i. Those that resemble the perfect insect, except in the relative proportions and number of some of their parts.
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].