke, on the rectal respiration of Culex, 465.

Rath, Vom, on larval Julus, 13.

Ratzeburg, on composition of head in Hymenoptera, 55.

Réaumur, on the cremaster, 637; on the double sexual openings of Ephemera, 489; on germs of wings, 128; on the heart, 403; on the mechanism of pupation, 661; on metamorphosis, 642; on the origin of legs of imago from those of the larva, 654; on rectal respiration, 463; on sense of smell, 264; on vision, 256.

Rectal glands, 318; tracheal gills, 463.

Rectum, 318; of embryo, 577.

Reinhard, on head of Hymenoptera, 55; on the median segment, 164.

Reproduction, organs of, 485; origin of, 575.

Repugnatorial glands, distribution of, 382.

Respiration, 430, 451; rectal, 463.

Respiratory system, 430; mechanism of, 451.

Resting stage, 707.

Retina, origin of, 568.

Retinula, cells of, 250, 253.

Rhabdites, 167, 517.

Rhabdom, 250.

Rhabdopoda, 176.

Rhipiphorus paradoxus, 697.

Rhopalum, 636; pupal spines of, 636.

Rib, Semper’s, 121.

Ribs of wings, 146.

Ridges, primitive, of nervous system, 554.

Riley, on the cremaster, 637; egg-burster, 585; hatching of seventeen-year Cicada, 584; life-history of Epicauta, 692.

Rods of eye, 253.

Rods, visual, of eye, origin of, 568.

Rombouts, on locomotion of insects on smooth surfaces, 114.

Ruptor ovi, 585.

Ryder, on loss of tarsi, 101; on Scolopendrella, 19.

Sacs, air, 456; use of, 457; coxal, 14; eversible, 369; hypodermal, 653.

Saliva, 324; poisonous, 359.

Salivary duct of Stomoxys, 446; glands, 331, 570; glands, formation of imaginal during metamorphosis, 683; homologues of coxal glands, 337; modified, 337; segmental arrangement of, 331.

Savigny on epipharynx, 43; on homologies of appendages, 39, 71.

Saw-fly, 374.

Scale-hairs, 198.

Scales, 187, 193, 202; battledore, 198; development of, 195; distribution of, 193; of fly’s wing, 124; scent, 198; flattened, 193; striæ of, 194, 202.

Scape, 57.

Scent-glands, 39.

Scent-scales, 198.

Scepsis, 618.

Schæffer, on blood, etc., in the pupal wings, 146; the fat-body, 420; leucocytes, 421; on origin of scales, 196; on the rudimentary wings, 128.

Schatz, on colors of butterflies, 202.

Schiemenz, on salivary glands of bees, 334.

Schindler, on urinary tubes, 351.

Schiödte on blood-gills, 475.

Schmidt, on the metamorphosis of male Coccidæ, 640; Scolopendrella, 21, 24.

Schneider, on the funnel of the proventriculus, 313; on spermatophores, 500.

Sciara, 348, 636.

Sclerites, cervical, 46.

Scolopendrella, 18; the ancestor of insects, 17; spinning glands, 346.

Scudder on the glazed eye of pupal butterflies, 631.

Scutellum, 87.

Scutum, 87.

Secretion, definition of, 327; mechanism of, 326; products of, 329.

Sectores coconis, 634.

Segment, antennal, 227; deutocerebral, 227; intercalary, 51; median, 90, 163; premandibular, 51, 52, 228; procerebral, 231.

Segmental arrangement of genital glands, 486.

Segments, number of, in head, 50, 68, 227, 229; optic, 231; origin of, 542.

Seirarctia, 618.

Selandria larva, mouth-parts of, 68.

Seminal ducts, 496.

Semipupa, 691.

Semper, ground-membrane of, 136; on origin of hair-scales, 195.

Sense organs, special, in flies, 293.

Sensory organs, 249.

Serosa, 532.

Setæ, 188.

Sexual differences, secondary, 59, 99, 101, 114; openings, double, 486, 490, 491.

Sharp, on causes of segmentation of Crustacea, 33; on cervical sclerites, 46; on sternites, 92; homologies of elytra, 126.

Sheep-tick, 507; post-embryonic changes of, 686.

Sialidæ, 602.

Sialis, 462, 468.

Siebold, organ of, 290; on spermatophores, 500.

Silk, 340; composition of, 346.

Silk-fibre, composition of, 346.

Silk-gland, 339; anal, 346; appendages of, 345; histology of, 334; modified coxal glands, 346; moulting of, 345.

Silkworm, 331, 339, 366; amount of food eaten by, 608; functional salivary glands of, 331; mode of escape of, from its cocoon, 635; Polyphemus, 621; voracity of, 608.

Simmermacher, on feet of insects, 113.

Simulium, 668, 678; hypopharynx of, 78; wing-germs of, 129.

Sinclair on double segments of Diplopods, 14.

Sisyra, abdominal appendages of, 164.

Sitaris, 691.

Slug-worm, 188.

Smell, experiments on, 269; organs of, 264, 271; physiology of, 268; sense of, 368.

Smith, on lack of fore-tarsi in a moth, 102; jointed structure and lacinia of mandibles, 61; maxilla, 65; on scent-glands, 391.

Sole, extensor, 116.

Somites, 30.

Sorby, on change of color in Aphides, 205.

Sounds, 293.

Specius, 585.

Spengel, on color sense, 260.

Spermatheca, 506.

Spermatid, 498.

Spermatocyte, 498.

Spermatogonium, 498.

Spermatophore cap, 499.

Spermatophores, 497, 499.

Spermatozoa, 497, 525; formation of, 498.

Sperm-nucleus, 525.

Sphinx, 456, 552; moulting of, 610.

Sphinx ligustri, changes during metamorphosis, 646.

Spilosoma, 391.

Spindle, directive, 525.

Spines, 187, 189; glandular, 190; helcodermatous, 612; locomotor, 612; moulting, 612; poisonous, 189.

Spinneret, 342; of caterpillars, 75.

Spinning apparatus, 340; at end of body, 346; glands, 339; process of, 340.

Spinules, 189, 197.

Spiracles, 437; types of, 438.

Spiral thread, 444; absence of, 447; origin of, 448.

Spraying apparatus, 370.

Spring of Collembola, 551.

Spuler, on pigments, 207, 208; on structure of scales, 195, 197.

Squama, 123.

Squamæ, 124.

Squamula, 124.

Squamule, 89.

Stadia of metamorphosis, 594.

Stage, carabidoid, 692; gastrula, 535; metabolous, 594; resting, 707; scarabæidoid, 692.

Stages, ametabolous, 594.

Stagmomantis carolina, embryo of, 584; hatching of, 584.

Staphylinus, 61, 454.

Stenobothrus sibiricus, swollen foretarsus in male, 113.

Stenosternus, 101.

Sternum, 87, 89.

Stigmata, 437; closed, 460; closing apparatus of, 441; mesothoracic, 462; number of, in the embryo, 554; number of pairs of, 439, 461; position of, 440; vestigial, 460.

Sting, bee’s, 172.

Stipes, 63.

Stokes, on the tænidia, 445; hairs in, 451.

Stomach, chyle, 314, 325; absorbent cells of, 328; glandular cells of, 327; formation of, in imago fly, 681, 682; origin of, 569.

Stomach-mouth, 309.

Stomodæum, 537, 547, 569.

Stomoxys, 446.

Strauss-Dürckheim, on the heart, 397; on muscles of cockchafer, 213.

Streak, embryonal, 531; primitive, 531.

Streblopus, 101.

Striæ of scales, 194, 202.

Styles, abdominal, 176; of ovipositor, 167.

Stylopidæ, 486; hypermetamorphosis of, 695.

Stylops childreni, triungulin larva of, 695.

Subgalea, 73.

Submentum, 54, 63, 68, 69.

Substance, fibrillar nerve, 238.

Sucking stomach, 302, 305.

Supra-anal plate, 181.

Supra-œsophageal ganglion, 231.

Supra-spinal vessel, 403.

Suranal plate, 181.

Surroundings, physical, 463.

Sustainers of the pupa, 638.

Swammerdam, on discovery of air-sacs, 456; on germs of wings, 128; on the mechanism of pupation, 661; on metamorphosis, 599; on rectal respiration, 463.

Swimming, act of, 116.

Symphyla, 18; characters of, 22.

Synaptera, 27, 534, 594, 705.

Syromastes, 372.

Tabanus, 629; mouth-parts of, 79.

Tænidia, 444; origin of, 447, 448.

Talæporia, 634.

Talocera, 57.

Tanypus, 472.

Tarsus, 96; changes of, from larva to imago of Lepidoptera, 655; reduction in or loss of, 101, 102.

Taste, organs of, 281.

Tegeticula yuccasella, 65, 66.

Tegmina, 124.

Tegula, 89, 124, 125.

Telea polyphemus, amount of food eaten by, 608; cocoon of, 621; moulting of, 610; thorax of, 88.

Teleas, hypermetamorphosis of, 703.

Telephorus, 111, 538; tenent hairs of, 99.

Tenebrio, 617.

Tenent hairs, 99.

Tentacle of maxilla, 65.

Tentorium, 49.

Tergum, 87.

Termen, 122.

Termes, abdomen, 162; origin of wings of, 140, 143.

Termes flavipes, 64.

Termopsis, 48, 64.

Testes, 487, 495; incipient eggs in the germ of the testis, 504.

Theory of incasement, 641.

Thomas, on origin of scent-scales, 199.

Thorax, 86, 95; gills on, 468.

Thread-plate, 575.

Thread, spiral, 444; origin of, 447, 448.

Thyridium, 124.

Thyridopteryx, 634.

Thysanoptera, nymph of, 597.

Thysanura, 72; cercopods of, 164; genital organs of, 486.

Tibia, 96; formation of, in imago of Lepidoptera, 655.

Tineid larva, wing-buds of, 652; wing-germs of, 129.

Tipula, 629; flight of, 151, 152; thorax of, 91.

Tissue, connective, 574.

Tongue, 70.

Torulus, 57.

Tracheæ, 431, 442; capillary, 655; distribution of, 432; end-cells, 437; hairs in, 451; moulting of, 612; origin of, 553; origin of, in worms, 442; size of, 433; of wings, 144.

Tracheal gills, of adult insects, 476; rectal, 463.

Tracheal system, amphipneustic type, 462; apneustic type, 459; closed, 459; holopneustic type, 459; metapneustic type, 461; peripneustic type, 462; propneustic type, 462; reformation of, in metamorphosis, 683.

Tracheoles, 126, 653.

Tract, optic, 253.

Trajectory made by wings, 150.

Tribolium, 617.

Trichodes, tracks of, 110.

Trichogen, 188, 199, 366.

Trichoptera, 469; appendages, 550; development of wings of, 142; embryology of, 537; eversible glands of, 375; haustellum of, 75; hypopharynx of, 74; pupal jaws of, 633; spinneret, 74.

Trilobita, 5.

Tritocerebrum, 231, 237.

Triunguline larva, 100, 693, 695.

Trochanter, 496; divided, 97.

Trochantine, 95.

Trogoderma, 617.

Trophi, 54, 59, 549.

Trouvelot, on the moulting fluid, 610; process of moulting of Telea, 610; spinning of cocoon by Telea, 621.

Truxalis, 422.

Tubercles, 187, 192.

Tubes, egg, 501; ovarian, 501; urinary, 316, 317, 348, 353, 572.

Tympanum, 289.

Typhlocyba, 616.

Uljanin, on the sexual cells of the honey bee, 582.

Ungues, 96.

Uranidin, 206.

Urates, 352.

Urea, 352.

Urech, on pigments, 206, 208.

Uric acid, 352.

Urinary bladder, 351; tubes, 316, 317, 348; absent in Collembola, 353; excretions of, 351; origin of, 572; primitive number of, 352; solid contents of, 352.

Urine, 206.

Urite, 163.

Uromeres, 163.

Uro-patagia, 183.

Urosome, 163.

Urosternites, 163.

Uterus, 507.

Utriculi, 487.

Uzel, on the embryology of Campodea, 51, 53; premandibular segment in Campodea, 51, 52.

Vagina, 507.

Valery-Mayet, on life-history of Sitaris, 691.

Vallisneri, on the cremaster, 637.

Valvule, cardiac, 312; proventricular, 313; pyloric, 315.

Van Bemmelen, on colors, 208.

Vanessa antiopa, 638.

Vanessa io, 381.

Vanessa urticæ, before pupation, 644; changes in nervous system during its metamorphosis, 646.

Van Rees, on post-embryonic changes of Muscidæ, 673, 674, 676, 680, 683, 684.

Vas deferens, 496; origin of, 579.

Vasa deferentia, 496; origin of, 579.

Vayssière, on lingua of Ephemera, 73.

Veins of wings, 121, 144.

Velum penis, 181.

Venomous glands, 358.

Vent, 319.

Ventriculus, 314.

Vermipsylla, tongue of, 77.

Verson, on serially arranged dermal glands, 366; vestigial stigmata, 460.

Vesicles, air, 456; use of, 457; frontal, 621; of head of semipupal fly, 677.

Vespa crabro, 217; olfactory organs of, 276.

Vessel, dorsal, 397; urinary, 316, 348.

Vestigial tracheæ, 460.

Viallanes, on brain, 231; head segments, 51.

Vision, mode of, by compound eyes, 256; by simple eyes, 255.

Vitelline membrane, 520.

Viviparous insects, 515.

Volucella, thorax of, 91.

Vosseler, on fœtid glands, 369.

Wagner, on the circulation, 419.

Walking, mechanics of, 103, 106.

Walter, on epipharynx of moths, 44; on hypopharynx of moths, 75.

Wasps, taste-organs of, 277, 286.

Wax, of butterfly, 364; of caterpillar, 364; of saw-fly larva, 364.

Wax-glands, 361, 364.

Weevil, embryology of, 538; bean and pea, hypermetamorphosis of, 700.

Weismann’s discovery of imaginal buds, 599, 643, 650; on formation of imago, 67; vesicle of semipupal fly, 677; on tracheæ, 434; on origin of tracheæ, 447; on origin of wings, 127, 129; theory of histolysis, 643.

West, Tuffen, on feet of fly, etc., 100; on walking, 99.

Westwood, on head of Hymenoptera, 55.

Wheeler, on embryonic abdominal appendages, 550; on the homologies of the ovipositor, 167; homology and primitive number of urinary tubes, 354, 355; on œnocytes, 423; on pleuropodia, 476, 550, 551; on the premandibular segment, 51; on structure of chorion, 521.

Wielowiejski, on blood tissue, 408; egg-tubes, 502; fat-body, 419; phosphorescence, 424; tracheæ and their endings, 436.

Williston, on anal glands, 372.

Will, on organs of taste, 282.

Wing-buds, or discs, 127, 129; rods, 146; sheaths, 124.

Wingless insects, 598.

Wings, 120; buds of, 664; cells, 121; circulation of blood in, 410; development of tracheæ of, 144; development of veins of, 144; embryonic development of, 126; folding of, 154, 156; imaginal buds of, 653; mechanism of, 153, 156; origin of, 138; primitive origin of, 137; as respiratory organs, 461; rudimentary, ground-membrane of, 136; spreading of, 155; theory of, 144; tracheæ in, 122; veins of, 121.

Wistinghausen, von, on tracheal endings, 436, 447.

Witlaczil, on honey dew, 364.

Wood-Mason, on gills of Paraponyx, 470; on Scolopendrella, 19, 22.

Xiphidium, embryo of, 534; indusium of, 534, 535.

Yersin, on results of section of commissures, 245.

Yolk, amount of, 524, 529; membrane, 520; segmentation of, 526; cells, 562; ridge, median, 563.

Zaitha, 431; pleuropodia of, 551.

Zone, annular, 653.

Footnote 1:

Zool. Anzeiger, xvi, 1893, pp. 271–5.

Footnote 2:

On the morphology of the Myriopoda, Proc. Amer. Phil. Soc. 1883, pp. 197–209.

Footnote 3:

Morphology and classification of the Pauropoda; also American Naturalist, 1897, p. 410.

Footnote 4:

The term which we proposed for this hypothetical ancestor of insects, “Leptus-like” or “Leptiform,” was an unfortunate one, since the name Leptus was originally given to the six-legged larva of a mite (Trombidium), the origin of the mites and other Arachnida being entirely different from that of the myriopods and insects.

Footnote 5:

Proc. Bost. Soc. Nat. Hist., xvi, 1873, p. 3.

Footnote 6:

American Naturalist, May, 1880, pp. 375, 376.

Footnote 7:

Zoologische Anzeiger, Bd. xx, 1897, pp. 125 and 129. He also states that Campodea resembles the myriopods, especially Geophilus, in the primitive band at first lying on the surface of the yolk, and in the absence of an amniotic cavity; also before hatching the abdomen is pressed against the thorax, as in myriopods.

Footnote 8:

“Scolopendrella has very remarkable antennæ; they may be compared each to a series of glass cups strung upon a delicate hyaline and extensible rod of uniform thickness throughout; so that, like the body of the creature, they shrink enormously when the animal is irritated or thrown into alcohol, and they then possess scarcely two-thirds the length they have in the fully extended condition, their cup-like joints being drawn close together, one within the other. Peripatus, Japyx, many (if not all) Homoptera, and the S. Asiatic relatives of our common Glomeris have all more or less extensible antennæ.” (Wood-Mason, Trans. Ent. Soc., London, 1879, p. 155.)

Footnote 9:

Lassaigne gave it the name of entomoline.

Footnote 10:

Miall and Denny ex Krukenberg; Kolbe gives the formula as C_{9}H_{15}NO_{6} or C_{18}H_{15}NO_{12}. As the result of his recent researches, Krawkow (Zeits. Biol., xxix, 1892, p. 177) states that the chemical composition of chitin may prove to be somewhat variable.

Footnote 11:

On allowing portions of a locust, a piece of the integument of Limulus, a scorpion, and a myriopod to soak for a month in white potash, neither were dissolved or affected by the reagent.

Footnote 12:

We may add, while correcting the proofs of this book, that the important summary, by Uzel, of his work on the embryology of Campodea appears in the Zoologischer Anzeiger for July 5, 1897. He observes that the premandibular segment in the embryo is very distinct, and that the two projections arising from it persist in the adult. “Campodea is now the first example where these appendages are present in the sexually mature insect and function as constituents of the completed mouth parts. I propose for these hitherto overlooked structures the name of intercalary lobes.” They each form a slightly developed chitinous lobe covering a gap between the base of the labium and the fused external lobe and palpus of the first maxillæ (which are inclined near the labium) in place of the mandibles which have sunken inward. Uzel also homologizes these appendages with two similar projections (Höcker) observed in the embryo of Geophilus by Zograf to be situated in front of the mandibles. Heymons has also detected this segment in the embryo of Lepisma.

Footnote 13:

While these pages are still in type, we may add, in confirmation of this view, that Uzel states, from his researches on the embryology of Campodea, that the maxillary tergites of the embryo only slightly share in building up the tergal region (occiput) of the head, but that they form the genæ of the maxillary segments. (Zool. Anzeiger, July 5, 1897, p. 235.)

Footnote 14:

Miall and Denny in their work on the cockroach, in describing the labium, remark: “The upper edge is applied to the occipital frame, but is neither continuous with that structure nor articulated thereto. By stripping off the labium upwards it may be seen that it is really continuous with the chitinous integument of the neck” (p. 95). This is, we think, a mistaken view, as proved by the embryology of the Odonata and of Nematus. Our statements on this subject were first published in part in 1871, and more fully in the third Report, U. S. Ent. Commission, 1883, pp. 284, 285. We also stated that all the gular region of the head probably represents the base of the primitive second maxillæ.

Footnote 15:

After we had arrived at this conclusion, and written the above lines, we received the Zoologischer Anzeiger for March 29, 1897, in which Dr. N. Léon publishes the same view, stating that each side of the submentum is the homologue of the cardo, and each side of the mentum corresponds to the stipes of a single maxilla (p. 74).

Footnote 16:

Miall and Denny were the first to homologize the paraglossæ with the galea and lacinia, showing the complete resemblance of the second maxillæ to the first pair, remarking that “the homology of the labium with the first pair of maxillæ is in no other insects so distinct as in the Orthoptera.” We have also independently arrived at a similar conclusion, but believe that the mentum corresponds to the first maxillary cardo, and the palpifer to the first maxillary stipes, the sclerite of each maxilla being fused to form the base of the labium, _i.e._ the unpaired mentum and submentum.

Footnote 17:

Uzel states that what is regarded as the ligula of Campodea is formed from the sternite of the first maxillary segment; while the two parts regarded as paraglossæ grow out from the sternite of the mandibular segment, and these three structures together he regards as the hypopharynx. (Zool. Anzeiger, July 5, 1897, p. 234.)

Footnote 18:

See, also, Breithaupt, Ueber die Anatomie und die Functionen der Bienenzunge, 1886. It confirms and extends Cheshire’s work.

Footnote 19:

Cholodkowsky, Zool. Anz., ix, p. 615; x, p. 102.

Footnote 20:

Zool. Anz., ix, p. 711.

Footnote 21:

Ent. Amer., v, p. 110, Pl. II, Fig. 7.

Footnote 22:

In his account of his studies on the locomotion of insects, De Moor states that he obtained the track of each of the feet in different colors by coating them with different pigments; the insect, as it moved, left its track on a strip of paper. (Archives de Biologie, Liège, 1890.)

Footnote 23:

Carlet and also De Moor (1890) confirm Graber’s statement that in beetles the first and last appendages on the same side are in contact with the ground, while the middle one is raised. On the other side of the body the middle appendage is on the ground and the first and last one raised.

Footnote 24:

Trans. Amer. Ent. Soc. xx, p. 168.

Footnote 25:

Proc. Ent. Soc. London. Feb. 19, 1896. Heymons also shows that the germs of the elytra of the larva of _Tenebrio molitor_ in the prepupal stage are like those of other insects. (Sitzungs-Ber. Gesell. natur f. Freunde zu Berlin, 1896, pp. 142–144.)

Footnote 26:

Zur Entwickelungsgeschichte und Reproductionsfähigkeit der Orthopteren. Von Vitus Graber. Sitzungsberichte d. math.-naturw. Classe der Akad. d. Wissensch., Wien. Bd. lv, Abth. i, 1867; also Die Insekten.

Footnote 27:

On the transformations of the common house fly, by A. S. Packard, Jr. Proceedings Boston Society of Natural History, vol. xvi, 1874. See Pl. 3, Figs. 12_a_, 12_b_.

Footnote 28:

See our Guide to the Study of Insects, p. 66, Figs. 65, 66.

Footnote 29:

Our Common Insects, 1873, p. 171.

Footnote 30:

Compare the observations of Palmén, Gerstäcker, Vayssière, and others.

Footnote 31:

Beiträge zur Kenntniss der Termiten. Jenaische Zeitschrift für Naturwissenchaft, Bd. ix, Heft 2, p. 253, 1875. Compare, however, Palmén’s Zur Morphologie des Tracheensystems, Helsingfors, 1877, wherein he opposes Müller’s view and adopts Gegenbaur’s. See p. 8, footnote.

Footnote 32:

Pancritius, who also adopted Müller’s views, lays much stress on the fact that in larvæ of some orders the tracheæ do not enter the rudimentary wings until the end of larval life, and hence the wings have not originated from tracheal gills, but were originally “perhaps only protective covers for the body.”

Footnote 33:

Reproduced from the author’s remarks in Third Report U. S. Ent. Commission, pp. 268–271, 1883.

Footnote 34:

Von Lendenfeld, however, points out the fact that Straus-Durckheim proved that the wings of beetles are moved by a complicated system of numerous muscles. “In the Lepidoptera I have never found less than six muscles to each wing, as also in the Hymenoptera and Diptera.” “The motions of the wings of Libellulidæ are the combined working of numerous muscles and cords, and of a great number of chitinous pieces connected by joints.”

Footnote 35:

Heymons, however, denies that the so-called cerci in Odonata are such, and claims that they are the homologues of the “caudal processes” (superior terminal appendages of Calvert), because they arise from the tenth abdominal segment.

Footnote 36:

Amer. Nat., iv, December, 1870.

Footnote 37:

Handbuch der Zoologie, p. 17, 1863, Fig. 162.

Footnote 38:

In my account of the anatomy of _Melanoplus spretus_, 1st Report U. S. Entomological Commission, p. 259, I have called these the infra-anal flaps or _uro-patagia_.

Footnote 39:

It has been suggested to us by A. A. Packard that the power possessed by insects of transporting loads much heavier than themselves is easily accounted for, when we consider that the muscles of the legs of an insect the size of a house-fly (¼ inch long), and supporting a load 399 times its own weight, would be subjected to the same stress (per square inch of cross-section) as they would be in a fly 100 inches long of precisely similar shape, that carried only its own weight; from the mechanical law that, while the weight of similar bodies varies as the cube of the corresponding dimensions, the area of cross-section of any part (such as a section of the muscles of the leg) varies only as the square of the corresponding dimensions. In short, the muscles of a fly carrying this great proportional weight undergo no greater tension than would be exerted by a colossal insect in walking.

Footnote 40:

This has been shown to be the case by Michels, who states that each commissure is formed of three parallel bundles of elementary nerve-fibres, which pass continuously from one end of the ventral or nervous cord to the other. “The commissures take their origin neither out of a central punctsubstanz (or marksubstanz), nor from the peripheral ganglion-cells of the several ganglia, but are mere continuations of the longitudinal fibres which decrease posteriorly in thickness, and extend anteriorly through the commissures, forming the œsophageal ring, to the brain.”

Footnote 41:

The following extract from Newton’s paper shows, however, that the infra or subœsophageal ganglion, according to Faivre, has the power of coördinating the movements of the body; still, it seems to us that the brain is primarily concerned in the exercise of this power, as the nerves from the subœsophageal ganglion supply only the mouth-parts. “The physiological experiments of Faivre in 1857 (Ann. des Sci. Nat. tom. viii, p. 245), upon the brain of Dyticus in relation to locomotion, are of very considerable interest, showing, as they appear to do, that the power of coördinating the movements of the body is lodged in the infraœsophageal ganglion. And such being the case, both the upper and lower pairs of ganglia ought to be regarded as forming parts of the insect’s brain.”—Quart. Jour. Micr. Sc., 1879, p. 342.

Footnote 42:

The arthropod protocerebrum probably represents the annelid brain (supraœsophageal ganglion). The antennal segment (deutocerebrum), with the premandibular (intercalary) segment (tritocerebrum) originally postoral, have, as Lankester suggests, in the Arthropoda moved forward to join the primitive brain. See Wheeler, Journ. Morphology, Boston, viii, p. 112.

Footnote 43:

Viallanes’ assertion that the instincts of the horse-flies and dragon-flies are “lower” than those of the locusts, may, it seems to us, well be questioned.

Footnote 44:

A. S. Packard, Experiments on the vitality of insects, Psyche, ii, 17, 1877.

Footnote 45:

Waterhouse, Trans. Ent. Soc., London, 1889, p. xxiv.

Footnote 46:

J. Müller, Physiology of the Senses. Trans. by Baly, copied from Lubbock, p. 176.

Footnote 47:

Hauser here uses the word _taster_, but this means palpus or feeler. It is probably a _lapsus pennæ_ for teeth (Kegeln).

Footnote 48:

In 1870 I observed these sense-pits in the antennæ and also in the cercopoda of the cockroach (_Periplaneta americana_). I counted about 90 pits on each cercus. They are much larger and much more numerous than similar pits in the antennæ of the same insect. I compared them to similar pits in the antennæ of the carrion-beetles, and argued that they were organs rather of the smelling than hearing. (Amer. Nat., iv., Dec. 1870.) Organs of smell in the flies (Chrysopila) and in the palpi, both labial and maxillary, of Perla were described in the same journal (Fig. 270). Compare Vom Rath’s account of the organs in the cercopods of Acheta (Fig. 271); also the singular organ discovered by him on the end of the palpus of butterflies, in which a number of hair-like rods (_sh_) are seated on branches of a common nerve (_n_, Fig. 272).

Footnote 49:

Forel, however (_Recueil Zoologique Suisse_, 1887), denies that these tympanic organs are necessarily ears, and thinks that all insects are deaf, with no special organs of hearing, but that sounds are heard by their tactile organs, just as deaf-mutes perceive at a distance the rumbling of a carriage. But he appears to overlook the fact that many Crustacea, and all shrimps and crabs, as well as many molluscs, have organs of hearing. The German anatomist Will believes that insects hear only the stridulation of their own species. Lubbock thinks that bees and ants are not deaf, but hear sounds so shrill as to be beyond our hearing.

Footnote 50:

Weismann, Die nachembryonale Entwicklung der Musciden. Zeitschr. für wissen. Zoologie, xiv, p. 196, 1864.

Footnote 51:

Plateau (1877) states that the digestive fluid of insects, as well as of Arachnids, Crustaceans, and Myriopods, has no analogy with the gastric juice of vertebrates; it rather resembles the pancreatic sugar of the higher animals. The acidity quite often observed is only very accessory in character, and not the sign of a physiological property. “Farther, I have found it in insects; Hoppe-Seyler has demonstrated in the Crustacea, and I have proved in the spiders, that the ferment causing the digestion of albuminoids is evidently quite different from the gastric pepsine of vertebrates; the addition of very feeble quantities of chlorhydric acid, far from promoting its action, retards or completely arrests it.” (Bull. Acad. roy. Belgique, 1877, p. 27.)

Footnote 52:

The word _grès_ we translate as the layer of gum. Not sure of the English equivalent for _grès_, I applied to Dr. L. O. Howard, U. S. Entomologist, who kindly answers as follows: “I have consulted Mr. Philip Walker, a silk expert, who writes me the following paragraph: ‘_Grès_, as I understand it, is the gum of the silk fibre, hence the French name for raw silk, _grèye_, which is in distinction to the silk that has been boiled out in soap after twisting, or throwing, as it is called. As I understand it, the silk fibre is composed of the _grès_ and fibroin. The former is soluble in alkali, like soap water, and the latter is not.’” While Blanc considers the _grès_ as the product of a special secretion of the wall of the reservoir, Gilson regards its production as simultaneous with that of the silk or of the fibroin (_l.c._ 1893, p. 74).

Footnote 53:

On cytological differences in homologous organs. Report 63d meeting of British Assoc. Adv. Sc. for 1893. 1894. p. 913.

Footnote 54:

See also Giard, Bull. Soc. Ent. France, p. viii, 1894.

Footnote 55:

“The contents of the Malpighian tubules may be examined by crushing the part in a drop of dilute acetic acid, or in dilute sulphuric acid (10 per cent). In the first case a cover-slip is placed on the fluid, and the crystals, which consist of oblique rhombohedrons or derived forms, are usually at once apparent. If sulphuric acid is used, the fluid must be allowed to evaporate. In this case they are much more elongated, and usually clustered. The murexide reaction does not give satisfactory indications with the tubules of the cockroach.” (Miall and Denny, The cockroach, p. 129, footnote.)

Footnote 56:

“There is a curious analogy between the excretory organs of these insects and the mesonephros of some vertebrates, where a second, third, etc., generation of tubules is added to the primitive metameric series. When the embryonic number of Malpighian vessels persists in insects, the demand for greater excreting surface is supplied by a lengthening of the individual vessels.”

Footnote 57:

For the mode of adhesion of Cynips eggs, see Adler in Deutsche Ent. Zeits. 1877, p. 320.

Footnote 58:

Mercaptan is a mercury, belonging to a class of compounds analogous to alcohol, having an offensive garlic odor. Methyl mercaptan is a highly offensive and volatile liquid.

Footnote 59:

Embryonic or temporary glands, the “pleuropodia” of Wheeler, viz. the modified first pair of abdominal legs, occur in Œcanthus, Gryllotalpa, Xiphidium, Stenobothrus, Mantis (occasionally a pair on the second abdominal segment, Graber); Blatta, Periplaneta, Cicada, Zaitha, Hydrophilus, Acilius, Melolontha, Meloë, Sialis, Neophylax. (See Wheeler, Appendages of the First Abdominal Segment, etc., 1890.)

Footnote 60:

These midges owe their phosphorescence to bacteria in their bodies during disease.

Footnote 61:

Untersuchungen zur Anatomie und Histologie der Tiere, 1884, p. 72.

Footnote 62:

Zelle und Gewebe, 1885, p. 43. (See also our p. 217.)

Footnote 63:

Studien über die Lampyriden, Zeits. für wiss. Zool., xxxvii, 1882. Both Wielowiejski and M. Wistinghausen have completely disproved the view of Schultze, that the tracheæ end in star-like cells, where respiration takes place, as the “star-like cells” are simply net-like expansions of the peritoneal membrane of the tracheæ.

Footnote 64:

The following summary compiled from Krancher, is translated, with some minor changes, from Kolbe’s work.

Footnote 65:

Miall and Denny state that in the cockroach the abdominal spiracles are permanently open, owing to the absence of a valve, but communication with the tracheal trunk may be cut off at pleasure by an internal occluding apparatus.

Footnote 66:

Zur Entwicklungsgeschichte der Biene, Zeitschr. wissens. Zoologie, xx, p. 519, 1870.

Footnote 67:

Die Entwicklung der Dipteren im Ei, Zeitschr. wissens. Zoologie, xiii, 1863.

Footnote 68:

Amer. Naturalist, May, 1886, p. 438.

Footnote 69:

Zeitschr. wissens. Zoologie, xl, 1884, Taf. xix, Fig. 8, _T_.

Footnote 70:

_Science_, 1893, pp. 44–46.

Footnote 71:

Art. Thorax, Todd’s Cycl. of Anat. and Phys.

Footnote 72:

The mesothoracic stigmata are open in Carabus, Potamophilus, Elmis, Macronychus, Buprestis, Elater, Lampyris, Lycus, Triphyllus, Eucinetus, Dascillus, Psephenus, Ergates, Micralymna, and probably many others. The metathoracic stigmata are open in Lycus and Elmis.

Footnote 73:

In the Hymenoptera the two pairs on the meso- and metathoracic segments are open in the Aculeata, also in the Siricidæ, among which sometimes that on the third segment is closed. In Pimpla and Microgaster (fully grown larvæ) only the mesothoracic stigmata are open.

Palmén adds that most dipterous larvæ are amphipneustic; Cecidomyia, the Mycetophilidæ, Bibionidæ, and Stratiomys are typically peripneustic. (p. 92.)

Moreover, a single insect, as Sialis, may be apneustic as a larva, peripneustic as a pupa, and holopneustic in the imago stage.

Footnote 74:

Mr. J. W. Folsom, who has made the accompanying sketch of the nymph of _Euphæa splendens_ in the Cambridge Museum, finds only seven pairs of gills, there being no traces of them on segments 1, 9, and 10. A stout trachea, he writes us, enters the base of each gill, and subdivides into several long branches, which course along the periphery. Hagen in his original account said there were eight pairs on segments 1–8 respectively.

Footnote 75:

Harris, Correspondence, p. 226, Pl. III., Fig. 7.

Footnote 76:

Nusbaum’s view has been questioned by Heymons, who, from his studies on the embryology of the cockroach (Periplaneta and Phyllodromia), Forficula, and Gryllus, concludes that the ectodermal ends of the sexual outlets owe their origin to an unpaired median hypodermal invagination, and that it is quite doubtful whether the ectodermal portions of the sexual passages of insects were ever paired (p. 104). On the other hand he appears, even throwing out the case of Ephemera, to have overlooked Nassonow’s discovery of paired outlets in the young of Lepisma.

Footnote 77:

Acta Acad. German., xxxiii, 1867, No. 2, p. 81. Quoted by Dr. Sharp, Insecta, p. 142.

Footnote 78:

Journ. Morph., iii, Boston, pp. 299, 300.

Footnote 79:

Proc. Boston Soc. Nat. Hist., xi, pp. 88, 89.

Footnote 80:

In the following general account of the embryology of insects, I have closely followed the admirable arrangement and description of Korschelt and Heider, in their Lehrbuch der vergleichenden Entwicklungsgeschichte der wirbellosen Thiere, pp. 764–846, often translating their text literally, though not omitting to state the results of other writers.

Footnote 81:

Korschelt and Heider state that no cellular embryonal membranes are present in Synaptera, Uljanin finding none in the Podurids. In the embryo of _Isotoma walkerii_ we, however, observed a membrane which we compared to the larval skin of many Crustacea, and both Sommer and Lemoine have detected in eggs of the same group a cuticular larval skin which is provided with spines for rupturing the chorion. The amnion is also wanting in Proctotrupids (Ayers), and is rudimental in Muscidæ (Kowalevsky, Graber), in viviparous Cecidomyidæ, according to Metschnikoff, who also states that in certain ants of Madeira the envelopes are represented only by a small mass of cells in the dorsal region.

Footnote 82:

In Diptera the stomodæum may be dorsal, Dr. Pratt tells us.

Footnote 83:

Will (Aphis) and also Cholodkowsky’s statement (Blatta), as well as Balfour and Schimkewitch’s statements that the brain is at first disconnected from the ventral cord, are apparently erroneous.

Footnote 84:

The description perhaps applies not only to the cockroaches, but, as seen from the similar but fragmentary notices of Heider and of Wheeler on the Coleoptera, may be common to insects in general.

Footnote 85:

Report on the Rocky Mountain locust, etc. Ninth Annual Report U. S. Geol. and Geogr. Survey of the Territories for 1875, pp. 633, 634.

Footnote 86:

Orthoptera Europæa, 1853, p. 37.

Footnote 87:

In his Für Darwin (1863), Fritz Müller gives his reasons for the opinion that the so-called “complete metamorphosis” of insects was not inherited from the primitive ancestor of all insects, but acquired at a later period.

Footnote 88:

For further details see the 1st Report of the U. S. Entomological Commission, 1878, pp. 279–281.

Footnote 89:

See Köppen ueber die Heuschrecken in Südrussland, 1862, pp. 22, 23.

Footnote 90:

In Samouelle’s The Entomologist’s Useful Compendium, 1819. See Westwood’s Class. Insects, i, p. 2; Leach’s Ametabolia comprised the Thysanura (Synaptera) and the lice.

Footnote 91:

From the Greek μανός, scanty; μεταβολή, change.

Footnote 92:

Greek, ἤρεμα, quiet; μεταβολή, change.

Footnote 93:

At the same date (March, 1869) we independently suggested that the insects had originated from some form like the hexapodous young of Pauropus and Podura. In November, 1870, we suggested that the Thysanura and the hexapodous Leptus may have descended from some Peripatus-like worm. Afterwards (1871) we proposed for the ancestral form the term _leptiform_, which was later abandoned for Brauer’s term _Campodea-form_.

Footnote 94:

Amer. Naturalist, i, p. 85, 1867.

Footnote 95:

First Rep. U. S. Ent. Commission, p. 281–283.

Footnote 96:

Trans. Ent. Soc. London, iii, p. xv. See also Ashton, R. J., Trans. Ent. Soc. London, iii, 1841–43, pp. 157–159.

Footnote 97:

Proc. Bost. Soc. Nat. Hist., x, 1866, p. 283.

Footnote 98:

See Max Braun’s article entitled Ueber die histologischen Vorgange bei der Hautung von _Astacus fluviatilis_, with a full bibliography, in Semper’s Arbeiten aus dem Zool. zoot. Institut in Würzburg, ii, pp. 121–166. Also Semper’s Animal Life, p. 20. Trouvelot also discovered the moulting fluid. (Amer. Nat., i, p. 37.)

Footnote 99:

American Naturalist, xvii, May, 1883, pp. 547, 548.

Footnote 100:

Le Pelletier. A. M. L., Bulletin de la Société Philomathique, Paris, April, 1813.

Footnote 101:

Heineken, Carl. Observations on the reproduction of the members in spiders and insects. (Zool. Journ., 1829, vi, pp. 422–432.)

Footnote 102:

Bees and Bee-keeping, pp. 21, 22.

Footnote 103:

Butterflies, their structure, changes, and life-histories. New York, 1881, pp. 37–42. Butterflies of the Eastern United States and Canada, 1888, 1889. Also, Frail children of the air, 1895, pp. 232, 233 _a_. Dr. Chapman, however, finds that this piece in micropupæ has no connection whatever with the head or eye, but belongs rather with the prothoracic segment. (Trans. Ent. Soc. London, 1893, p. 102.) We have been able to confirm his statements, but still this piece is peculiar to the pupal state.

Footnote 104:

Rep. Ent. U. S. Dept. Agr., 1879, pp. 228, 229, Pl. IV, Fig. 4.

Footnote 105:

Monograph of bombycine moths, Pt. I, 1897. Figs. 24, 28, 29, 33, 34, 40, 77.

Footnote 106:

Amer. Naturalist, xii, pp. 379–383.

Footnote 107:

_Hybocampa milhauseni_, Dr. Chapman tells me, has a pupal spine (imperfectly present in Cerura) with which it cuts out a lid of the cocoon.

Footnote 108:

Riley’s Report for 1892, p. 203.

Footnote 109:

Philosophy of the pupation of butterflies, and particularly of Nymphalidæ, by Charles V. Riley. (Proc. Amer. Assoc. Adv. Science, xxviii, Saratoga Meeting, August, 1880, pp. 455–463.)

Footnote 110:

The homology of the suranal plate of the larva with the cremaster of the pupa, established by Riley in 1880, is also affirmed by Jackson (1888) and by Poulton, and for some years we have been satisfied that this is the correct view; Professor Hatchett-Jackson discovered it, he states, in 1876.

Footnote 111:

In his remarkable studies on the morphology of the Lepidoptera, Professor W. Hatchett-Jackson states his belief that Riley’s homology of the sustentors with the soles or plantæ of the anal prolegs, and the sustentor ridges with their limbs, is wrong, and that the eminences on either side the anal furrow, or the “anal prominences,” as they are termed by Riley, represent the prolegs, and that the sustentor ridges and sustentors are probably peculiar developments of the body of the 10th somite, found only in some Lepidoptera. From our examination of pupa of different families of moths, we are satisfied that Jackson’s view is the correct one. We have not found the sustentors and their ridges in the pupæ of the more generalized moths, but the vestiges of the anal legs are almost invariably present, their absence in the pupa of Nola and Harrisina being noteworthy.

Footnote 112:

We copy from Kirby and Spence their abstract of Herold’s conclusions: “The successive skins of the caterpillar, the pupa-case, the future butterfly, and its parts or organs, except those of sex, which he discovered in the newly excluded larva, do not preëxist as germs, but are formed successively from the _rete mucosum_, which itself is formed anew upon every change of skin, from what he denominates the _blood_, or the chyle after it has passed through the pores of the intestinal canal into the general cavity of the body, where, being oxygenated by the air-vessels, it performs the nutritive functions of blood. He attributes these formations to a _vis formatrix_ (bildende Kraft).

“The caul or epiploon (_fett-masse_), the _corps graisseux_ of Réaumur, etc., which he supposes to be formed from the superfluous blood, he allows, with most physiologists, to be stored up in the larva, that in the pupa state it may serve for the development of the imago. But he differs from them in asserting that in this state it is destined to two distinct purposes: first, for the production of the muscles of the butterfly, which he affirms are generated from it in the shape of slender bundles of fibres; and, secondly, for the development and nutrition of the organs formed in the larva, to effect which, he says, it is dissolved again into the mass of blood, and being oxygenated by the air-vessels, becomes fit for nutrition, whence the epiploon appears to be a kind of concrete chyle.” (Entwickelungsgeschichte der Schmetterlinge, pp. 12–27.) It seems that Herold was right in deriving the pupa and imago from the hypodermis (his _rete mucosum_), but wrong in denying that the germs did not preëxist in the young caterpillar, and wrong in supposing that the latter originated from the blood, also in supposing that the muscles owe their origin to the fat-body. Swammerdam, and also Kirby and Spence, were correct in supposing that the imago arose from “germs” in the larva, though wrong in adopting the “emboîtement” theory.

Footnote 113:

In the regions where the imaginal buds are not present (dorsal aspect of the prothorax, and abdomen), the epithelium (hypodermis) may proliferate independently of these buds.

Footnote 114:

We shall translate portions and, when the text allows, make an abstract of parts of Gonin’s clear and excellent account, often using his own words.

Footnote 115:

C. Herbert Hurst, The Pupal Stages of Culex.

Footnote 116:

Lowne on the Blow-fly, new edit., pp. 2, 41, Fig. 7.

Footnote 117:

Miall, Natural History of Aquatic Insects, pp. 136–138. Also Trans. Linn. Soc. London, V, Sept., 1892.

Footnote 118:

This account is translated from Korschelt and Heider, with some omissions and slight changes.

Footnote 119:

Westwood in his excellent account of this group remarks: “Hence, as well as from the account given by Jurine, it is evident that the pupa of the Stylops is enclosed in a distinct skin, and is also in that state enveloped by the skin of the larva, contrary to the suggestion of Mr. Kelly.” (Class. Insects, II. 297.) This is all we know about the supernumerary larval stages.

Footnote 120:

Some facts towards a life history of _Rhipiphorus paradoxus_. Annals and Magazine of Natural History for October, 1870.

TRANSCRIBER’S NOTES

● P. 316, changed “abdominal cells” to “absorbent cells”. ● Silently corrected typographical errors and variations in spelling. ● Archaic, non-standard, and uncertain spellings retained as printed. ● Enclosed italics font in _underscores_. ● Enclosed bold font in =equals=. ● Superscripts are denoted by a caret before a single superscript character or a series of superscripted characters enclosed in curly braces, e.g. M^r. or M^{ister}. ● Subscripts are denoted by an underscore before a series of subscripted characters enclosed in curly braces, e.g. H_{2}O.