ii. Those which resemble the perfect insect, except that they

are apterous, or not yet furnished with organs of flight.

i. Spiders, _Phalangia_, scorpions, lice, _Poduræ_, sugar-lice (_Lepisma_), mites, centipedes, millepedes, &c. come under the _first_ subdivision. The larvæ of the first six tribes here mentioned differ at their birth from the perfect insect, only in size and the proportions of their parts. Thus the larvæ of spiders have their legs of a different relative length from that which they subsequently acquire; and the palpi in the males, which previously to the discoveries of Treviranus were regarded as their sexual organs, are not yet fully developed[207]: and a similar difference takes place in the legs of _Phalangia_. The general form too of the body undergoes slight alterations, and the colour very considerable ones, with each change of the skin--a change to which all these tribes are subject.

The larvæ of the three last-mentioned tribes (the mites, centipedes, and millepedes) differ from the perfect insect not only in the proportion but also in the number of their parts. Leeuwenhoeck states (and De Geer confirms his assertion, extending it to other species of mites[208]), that the common cheese-mite, which in its perfect state has _eight_ legs, when first excluded from the egg has but _six_, the third pair being wanting[209]. Some however are born with _eight_ legs, for instance _A. eruditus_ of Schrank, which he saw come from the egg itself with that number[210]. Others again have never more than six legs: this is the case with Latreille's genera--_Caris_, _Leptus_, _Atoma_, and _Ocypetes_ of Dr. Leach[211]. In the centipedes (_Scolopendridæ_) and millepedes (_Iulidæ_) differences still more remarkable, as I have stated in a former letter, have been observed by De Geer; these animals, in their progress to the perfect state, not only gain several additional pairs of _legs_, but also several additional _segments_ of the body. This illustrious Entomologist found that _Pollyxenus lagurus_ (_Scolopendra_ L.) was born a hexapod, with but three segments and as many pairs of feet, but successively acquired five additional segments with other appendages, and nine more pairs of feet[212]. A species of millepede (_Iulus terrestris_ L.), which he also traced from its birth, and which begins the world at first with only eight segments and six feet, by a successive development at length acquires, in its perfect state, 50 segments and not less than 200 feet[213]. The nature of these very singular accretions, which Latreille and Mr. Wm. MacLeay have also observed in the centipedes[214], seems not well understood. If, as is most probable, though De Geer could not find any exuviæ[215], the larvæ cast a skin before each change, they do not essentially differ from the metamorphosis of other insects. The legs that these insects thus acquire are affixed to the abdomen, the six that they set out with being attached to the part representing the trunk, so that the former may be regarded as analogous to the prolegs of caterpillars. These animals therefore, as I have before intimated, invert the order of Nature, and from _perfect_ degenerate into _imperfect_ insects.

ii. If you examine the _cockroach_, _cricket_, or _grasshopper_, in different stages of their growth, you will find that the larva does not vary essentially from the perfect insect, except in wanting wings and elytra. The case is the same in almost all the Linnean genera of the modern order--_Hemiptera_; and with _Raphidia_, _Termes_, and _Psocus_, in the _Neuroptera_. Some of these, however, exhibit slighter discrepancies in the proportion of some of their parts, but without affecting the general resemblance. Thus the larvæ of the common ear-wig have at first only _eight_, and subsequently _nine_ joints to their antennæ, whereas the perfect insect has _fourteen_[216]; and the forceps is quite different, resembling rather two straight styles than what its name implies. In those also of many bugs (_Coreus marginatus_ F. &c.), the joints of the antennæ are of a shape dissimilar to that which obtains in the perfect insect. In that of the common water-scorpion, the anal air-tube, which is so long in the imago, is scarcely visible[217]. In the _Cicada_ tribe, so celebrated for their song[218], neither the larva nor the imago have the enormous thigh armed below with strong teeth, the tibiæ terminating in a fixed incurved claw, probably for the purpose of digging the holes into which they retire till they disclose the fly, which distinguish the pupæ of some species, and is particularly conspicuous in one commonly brought from China[219]. These often exhibit also other minor differences.

II. In treating of the second great division of larvæ, those that are wholly unlike the parent insect,--which includes, with few exceptions[220], the whole of the Linnean orders, _Coleoptera_, _Lepidoptera_, _Hymenoptera_, _Diptera_, the majority of the _Neuroptera_, _Coccus_ and _Aleyrodes_ in _Hemiptera_, and the genus _Pulex_ in _Aptera_,--I shall advert to their characters, under several distinct heads; and to avoid unnecessary circumlocution, I shall in what follows wholly leave out of consideration the _first_ division already explained, and use the term _larvæ_ with reference only to those of the _second_. The heads under which I propose to treat of them are: The _substance_ of their body, its _parts_, _shape_, or figure, _clothing_, _colour_. Also the _Economy_ or mode of life of these creatures: their _food_, _moultings_, _growth_, _age_, _sex_, and their preparations for assuming the _Pupæ_.

i. _Substance_, with the exception of the head and six fore-feet, which are usually corneous, the exterior integument or skin of larvæ is commonly of a membranous texture, and the body is of a much softer consistence than in the perfect insect. In those, however, of some _Staphylinidæ_ and other _Coleoptera_, the dorsal part of the three first pieces, which represent the trunk of the perfect insect, is hard and horny. Some also have their whole skin coriaceous, as the tortoise-shell butterfly (_Vanessa polychloros_); and some few, as the wire-worm (_Elater segetum_), and other _Elateres_, very hard. I possess a very remarkable larva from Brazil, from the extreme flatness of its body, and from its having cavities to receive its legs when unemployed, probably living under bark, the skin of which is still harder than that of the grub of the Elaters. Perhaps it has to resist great pressure; and on that account is gifted with this quality, so seldom to be met with in other kinds of larvæ. The interior of the body of these animals is generally of a softer consistence than in the perfect insect. Their intestines, and other internal organs, are usually wrapped in a voluminous substance of a fatty nature, which is regarded as analogous to the _epiploon_, _omentum_, or _caul_, which envelops the viscera of quadrupeds, &c., and is called by Reaumur the _corps graisseux_. The use of this general flexibility of larvæ is obvious; for, their bodies being mostly long and narrow, a hard rigid covering would have been very inconvenient, and a considerable impediment to their motions. When a caterpillar is feeding, it has occasion to apply its body to any part of the margin of a leaf so as to support itself by its prolegs, and when moving it wants to give it all the curves that are necessary to enable it to avoid obstacles, and thread its way through the sinuous labyrinths which it must often traverse. On the other hand, the hardness of the substance of its head affords a strong fulcrum to the muscles which keep its powerful jaws in constant play. The larvæ, indeed, of some _Diptera_ have a membranous head; but their mandibles, which serve also as legs, are not grinders, but merely claws, the muscles of which require less powerful support[221]. Under this head it may be proper to observe, that generally larvæ are opaque; but some, as those of ants, and a few _Lepidoptera_[222], are diaphanous. That of _Corethra crystallina_ (_Tipula_ De Geer) is so beautifully transparent as to resemble a piece of crystal, and scarcely to be distinguished from the water in which it lives[223].

ii. _Parts._ The body of each larva consists of the _head_, including its different organs, and of the _succeeding segments_, of which the three first may usually be denominated the _trunk_, and have the six anterior feet, when present, attached to their under side: the remainder is the _abdomen_. The latter includes in some species a variable number of membranous feet, as well as various appendages affixed usually to its tail and sides. No larva is ever furnished with wings[224]. Each of these greater divisions, and the organs which they include, require separate consideration.

1. _Head._ This, as was lately observed, is exteriorly of a horny substance, or at least harder than the rest of the body, in most larvæ; and on this account, though rarely separated from it by any visible distinct neck[225], is, if the larva be of a tolerable size, distinguished at the first view. In those of many Dipterous insects, however, the head is covered with the same flexible membranous skin with the rest of the body, from which it is often scarcely to be distinguished. In these, except that it contains the organs of manducation, it wears no more the appearance of a head than any other segment of the body, and scarcely so much as the last or anal one. The head of these larvæ is also remarkable for another peculiarity,--that it is capable of being extended or contracted, and assuming different forms at the will of the insect: a property which the head of no superior animal can boast. It is probable that there is a considerable variety in the shape and circumstances of the heads of larvæ; but since, with the exception of those of _Lepidoptera_, they have had less attention paid to them than they deserve (indeed in a vast number of cases, from the difficulty of meeting with them, these variations, except in a few instances, have not been described), I will here mention a few of the most remarkable. The head of the young larva at its first exclusion from the egg is usually the most dilated part of the body, but it does not often continue so. In that of _Cicindela campestris_, however,--the beautiful green beetle sometimes found in sandy banks,--and also in several caterpillars of _Lepidoptera_, it is much larger than any of the following segments[226], which, in conjunction with the animal's formidable jaws, gives it a most ferocious appearance. In some lepidopterous larvæ the head is of the same diameter with the rest of the body, but in insects in general it may, I think, be stated as less; and occasionally it bears no proportion whatever to it. This is the case with the subcortical one from Brazil lately mentioned. It is more commonly longer than broad; but in some, as in the larvæ of carrion beetles (_Silphæ_), the reverse of this takes place. Its shape varies from triangular to orbicular, the mouth of the animal forming the vertex of the triangle. In some larvæ of _Hemerobii_, however, the head is narrowest behind. That of the grub of a gnat noticed above (_Corethra crystallina_) forms a kind of sharp horn or claw, terminating the body anteriorly[227]. The contour of the head of larvæ is usually intire and unbroken; but in the caterpillars of some _Lepidoptera_, as the butterfly called the grand admiral (_Vanessa Atalanta_), the Glanville fritillary (_Melitæa Cinxia_), &c. it is divided into two lobes[228]. In the Brazil flat larvæ it is trilobed, each lateral lobe being divided into three smaller ones: in which circumstance it somewhat resembles the head of some subcortical _Cimicidæ_. Although the part we are treating of is generally without _horns_, yet in some tropical butterflies of the tribe of _Nymphales_, it is singularly armed with them. Thus _Papilio Anchises_ is distinguished, according to Madame Merian[229], by two in the occiput, which it has the power of retracting. In the purple highflier (_Apatura Iris_), a British species, the two lobes of the head, I am informed, terminate behind in two horns; as they do likewise in the brilliant _Morpho Menelaus_[230], the lobes assuming the form of a pear, and the horn representing the stalk. In a caterpillar I found amongst Mr. Francillon's larvæ, the head is bilobed, with a very long recurving subcapitate subramose spine. In _Satyrus Cassiæ_, the head is armed with three occipital stout spines[231]. The larva of _Nymphalis Amphinome_ Latr. (_Limenitis_ F.) is crowned with a coronet of eight occipital stout acute spines, the intermediate ones being the longest[232]; and that of _Morpho Teucer_ has a similar coronet, consisting of only seven blunt _rays_, seemingly, rather than spines[233]. With regard to the articulation of the head with the trunk, it is generally by its _whole_ diameter; but in some instances, only by a _part_ of it. This is the case with one of a sphinx figured by Mad. Merian[234]; and I have another, probably belonging to the nocturnal _Lepidoptera_ (_Phalæna_ L.)[235]. In both these, the head is vertical and triangular; and in the latter (which is a remarkable creature, the tail itself being more like a head, and furnished with what resemble two prominent black eyes) the vertex of the triangle is considerably higher than the back of the animal. Whatever may be the clothing of the body, the head is usually _naked_. Sometimes, however, it is itself beset with very small simple spines, as in the butterfly of the mallow (_Hesperia Malvæ_); or with longer compound ones, such as are found on the rest of the body. This is the case with one of a butterfly named by Rösel _Papilio morsa_. The most common _colour_ of the head of larvæ, where it differs from the rest of the body, is a darker or lighter reddish brown, or piceous. This is particularly observable in those of Coleopterous insects, but it is very commonly in other tribes of the same hue. Sometimes, amongst the _Lepidoptera_, the head is of a different colour from the rest of the body; especially where a contrast renders it striking. I can show the caterpillar of some insect, probably of the hawk-moth tribe (_Sphingidæ_), from Georgia, remarkable for the length of its anal spine, in which the body is black, and the head red: another has a white head and a brown body. In the larvæ of some _Lepidoptera_, _Coleoptera_, and _Diptera_, the head can be wholly or nearly withdrawn within the first segment of the body. This may be readily seen in that of the common glow-worm; and that of a small gnat (_Tipula replicata_ De Geer) withdraws it so completely that the anterior margin of that segment closes the orifice, so that the animal appears to have no head[236].--The parts of the head which require distinct consideration are, the _eyes_, _antennæ_, and the _mouth_: consisting of various organs, which will be specified. Some of these parts and organs are peculiar to larvæ of one order, others to those of another, and some are furnished with them all.

_Eyes._ The larvæ of many insects have no eyes. Those with antennæ which terminate in a lamellated clava (_Scarabæus_ L.), and capricorn beetles also (_Cerambyx_ L.), amongst the _Coleoptera_, are without them, and probably several others; and amongst the _Diptera_, all those with a membranous or variable head. Those of the remaining orders, with the exception, perhaps, of some _Hymenoptera_ and _Lepidoptera_, are furnished with these organs; and in the _Coleoptera_ all the predaceous tribes, as well as most of those that are herbivorous or granivorous, and the Gnats and other Tipulidans (_Tipulariæ_ Latr.) in the _Diptera_, are also distinguished by them. In the larvæ of the dragon-flies (_Libellula_ L.), and other _Neuroptera_, they are composed of many facets as in those of the perfect insect, from which they differ chiefly in being smaller. But in the other insects of this description they are simple, and resemble those of the _Arachnida_, and many aptera. These simple eyes vary in their number, in different genera and tribes, from one to six on each side of the head. Thus the larva of _Telephorus_, and the saw-flies, has only _one_[237]; that of _Cicindela three_, the two posterior ones being large with a red pupil surrounded by a paler iris, which adds to the fierce aspect of this animal; and the anterior one very minute. Those of the tortoise-beetles also (_Cassida_) have _three_[238]; of _Staphylinus, four_; of _Timarcha_ (the bloody-nosed beetle) _five_; of _Carabus_, and the _Lepidoptera_ in general, _six_. In the last they are of different sizes, and generally arranged in a circle: in that of _Hemerobius_ there are five in a circle, with one central one[239]. The appearance of these globules, which are often not visible but under a powerful lens, is so different from that of the eyes of a butterfly or moth, or other perfect insect, that it has been doubted whether they actually perform the office of _eyes_, but without reason. They occupy the usual station of those organs, being situated in many instances upon a protuberance which appears to incase them; and seem of a construction closely analogous to that of the eyes of spiders, and the _stemmata_ or _ocelli_ of _Hymenoptera_, which have been satisfactorily proved to be organs of vision. In the larva of a moth not yet ascertained to exist in this country, _Attacus Tau_, and probably other species, the eyes, after the skin has been changed a few times, are no longer to be seen[240].

_Antennæ._ Most larvæ are provided with organs near the base of the mandibles, which from their situation and figure may be regarded as antennæ. Fabricius has asserted that the larvæ of the saw-flies (_Tenthredo_ L.) have no antennæ; but in this he was mistaken, for though very short, they are discoverable in them, as he might have learned by consulting De Geer[241]. In the majority of _Neuropterous_ larvæ, they almost precisely resemble those of the perfect insect. In all the rest they are very different. The antennæ of Coleopterous larvæ are usually either filiform or setaceous, consisting of four or five joints, nearly equal in length. Those of Lepidopterous larvæ are commonly conical, as are those likewise of _Chrysomela_ and _Coccinella_ &c. amongst the _Coleoptera_, and very short, composed of two or three joints, of which the last is much thinner than the first, and ends in one or two hairs or bristles. These antennæ the larva has the power of protruding or retracting at pleasure. Lyonnet informs us, that the caterpillar of the great goat-moth (_Cossus ligniperda_) can draw the joints of its antennæ one within the other, so as nearly to conceal the whole[242]. The larva of the common gnat has two long incurved setaceous antennæ, fringed with hairs at some distance from their apex, which consist only of a single joint[243]. The greater number of Dipterous larvæ, however, all indeed except the Tipulidans (_Tipulariæ_ Latr.), and many belonging to the _Coleoptera_ and _Hymenoptera_ orders (as those of _Curculio_, _Apion_, _Apis_, &c.), are wholly deprived of antennæ. It is a general rule, that the antennæ of larvæ are shorter than the same organs in the perfect insect, the tribe _Ephemerina_ perhaps affording the only example in which the reverse of this takes place[244].

_Mouth._ All larvæ have a mouth situated in the head, by which they receive their food, and furnished with one or more instruments for the purpose of mastication and deglutition. These instruments, in all the orders except _Lepidoptera_, some _Neuroptera_ and _Diptera_, bear a general resemblance to the same parts in the perfect insect. In larvæ of the Coleopterous, Lepidopterous, and Hymenopterous orders, we can distinguish for the most part an upper and under lip; two pairs of jaws answering to the mandibulæ and maxillæ; and two, four, or six palpi[245]: and some of these instruments may be found in most _Diptera_. Each of these parts require separate notice.

_Upper-lip_ (LABRUM). The mouth of almost all larvæ, except some of the order _Diptera_, are provided with a distinct _upper-lip_, for retaining their food during mastication. As the construction of this part does not widely differ from that of the perfect insect, which will hereafter be more fully described, it is only necessary to observe, that it is usually a transverse moveable plate, attached posteriorly to the nasus (_clypeus_ F.), and situated just above the mandibles[246].

_Upper-jaws_ (MANDIBULÆ). The most usual figure of these, which are of a hard horny consistence[247], is that of two slightly concave, oblong, or triangular plates, often at their lower extremity of considerable thickness, and of very irregular form, the base of which is filled with powerful muscles, and planted in the side of the mouth so as to move transversely. The other extremity can be made to meet or diverge like the claws of pincers, and are divided into one or more tooth-like indentations, by means of which the food of the larva is cut[248]. This is their construction in the larvæ of all _Lepidoptera_, and in many of those of the other orders. They frequently, however, assume a different form, though their situation is always the same. Thus in the larvæ of the capricorn beetles (_Cerambyx_ L.) and of other wood-boring species, they are shaped like the half of a cone, the inner sides of which, applying close to each other, form a pair of powerful grindstones, capable of comminuting the hardest timber[249]. M. Cuvier has observed, with regard to the mandibulæ of those of stag-beetles (_Lucanus_), that besides their teeth at the extremity, they have towards their base a flat striated molary surface; so that they both cut and grind their ligneous food[250]. It seems to have escaped him, that a similar structure takes place in many perfect insects of the lamellicorn tribe, as I shall hereafter show you. In the larvæ of the water-beetles (_Dytiscus_ L.), ant-lions (_Myrmeleon_ L.), and lace-winged flies (_Hemerobius_ L.), they resemble somewhat the forceps at the tail of an ear-wig, being long and incurved; and, what is more remarkable, hollow and perforated at the end, so as to serve as a channel for conveying into the larva's mouth the juices of the prey which by their aid it has seized. Reaumur even asserts, that the larva of _Myrmeleon_ has no other entrance into its throat than through these tubular mandibles[251]. That of the rove-beetles (_Staphylinus_ L.), and of many other Coleopterous genera, have these organs of this forcipate construction, without being perforated[252]. In the larva of the carnivorous flies, and many other _Diptera_, are two black incurved subulate parts, connected at the base, and capable of being protruded out of, and retracted into, the head, through the skin of which they are usually visible. As I informed you in a former letter[253], these mandibles are used for _walking_ as well as _feeding_: they are parallel to each other, and are neither formed for cutting nor grinding like the mandibles of other insects, but merely detach particles of food by digging into it and tearing the fibres asunder. In this operation they are probably assisted by an acutely triangular dart-like instrument of a horny substance, which in some species (_Musca vomitoria_) is placed between the two. In others this part is wanting. Some Dipterous larvæ have two similar mandibles, but instead of being parallel, they are placed one above the other; others (_Musca domestica_ and _meridiana_) have but one such mandible, and some have no perceptible mandible of any kind. The mandibles of the larva of the crane-flies (_Tipula_), which are transverse and unguiform, do not act against each other, but against two other fixed, internally concave and externally convex, and dentated pieces[254].

_Under-jaws_ (MAXILLÆ). These are a pair of organs, usually of a softer consistence, placed immediately under the upper-jaws; but as they are usually so formed and situated as not to have any action upon each other, it is probable that in general they rather assist in submitting the food to the action of the mandibulæ, than in the comminution of it. In Lepidopterous larvæ they appear to be conical or cylindrical (at least in that of the cossus so admirably figured by Lyonnet[255]), and to consist of two joints; which may, I imagine, be analogous to the upper and lower portions of which the maxillæ of perfect insects usually consist. The last of these joints is surmounted by two smaller jointed palpiform organs. If any part of the maxillæ can act upon each other, it is these organs or palpi; but it is evident they are not calculated for mastication, although they may assist in the retention of the substance to be masticated. In a figure given by Reaumur of the under side of the head of another lepidopterous larva (_Erminea Pomonella_), the maxillæ consist of a single joint, and appear to be crowned by chelate palpi[256]: a circumstance which is also observable in that of a common species of stag-beetle (_Lucanus parallelipipedus_), the weevil of the water-hemlock (_Lixus paraplecticus_[257]), and other insects. In general the maxillæ of larvæ are without the lobe or lobes discoverable in those of most perfect insects, this part being usually represented by a kind of nipple, or palpiform jointed process, strictly analogous to the interior maxillary palpi of the predaceous coleoptera; but in most of the lamellicorn beetles the lobe exists in its proper form[258], as it does likewise in that of the capricorn-beetle before noticed (_Callidium violaceum_[259]). In the former instance, it is armed with spines or claws; but in the latter it is unarmed, and rounded at the end. In the larva of _Cicindela campestris_, the base of the maxilla runs in a transverse direction from the mentum, to which, as is usually the case, it is attached. From this at right angles proceeds the lobe, from the outer side of which the feeler emerges; and the inner part terminates in an unguiform joint, ending in two or three bristles. The structure in the larvæ of water-beetles (_Dytiscus_ L.) is different, for they appear to be without maxillæ[260]; but the case really seems to be, that these organs are represented by the first joint of what M. Cuvier calls their _palpi_[261]; from which proceed the real palpi, the interior one being very short, and consisting only of a single joint. These maxillæ of larvæ were regarded by Reaumur and other writers as parts of the under-lip, on each side of which they are situated; and indeed, as well as those in the perfect insect, they form a part of the same machine, being connected by their base with the mentum, which is part of the labium, but they are clearly analogous to the maxillæ of the imago. They are not to be found in the larvæ of many Dipterous insects, and perhaps in some species belonging to other orders. In some Neuropterous larvæ, as those of the _Libellulina_ MacLeay, the maxillæ are of a substance quite as solid and horny as the mandibles, which in every respect they resemble[262].

_Under-lip_ (LABIUM). Between the two maxillæ in the larvæ of most of the insects under consideration is a part termed by Reaumur the middle division of the under-lip, but which is in fact analogous to the whole of that organ in the _imago_. This organ varies in shape, being sometimes quadrangular, often conical, &c. Interiorly it is frequently connected with a more fleshy protuberance, called the tongue by Reaumur[263], and supplying the place of the _ligula_ in the perfect insect. On each side of the apex of the under-lip is a minute feeler, and in the middle between these in the _Lepidoptera_ and many others, is a filiform organ, which I shall call the _spinneret_ (_Fusulus_), through which the larva draws the silken thread employed in fabricating its cocoon, preparatory to assuming the pupa state, and for other purposes[264]. This organ is found only in those larvæ which have the power of spinning silk; that is, in all _Lepidoptera_, most _Hymenoptera_, _Trichoptera_, some _Neuroptera_, and even a _Dipterous_ insect[265]. This tube, Lyonnet had reason to believe, is composed of longitudinal slips, alternately corneous and membranous, so as to give the insect the power of contracting its diameter, and thus making the thread thicker or smaller. There is only a single orifice at the end, which is cut obliquely, somewhat like a pen, only with less obliquity, and without a point, the opening being below, so as to be conveniently applicable to the bodies on which the larva is placed. Reaumur conceived that this spinneret had two orifices; but Lyonnet ascertained this to be a mistake, the two silk tubes uniting into one before they reach the orifice. From the contractile nature of the sides and the form of the orifice, combined with the power the insect has of moving it in every direction, results the great difference which we see in the breadth and form of the threads, some being seven or eight times as thick as others, some cylindrical, others flat, others channelled, and others of different thickness in different parts[266]. In the larvæ of many _Diptera_ the under-lip is merely a small tubercle, which can be protruded from the insect's mouth by pressure[267].

One of the most remarkable prepensile instruments, in which the art and skill of a DIVINE MECHANICIAN are singularly conspicuous, and which appears to be without a parallel in the insect world, may be seen in the under-lip of the various species of dragon-fly (_Libellula_ L.). In other larvæ this part is usually small and inconspicuous, and serves merely for retaining the food and assisting in its deglutition; but in these it is by far the largest organ of the mouth, which when closed it entirely conceals; and it not only retains but actually seizes the animal's prey, by means of a very singular pair of jaws with which it is furnished. Conceive your under-lip (to have recourse, as Reaumur on another occasion[268], to such comparison,) to be horny instead of fleshy, and to be elongated perpendicularly downwards[269], so as to wrap over your chin and extend to its bottom,--that this elongation is there expanded into a triangular convex plate[270], attached to it by a joint[271], so as to bend upwards again and fold over the face as high as the nose, concealing not only the chin and the first-mentioned elongation, but the mouth and part of the cheeks[272]: conceive, moreover, that to the end of this last-mentioned plate are fixed two other convex ones, so broad as to cover the whole nose and temples[273],--that these can open at pleasure, transversely like a pair of jaws, so as to expose the nose and mouth, and that their inner edges where they meet are cut into numerous sharp teeth or spines, or armed with one or more long and sharp claws[274]:--you will then have as accurate an idea as my powers of description can give, of the strange conformation of the under-lip in the larvæ of the tribes of _Libellulina_; which conceals the mouth and face precisely as I have supposed a similar construction of your lip would do yours. You will probably admit that your own visage would present an appearance not very engaging while concealed by such a mask; but it would strike still more awe into the spectators, were they to see you first open the two upper jaw-like plates, which would project from each temple like the blinders of a horse; and next, having by means of the joint at your chin let down the whole apparatus and uncovered your face, employ them in seizing any food that presented itself, and conveying it to your mouth. Yet this procedure is that adopted by the larvæ provided with this strange organ. While it is at rest, it applies close to and covers the face. When the insects would make use of it, they unfold it like an arm, catch the prey at which they aim by means of the mandibuliform plates, and then partly refold it so as to hold the prey to the mouth in a convenient position for the operation of the two pairs of jaws with which they are provided. Reaumur once found one of them thus holding and devouring a large tadpole;--a sufficient proof that Swammerdam was greatly deceived in imagining earth to be the food of animals so tremendously armed and fitted for carnivorous purposes. Such an under-lip as I have described is found in the tribe of dragon-flies (_Libellulina_); varied, however, considerably in its figure in the different genera. In the larva of _Libellula_ Fab., such as _Libellula depressa_, &c. it is of the shape above described; so exactly resembling a mask, that if Entomologists ever went to masquerades, they could not more effectually relieve the insipidity of such amusements and attract the attention of the _demoiselles_, than by appearing at the supper table with a _mask_ of this construction, and serving themselves by its assistance. It would be difficult, to be sure, by mechanism to supply the place of the muscles with which in the insect it is amply provided: but Merlin, or his successor, has surmounted greater obstacles. In the larva of the Fabrician _Æshnæ_ (_Libellula grandis_, &c. L.), this apparatus is not convex but flat: so that, though it equally conceals the face, it does not so accurately resemble a mask; and the jaws at its apex are not convex plates, but rather two single conical teeth[275]. It is, as to its general shape, similarly constructed in _Agrion_ Fab. (_L. Virgo_, &c. L.); but the first joint is more remarkably elongated, the jaws more precisely resemble jaws than in any of the rest, and are armed with three long, very sharp teeth: between them also there is a lozenge-shaped opening, through which, when the apparatus is closed, is protruded a circular sort of nipple, apparently analogous to the ligula[276]. _Libellula ænea_, L., which is the type of another tribe (_Cordulia_ Leach), has a mask somewhat different from all the above, the jaws being armed with a moveable claw and an internal tooth[277]. You will admire the wisdom of this admirable contrivance, when you reflect that these larvæ are not fitted to pursue their prey with rapidity, like most predaceous animals; but that they steal upon them, as De Geer observes[278], as a cat does upon a bird, very slowly, and as if they counted their steps; and then, by a sudden evolution of this machine, take them as it were by surprise, when they think themselves safe. De Geer says, it is very difficult for other insects to elude their attacks, and that he has even seen them devour very small fishes[279]. As these animals are found in almost every ditch, you will doubtless lose no time in examining for yourself an instance of so singular a construction.

_Feelers_ (PALPI). In the orders _Diptera_ and _Hymenoptera_ are many larvæ in which these organs have not been certainly discovered; yet Reaumur in that of a common fly (_M. meridiana_ L.) found four retractile nipples[280] which seem analogous to them; and Latreille has observed, that below the mandibles of those of ants are four minute points, two on each side[281]: but in all other larvæ their existence is more clearly ascertained. The _maxillary_ palpi vary in _number_, many having _two_ on each maxilla and others only _one_. In the perfect insect the former is one of the distinguishing characters of the predaceous beetles (_Entomophagi_ Latr.), but in the larvæ it is more widely extended; since even in the caterpillars of _Lepidoptera_ the inner lobe of the maxilla which represents this feeler is jointed, which is precisely the case with the beetles just named. Cuvier has observed this circumstance in the larva of the stag-beetle[282]; and it belongs to many other _Coleoptera_ that have only a pair of maxillary palpi in the perfect state. The _labial_ palpi are always two, emerging usually one on each side from the apex of the under-lip. With regard to the _form_ of the palpi, those of the _Lepidoptera_ are mostly conical; in other orders they are sometimes setaceous and sometimes filiform. Their termination is generally simple, but sometimes the last joint is divided. They are for the most part very short, and the labial shorter than the maxillary. The latter never exceed _four_ joints[283], which seems the most natural number; and the former are limited to _three_. Both vary between these numbers, and _one_ joint. The joints, though commonly simple, are sometimes branched. This is the case with one I met with in considerable numbers upon the Turnip, in October 1808, the second joint of the palpi of which sends forth near the apex an internal branch. In the larva of the _Cossus_, as Lyonnet informs us[284], the joints of the palpi are retractile, so that the whole of the organ may be nearly withdrawn.

* * * * *

After thus describing the _head_ of larvæ, and its principal organs, we must next say something upon the remainder of the body, or what constitutes the

2. _Trunk and Abdomen_: which I shall consider under one article. These are composed of several _segments_ or rings, to which the feet and other appendages of the body are fixed. The form of these segments, or that of their vertical section, varies considerably: in many _Lepidoptera_, the wire-worm, &c., it would be nearly circular; in others a greater or less segment of a circle would represent it; and in some, perhaps, it would consist of two such segments applied together. Their lower surface is generally nearly plane. Their most natural _number_, without the head and including the anal segment, is twelve: this they seldom exceed, and perhaps never fourteen. The three first segments are those which represent the _trunk_ of the perfect insect, and to which the six anterior legs when present are affixed. In general, they differ from the remaining segments only in being shorter, and in many cases less distinctly characterized; but in Neuropterous larvæ, those of _Dytisci_, and some other _Coleoptera_, they are longer than the succeeding ones, and pretty nearly resemble the trunk of the animal in its last state. The surface of the trunk and abdomen will be considered under a subsequent head; I shall not, therefore, describe it here. The _conformation_ of the different segments varies but little, except of the terminal one, or tail, which in different larvæ takes various figures. In most, this part is obtuse and rounded; in others acute or acuminate; in others truncate; and in others emarginate, or with a wider sinus, and with intermediate modifications of shape which it would be endless to particularize. In some, also, it is simple and unarmed; in others beset with horns, spines, radii, and tubercles of different forms, some of which will come under future consideration. The parts connected with the trunk and abdomen which will require separate consideration, are the _legs_, the _spiracles_, and various _appendages_.

_Legs._ It may be stated generally that the larvæ of the orders _Coleoptera_, _Lepidoptera_, and _Neuroptera_, have legs; and that those of the orders _Hymenoptera_ and _Diptera_ have none. This must be understood, however, with some exceptions. Thus the larvæ of some _Coleoptera_, as the weevil tribes (_Curculio_ L.) have no legs, unless we may call by that name certain fleshy tubercles besmeared with gluten, which assist them in their motions[285]; while those of _Tenthredo_ and _Sirex_ in the order _Hymenoptera_ are furnished with these organs. At present I know no Dipterous larva that may be said to have _real_ legs, unless we are to regard as such certain tentacula formed upon a different model from the legs of other larvæ[286]. Rösel has, I think, figured a Lepidopterous apode. No Neuropterous one has yet been discovered.

The legs of larvæ are of two kinds; either horny and composed of joints, or fleshy and without joints[287]. The first of these, as I observed in a former letter[288], are the principal instruments of locomotion, and the last are to be regarded chiefly as props and stays by which the animal keeps its long body from trailing, or by which it takes hold of surfaces; while the other legs, or where there are none, the annuli of its body, regulate its motions. The former have been commonly called _true legs_ (_pedes veri_), because they are persistent, being found in the perfect insect as well as in the larva; and the latter _spurious legs_ (_pedes spurii_), because they are caducous, being found in the larva only. Instead of these not very appropriate names, I shall employ for the former the simple term _legs_, and for the latter _prolegs_ (_propedes_)[289].

The legs, when present, are always in number _six_, and attached by pairs to the underside of the three first segments of the trunk. They are of a horny substance, and consist usually of the same parts as those of the perfect insect; namely, _coxa_, _trochanter_, _femur_, _tibia_, and _tarsus_, suspended to each other by membranous ligaments: these parts are less distinctly marked in some than in others. Thus in the legs of a caterpillar, or the grub of a capricorn-beetle, at first you would think there were only three or four joints besides the claw; but upon a nearer inspection, you would discover at the base of the leg the rudiments of two others[290], in the latter represented indeed by the fleshy protuberance from which the legs emerge. In the larvæ of the predaceous _Coleoptera_, the hip and trochanter are as conspicuous nearly as in the perfect insect; and the tarsus, which still consists of only a single joint, is armed with two claws[291]. In those of the _Neuroptera_ order, in which all the joints are very conspicuous, the tarsi are jointed, as well as two-clawed[292]. The legs of larvæ are usually shorter than those of the perfect insect, and scarcely differ from each other in shape, for they all gradually decrease in diameter from the base to the apex. This is the most usual conformation of them in Lepidopterous, Hymenopterous, and some Coleopterous larvæ, (those of the capricorn-beetles are very short and minute, so as to be scarcely visible,) in which they are so small as to be concealed by the body of the insect[293]. In Neuropterous larvæ, however, and several _Coleoptera_, as those of _Dytiscus_, _Staphylinus_, _Coccinella_, &c., they more resemble the legs of the perfect insect, the joints being more elongated, and the femoral one projecting beyond the body[294].

You will find no other than true legs in most Coleopterous, Neuropterous, and Hymenopterous larvæ. But those of the saw-flies (_Tenthredo_ L.), and all caterpillars, have besides a number of _prolegs_: a few Dipterous larvæ also, are provided with some organs nearly analogous to them. These _prolegs_ are fleshy, commonly conical or cylindrical, and sometimes retractile protuberances, usually attached by pairs to the underside of that part of the body that represents the abdomen of the future fly[295]. They vary in conformation and in number; some having but one, others as many as eighteen.

With regard to their _conformation_, they may be divided into two principal sections: first, those furnished with terminal claws; and secondly, those deprived of them. Each of which may be divided into smaller sections, founded on the general figure of the prolegs, and arrangement of the claws or hooks.