vi. Though the pupæ of the second great division are usually not

locomotive, yet I must not omit some notice of their _motions_. As the legs of insects in this state are folded within a common or partial integument, of course none of the pupæ now under consideration, with the exception of those of the _Trichoptera_ order, can walk: _coarctate_ ones are even incapable of the slightest motion, and exhibit no symptom whatever of animation. Some of those that are termed _incomplete_, however, and most chrysalises, have the power of communicating to their bodies a slight movement, extending more or less in different species, which is effected by the abdominal segments solely. The latter, during the first twelve hours of being pupæ, when their skin is soft, frequently turn themselves, that the side on which they lie may not be flattened; afterwards by far the majority merely wriggle or twist their abdomen when touched, or in any way incommoded or disturbed. We learn from De Geer, that the pupa of the ghost-moth (_Hepialus Humuli_), the cocoon of which is more than twice the length of the chrysalis, moves in it from one end to the other[645]. Bonnet observed one of a moth (perhaps _Lasiocampa Quercus_), which alternately fixed itself at the top and bottom of its spacious and obliquely-fixed cocoon; descending slowly, but ascending as quickly, and almost in the same manner, as a chimney-sweeper in a chimney[646]. The pupa of the weevil of the water-hemlock (_Lixus paraplecticus_) will move from one end of the interior of a branch to another by means of its _adminicula_, aided by the motion of its abdominal segments[647]. But the most locomotive of pupæ of the second division are those of gnats, and many Tipulidans, which pass this state in the water. These will move from the bottom to the surface, and back again, with great facility and velocity. I have before mentioned several other motions of pupæ[648], which I shall not repeat here, by which they extricate themselves from their several places of intermediate repose, before they leave the puparium: if the imago were to be disclosed in the interior of a tree, or in the earth, its wings would be materially injured in forcing its way out. The object of several of the above motions may be to alarm insects that might attack these defenceless beings. The twirling motion in particular, formerly noticed[649], in some species, by causing a rustling against the sides of the cocoon, makes a considerable noise--so singular in that of a red underwing-moth (_Noctua pacta_), that Rösel tells us, (who by the by was more timid than becomes a philosopher,) that the first time he heard it, he had nearly thrown away the box that contained it, in his fright[650].

vii. We are next to consider _The extrication of the perfect insect from the puparium, or pupa-case, and from the cocoon_. The period when the pupa has attained maturity, and the inclosed insect is ready to burst the walls of its prison, may be often ascertained. Just at this time the colour frequently undergoes an alteration, the golden or silver tint of the gilded chrysalises vanishes; and those which are transparent, usually permit the form and colours of the insect within and the motions of their limbs to be distinctly seen through them. In the _Libellulina_ the eyes become more brilliant[651]. The mature pupæ of the moth lately mentioned (_Eriogaster lanestris_) have a particular swell of the abdominal segments, not apparent in those that are to continue till another season, or longer[652]. Those of the case-worms (_Trichoptera_) push off the grates from the cases which they have hitherto inhabited, and swim about[653]. Other signs and motions doubtless predict the approach of this great change in other species, which have not been recorded.

The mode in which insects make their way out of the _puparium_ differs in different orders. In _obtected_ pupæ, the struggles of the included butterfly or moth first effect a longitudinal slit down the middle of the thorax, where there is usually a suture for the purpose. The slit rapidly extends along the head, and down the parts which compose the breast, and the insect gradually withdraws itself from its case. It is not, however, from the outer skin merely that it has to disengage itself, but also from a series of inner membranous cases, which separately inclose the antennæ, proboscis, feet, &c., as a glove does the fingers; and similar cases inclose the parts of the perfect insect in pupæ of all the other orders. This is sometimes a work of difficulty, but ordinarily it is effected with ease.

_Incomplete_ and _semicomplete_ pupæ undergo nearly the same process, save that in them the body is not swathed up in a common case; and therefore they have only to liberate themselves from the partial cases that envelop the several parts of their body.

In _coarctate_ pupæ, as those of _Muscidæ_, _Syrphidæ_, _Œstridæ_, &c., the process is different. Their outer-case is ordinarily more rigid and destitute of the sutures, which in the former tribes so easily yield to a slight effort. Yet in these, at the anterior end under which the head of the fly lies, and from which it always issues, there is commonly a sort of lid, joined by a very indistinct suture to the rest, which can be pushed off, leaving a sufficient opening for the egress of the insect. In the pupæ of many of this tribe this lid is composed of two semicircular pieces, which can be separately removed. Many species seem to be able to force off the lid of their puparium, by merely pushing against it with their heads: but the common flesh-fly and many other _Muscidæ_, which are perhaps too feeble to effect this, or whose puparia are stronger than ordinary, are furnished with a very remarkable apparatus for this express and apparently sole purpose. They are gifted with the power of introducing _air_ under the middle part of the head, to which the antennæ are fixed, and of inflating that part into a sort of membranous vesicle as big as the head itself; by the action of which against the end of the pupa-case, the lid is soon forced off. So powerful is this singular lever, that it is even sufficient to rupture the fibrous galls in which the pupæ of the gay-winged _Tephritis Cardui_[654] are inclosed. That it is designed by Creative Wisdom to answer this sole purpose seems proved, from its disappearing soon after the disclosure of the fly, whose head shortly becomes all alike hard. Reaumur suspects that it may also be intended to promote the circulation of the insect's fluids; but to me his reasons appear not conclusive[655]. In one instance a mode still more unexpected obtains. The illustrious naturalist just named found that the fly which proceeded from one of the rat-tailed grubs (_Elophilus_ Latr.) had actually the power of completely reversing its situation in its narrow case; and that it then employed its _tail_ in pushing off the lid, which other species remove by means of their _heads_[656].

The extrication of insects whose pupæ are above ground, like those of butterflies, many beetles, flies, &c., is comparatively a simple operation. But what, you will ask, becomes of those species whose pupæ are concealed deep in the earth, or in the heart of the trees on which their larvæ have fed? Of this you shall be informed.--_Coleopterous_ insects disclosed from pupæ thus circumstanced, wait until their organs have acquired strength, and their elytra are sufficiently hardened to protect their filmy wings from damage in forcing their way through the earth or wood which covers them. Thus _Oryctes nasicornis_, a rhinoceros beetle common on the Continent, is a full _month_ before it reaches the surface of the earth, after quitting its puparium. But it is evident that no delay would enable _lepidopterous_ or _dipterous_ insects, which are without elytra, to make their way out of such situations, without irreparable injury to their delicate wings. Many of these, therefore, while still within the hard case of the pupa, have the precaution, a few days previously to their exclusion, to force themselves up to the surface of the earth, or, when they reside in the interior of trees, to the entrance of their hole. This is effected by a successive wriggling of the abdominal segments, which in several species, of the _Coleoptera_, _Lepidoptera_, and _Diptera_ orders, for this purpose, as has been more than once observed[657], are furnished with sharp points (_adminicula_), admitting a progressive, but not a retrograde motion. The puparia of the great goat-moth (_Cossus ligniperda_) may be often seen projecting from orifices in willow-trees; and those of the common crane-fly (_Tipula oleracea_) from the surface of the earth, to which they have thus made their way from a depth of several inches.

In all the preceding instances the exclusion of the perfect insect is complete, as soon as it has withdrawn itself from the puparium. But to a very large number, even after this is effected, the arduous task still remains of piercing the cocoons of leaves, of thick silk, of tough gum, or even of wood, in which the pupæ are incased. We can readily conceive how the strong jaws of _coleopterous_ and _hymenopterous_ species may be employed to release them from their confinement. But what instruments can be used for this purpose by _moths_ in a state of great debility, whose mouth has nothing like jaws--merely a soft membranous proboscis? How shall the silkworm-moth (_B. Mori_) force its way through the close texture of a silken ball, through which the finger could not be easily pushed? Or the puss-moth (_Cerura Vinula_) pierce the walls of its house of glue and wood, which scarcely yield to the knife? You will not doubt that these difficulties have been foreseen by INFINITE WISDOM, and provided against by INFINITE POWER. The egress of moths from their cocoons is secured in two ways;--either by some peculiarity in the first construction of the cocoon by the caterpillar, or by some process which the pupa or perfect insect is instructed to perform. As examples of each, several curious instances may be cited.

The larva of the moth which about 1760 made such havoc in the province of Angoumois in France, becomes a pupa in the interior of the grain of wheat which it has excavated; but the opening by which it first entered is not bigger than a pin's point, and is quite insufficient for the egress of the moth. How, then, is the latter to force its way through the tough skin which surrounds it? The larva, previously to assuming the pupa state, gnaws out a little circular piece at that end of the grain where the head of the future moth would lie, taking care not to detach it entirely. At this little door, which is sufficient to protect it from intruders, the moth has but to push, when it falls down, and leaves a free passage for its exit. A contrivance almost similar is adopted by a caterpillar which feeds in the interior of the heads of a species of teazel (_Dipsacus_ L.), for a minute and interesting history of which we are indebted to Bonnet. This caterpillar previously to its metamorphosis actually cuts a circular opening in the head, sufficiently large for the egress of the future moth; but to secure this sally-port during its long sleep, it artfully closes it with fibres of the teazel, closely but not strongly glued together[658]. Another small caterpillar described by the same author, resides in the leaf of an ash curiously rolled up into a cone, and then assumes the pupa, which is inclosed in a silken cocoon, ingeniously suspended by two threads like a hammock in the middle of its habitation, and of so slight a texture that it presents no obstacle to the extrication of the moth. It is the closely-joined sides of its leafy dwelling that form a barrier, which, were it not for the precaution of the larva, would be impenetrable to so small and weak an animal. The little provident creature, before its change to a pupa, gnaws in the leaf a round opening, taking care not to cut through the exterior epidermis. This door is to serve the moth for its exit, like that formed by the wheat-caterpillar. But in proportion to its bulk its verdant apartment is of considerable size. How then shall the moth know the exact place where its outlet has been traced? How, without a clue, shall it discover in its dark abode the precise circle which requires only a push to throw it down? Even this is foreseen and provided against. Out of twenty positions in which its hammock might have been slung, the caterpillar has been directed so to place it, that the silken cord that suspends the head is fastened close to the side of the door which it has previously constructed; and the moth, guided by this _filum ariadneum_, at once makes its way out of an apartment which, but for this contrivance, might have been to it a labyrinth as inextricable as that of Minos[659].

The mode in which other caterpillars provide for their extrication, when become moths, from their silken cocoons, is not less ingenious. Those of _Eriogaster lanestris_ (of which I have lately said so much,) and others, form oblong cocoons, which, viewed externally, you would at the first glance assert were of one solid piece: but on examining them more narrowly, you perceive one end of them to be a distinct lid, of a size large enough to permit the moth to issue out; and that it is kept in its place by a few slight threads, easily broken by pressure from within[660]. A few pages back[661] I mentioned a cocoon formed by the larva of _Tortrix prasinana_, of the shape of a boat reversed, composed of two inclined walls fastened together at the top and ends. In constructing this cocoon, it firmly glues to each other the top and one end, so as to form an impermeable suture; but the other end, at which the moth is to issue, though externally it seems as strong as the rest, is merely drawn close by a slender thread or two fastened on the inside, and easily broken from within. And, what is particularly singular in the construction of this ingenious habitation, the sides forming the end last mentioned, though originally requiring force to draw them into their required position, become so elastic as to close again when the moth has passed between them and made her escape; the cocoon preserving its usual shape, even when deprived of its inhabitant[662]. A similar cocoon is constructed by another leaf-rolling caterpillar, that of _Tortrix chlorana_[663]. Many similar proofs of contrivance in the construction of silken cocoons might be adduced, but I shall confine myself to one more only--I mean that furnished by the flask-shaped brown one of _Saturnia Pavonia_, and some other moths. If you examine one of these cocoons, which are common enough in some places on the pear-tree or the willow, you will perceive that it is generally of a solid tissue of layers of silk almost of the texture of parchment; but at the narrow end, or that which may be compared to the neck of the flask, that it is composed of a series of loosely-attached longitudinal threads, converging, like so many bristles, to a blunt point, in the middle of which is a circular opening[664]. It is through this opening that the moth escapes. The silk of its cocoon is of so strong a texture and so closely gummed, that had both ends been similarly closed, its egress would have been impracticable; it finds, however, no difficulty in forcing its way through the aperture of a sort of reversed funnel, formed of converging threads that readily yield to pressure from within. But an objection will here probably strike you. You will ask, Is not this facility of egress purchased at too dear a rate? Must not a chrysalis in an open cocoon be exposed to the attacks of those ichneumons of which you have said so much, and of numerous other enemies, which will find admittance through this vaunted door? Our caterpillar would seem to have foreseen your dilemma; at least, under heavenly guidance, she has guarded against the danger as effectually as if she had. If you cut open the cocoon longitudinally, you will see that within the exterior funnel-shaped end, at some distance she has framed a second funnel, composed of a similar circular series of stiff threads, which, proceeding from the sides of the cocoon, converge also to a point, and form a sort of cone exactly like the closed peristome of a moss; or, to use a more humble though not less apt illustration, like the wires of certain mousetraps[665]. In this dome not the slightest opening is left, and from its arched structure it is impenetrable to the most violent efforts of any marauders from without; whilst it yields to the slightest pressure from within, and allows the egress of the moth with the utmost facility. When she has passed through it, the elastic threads resume their former position, and the empty cocoon presents just the same appearance as one still inhabited. Rösel relates with amusing naïvété how this circumstance puzzled him the first time he witnessed it: he could scarcely help thinking that there was something supernatural in the appearance of one of these fine moths in a box in which he had put a cocoon of this kind, but in which he could not discover the slightest appearance of any insect having escaped from it, until he slit it longitudinally[666]. But from an observation of Meinecken, it appears that these converging threads serve a double purpose; being necessary to compress the abdomen of the moth as it emerges from the cocoon, which forces the fluids to enter the nervures of the wings, and give them their proper expansion. For he found, that when the pupa is taken out of the cocoon, the moth is disclosed at the proper time, but remains always crippled in its wings; which never expand properly, unless the abdomen be compressed with the finger and thumb, so as to imitate the natural operation[667].

I am next to give you some account of the _second_ mode in which the release of the perfect insect from its cocoon is effected--that, namely, wherein its own exertions chiefly accomplish the work. I shall from a large number select only a few instances. The texture of the cocoon of the silkworm-moth is uniform in every part, and the layers of silk are equally thick at both ends. The moth makes its way out by cutting or breaking these threads at the end opposite to its head: an operation which, as it destroys the continuity of the silk, those who breed these insects are particularly careful to guard against, by exposing the cocoon to heat sufficient to destroy the included pupa. The question is--What instruments does the moth employ to effect this? And this we are not able to answer satisfactorily. Malpighi asserts that the animal first wets the silk with a liquid calculated to dissolve the gum that connects the threads, and then employs its lengthened head to push them aside and make an opening[668]. But, as Reaumur has observed, besides that so obtuse a part as the head of a moth is but ill fitted to act as a wedge, we find the threads not merely pushed to each side, but actually cut asunder. He therefore infers that the eyes, which are the only hard organs of the head, are the instruments by which the threads are divided--their numerous minute facets serving the purpose of a fine file[669]. It should be observed, however, that Mr. Swayne confirms Malpighi's assertion, that the silkworm does not cut, but merely pushes aside, the threads of its cocoon; and he informs us that he has proved the fact, by unwinding a pierced cocoon, the thread of which was entire[670]. Yet Reaumur's correctness cannot be suspected: and he affirms, that from observation there can scarcely be a doubt that most of the threads are broken[671]; which is further confirmed in an account of the breeding of silk-worms published in the _American Philosophical Transactions_: in which it is expressly stated, that cocoons out of which the fly has escaped, cannot be wound[672]. Analogy, it must be confessed, is against Reaumur's opinion; since other kinds of silkworms make their escape by means of a _fluid_. Thus we are informed by Dr. Roxburgh, that _Attacus Paphia_, when prepared to assume the imago, discharges from its mouth a large quantity of liquid, with which the upper end of the case is so perfectly softened, as to enable the moth to work its way out in a very short space of time,--an operation which, he says, is always performed in the night[673]. Perhaps the two opinions may be reconciled, by supposing the silkworm first to moisten and then break the threads of its cocoon. In those that are of a slighter texture, a mere push against the moistened end is probably sufficient: and hence we find in so many newly disclosed moths the hair in that part wet, and closely pressed down[674]. If it be apparently difficult for the silkworm-moth to effect an opening in its cocoon, how much harder must seem the task of the puss-moth (_Cerura Vinula_) to pierce the solid walls of its wood-thickened case? Here the eyes are clearly incompetent; nor could any ordinary fluid assist their operation, for the gum which unites the ligneous particles is indissoluble in _aqueous_ menstrua. You begin to tremble for the fate of the moth incarcerated in such an impervious dungeon--but without cause: what an _aqueous_ solvent cannot effect, an _acid_ is competent to: and with a bag of such acid our moth is furnished. The contents of this she pours out as soon as she has forced her head through the skin of the chrysalis, and upon the opposite end of the cocoon. The acid instantly acts upon the gum, loosens the cohesion of the grains of wood, and a very gentle effort suffices to push down what was a minute ago so strong a barrier. How admirable and effectual a provision! But there is yet another marvel connected with it. Ask a chemist, of what materials a vessel ought to be to contain so potent an acid: he will reply,--of glass. Yet our moth has no glass recipient: her bottle is a membranous bag; but of so wonderful a fabric as not to be acted upon by a menstruum which a gum, apparently of a resinous nature, is unable to resist! This fact can only be explained by the analogous insensibility of the stomach to the gastric juice, which in some animals can dissolve bone,--and it is equally worthy of admiration. In both cases, the vitality of the membranous or fleshy receptacle secures it from the action of the included fluid; but _how_--who shall explain?

Ordinarily it is the moth that breaks the cocoon; but in the goat-moth and many _Tortrices_ it is the pupa itself that performs the work, either wholly or partially. The pupa of the former is for this purpose furnished with sharp points upon the head, capable of effecting this object[675]. The locust-moth, another species of _Cossus_ (_C. Robiniæ_ Peck), whose history has been admirably detailed by Professor Peck, has a different process. "In the silk-moth," says he, "and all others which I have had opportunity to observe, the chrysalis bursts _in_ the cocoon, and the fluid which surrounded the new insect in it escaping at the same time, so weakens or dissolves the fibre and texture of the silk, that the moth is able to extricate itself, leaving the chrysalis behind it; but this is not the manner in the locust-moth. After remaining till all its parts are fully grown and it is ready to quit its prison, a certain quantity of exercise is necessary, to break the ligaments which attach the moth to the shell of the chrysalis, and to loosen the folds of the abdomen. In taking this exercise, it can only move the abdomen in various directions: as one side of the rings is moved forward, the hooks in the serrated lines above mentioned (the _adminicula_) take hold of the silk, and prevent their sliding back; the next flexure brings forward the opposite side of the rings, which are prevented by the points on that side from slipping back in the same manner, and the chrysalis is forced out of the slightly woven extremity of the cocoon, and through the silk-lined cavity, till it is protruded for about one-third of its length out of the opening in the bark, and into the air[676]."

An exception to the general rule--that the rupturing of the cocoon is the business of the inclosed insect itself--is met with amongst ants; the workers of which not only feed the young, but actually make an aperture in their cocoons, cutting the threads with their mandibles with admirable dexterity and patience, one by one, at the time they are ready to emerge, the precise period for which these indefatigable nurses are well aware of, that they may meet with no obstacle. Without this aid, the young ant would be unable to force its way through the strong and dense coating of silk that infolds it[677]. And a proceeding somewhat akin to this was observed by the Hon. Captain Percy, R.N., who himself related it to me. Being fond of the study of insects, he was in the habit of attending to their motions; and in the beginning of September 1821 noticed those of a number of female _Tipulæ_, probably _T. oleracea_ L., busily engaged in depositing their eggs amongst the roots of grass. While observing these proceedings, he at the same time saw one quitting its pupa-case, which had already by its own efforts got its head, thorax, and anterior legs out of it. It was then joined by two _male_ flies; which, with their anal forceps and posterior legs taking hold of the pupa-case, appeared with their mouths and anterior legs to push the little prisoner upwards, moving her backwards and forwards; and as they kept raising her, shifting their hold of the skin till she was entirely extricated, when they left her to recover her strength by herself. Probably the extreme length of the two pair of hind-legs of these animals may render such assistance necessary for their extrication.

There remains yet to be explained under this head the manner in which the perfect insect is excluded from certain aquatic pupæ; such as those of _Phryganeæ_, gnats, and one of those _Tipulidæ_ that resemble gnats. These pupæ (perhaps that they may be safe from the attack of birds) are destined to remain during the greater part of their existence in this state at the bottom of the water. But it is obvious that if the perfect insects were there to be disclosed, their wings would be wetted, and they would be drowned. It is the provision by which this result is obviated that now calls for your attention.

You have already been told that the larvæ of _Phryganeæ_ inclose themselves in cases of different materials, open at each end[678]. You have also learned, that in becoming pupæ, they secure each end of their cases with a grating of silk[679]. When that change has occurred, they remain motionless at the bottom of the water. Now how are these pupæ, encased in tubes of a greater specific gravity than the surrounding fluid, to make their way to the surface when the time has arrived for their becoming denizens of the air? This they accomplish in the following manner:--The pupa is furnished with two strong exterior moveable mandibuliform processes, and has the power of moving its four anterior legs and antennæ while in the pupa-case. With these temporary _jaws_ it makes an opening in one of the silken doors of its case, forces its way out at that end, and then by moving its legs, the cases of which in some species are ciliated for this very purpose, swims to the surface, where its skin splits, and discloses the included insect. That these jaws are given for the express and exclusive purpose of being thus applied, seems undeniable. The pupa eats nothing--they are therefore in every other point of view superfluous. They are given to it alone of all other similar pupæ, because unnecessary to all others; and they are cast off along with the rest of the puparium, the perfect insect having no vestige of jaws[680].

The _gnat_ has to undergo its change on the surface of the water--How is it to accomplish this without being wetted? In the pupa state they usually remain suspended with the posterior end of the body turned downwards: but when the period for its change is arrived, it stretches it out upon the surface, above which its thorax is elevated. Scarcely has it been a moment in this position, than, swelling out the interior and anterior parts of the thorax, it causes it to split between the two respiratory horns. Through this opening the anterior part of the gnat then emerges. As soon as the head and trunk are disengaged, it proceeds with its labour, and gets out more and more; elevating itself so as to appear in the puparium like a mast in a boat. As it proceeds, the mast is more and more elevated and lengthened, till it becomes nearly perpendicular--just as the mast of a boat is gradually raised from a nearly horizontal to a vertical position: at this period a very small portion of the abdomen remains in the puparium. Neither its legs nor wings are of any use in maintaining it in this position. The latter are too soft, and, as it were, folded; and the former are stretched out along the abdomen--the segments of this last part are the only agents. The observer who sees how the little boat gradually sinks, and how its margin approaches the water, forgets the mischievous insect it contains, which at another time he would crush without remorse, and becomes interested for its fate; especially should wind agitate the water. A very little is sufficient to drive about rapidly the little voyager, since it catches the wind in some degree as a sail. If it should be upset, it would be all over with it;--and numbers do thus perish. The gnat, after having fixed itself thus perpendicularly, draws first its two anterior legs out of their case, and moves them forward, and next the two intermediate ones; then inclining itself towards the water, it rests its legs upon it, for water is to them a soil sufficiently firm and solid to support them, although surcharged with the weight of the insect's body. As soon as it is thus upon the water, it is in safety; its wings unfold themselves and are dried, and it flies away. All this is the work of an instant[681].

The pupæ of _Chironomus plumosus_ proceed from those red worm-like larvæ so common throughout the summer in tubs of rain-water, &c., described by Reaumur[682]. They are not inclosed in cases, but are of a greater specific gravity than the water at the bottom of which they reside, until within a few hours of the exclusion of the fly. They have the power of swimming, however; and by moving the tail alternately backwards and forwards, can slowly raise themselves to the top of the water. But here occurs a difficulty. For the extrication of the imago it is necessary that they should remain quietly suspended at the surface; and moreover that the thorax, in which the opening for its exit is to be made, should be at least level with it: and this is precisely what takes place. If you watch one of these pupæ when it ascends from the bottom, you will see that as soon as it has reached the top it remains suspended there motionless; and that its thorax is the highest part of the body, and level with the surface. Now the question is, in what way this is accomplished? How can a pupa of greater specific gravity than water, remain suspended without motion at its surface? and how can its thorax, which is at its heaviest end, be kept uppermost?--By a most singular and beautiful contrivance, which I shall explain; the more particularly because it has escaped Reaumur, and, as far as I know, all other entomological observers. The middle of the back of the thorax has the property of repelling water--apparently from being covered with some oily secretion. Hence, as soon as the pupa has once forced this part of its body above the surface, the water is seen to retreat from it on all sides, leaving an oval space in the disk, which is quite dry. Now though the specific gravity of the pupa is greater than that of water, it is but so very slightly greater, that the mere attraction of the air to the dry part of the thorax, when once exposed to it, is sufficient to retain it at the surface; just as a small dry needle swims under similar circumstances. That this is a true solution of the phænomenon, I am convinced by the result of several experiments. If, when the pupa is suspended at the surface, a drop of water be let fall upon the dry portion of the thorax, it instantly sinks to the bottom,--the thorax, which belongs to the heaviest half, being the lowest; and if the pupa be again brought to the surface, so that the fluid is repelled from its disk, it remains suspended there without effort, as before. Just previously to the exclusion of the fly, the dry part of the thorax is seen to split in the middle. The air enters, and forms a brilliant stratum resembling quicksilver, between the body of the insect and its puparium; and the former pushing forth its head and forelegs, like the gnat, rests the latter upon the water, and in a few seconds extricates itself wholly from its envelope.

Before I close this letter, I must state a fact connected with the subject of it that deserves to be recorded. It is a general rule, that _one_ pupa-case incloses only _one_ insect; but Kleesius, a German entomologist, asserts that he had once _two_ specimens of _Gastropacha quercifolia_ produced from _one_ pupa; which was large, being full two inches long, and one thick.

FOOTNOTES:

[562] In the _Hemiptera_ the male _Cocci_ (Reaum. iv. 32.) and _Aleyrodes_ (Ibid. ii. 311.) belong to the second division.

[563] The terms _larva_ and _pupa_, applied to the insects of this subdivision, are perhaps not strictly proper.

[564] The larvæ and pupæ of many of the _homopterous_ section of _Hemiptera_ differ often from the imago, not only in their fore-legs (PLATE XVI. FIG. 4.), but also in other respects. I have the larva of a _Centrotus_ from Canada, given me by Dr. Bigsby, which has a long anal process or tail.

[565] See above, p. 125--.

[566] iii. 135.

[567] The pupæ of _Cassida_, _Imatidium_, &c. seem to vary somewhat from this type, the upper part being neither membranous nor exhibiting distinctly the form of the inclosed imago.

[568] The following arrangement of pupæ is perhaps in some respects better than that above given. But it is scarcely possible to propose one free from objections.

I. Capable of eating and walking.