Part 9

[140] Compare _N. Dict. d'Hist. Nat._ xvi. 246. with xx. 352-; but as the _Amnios_ immediately envelops the fœtus, the pellicle seems most analogous to it, and the shell to the _Chorion_.

[141] Swamm. _Bibl. Nat._ ed. Hill. l. 133. a. Comp. _N. Dict. d'Hist. Nat._ xvi. 246.

[142] Swamm. _Ibid._

[143] Sepp. iv. _t._ iii. _f._ i. _c._ v. _t._ iv. _f._ 2.

[144] See above, VOL. I. p. 208: it is there called an _Aphis_, but it is a distinct genus.

[145] De Geer iii. 245. _t._ xiii. _f._ 20-22.

[146] Sepp. iv. _t._ xiii. _f._ 2. 3.

[147] The sturgeon is said to lay 1,500,000 eggs, and the cod-fish 9,000,000.

[148] Reaum. iv. 392.

[149] See above, VOL. I. p. 350.

[150] De Geer vii. 159.

[151] See above, VOL. II. p. 109.

[152] Ibid. 159. 166.

[153] Ibid. 36--.

[154] iv. _t._ xviii. _f._ 4. 5.

[155] De Geer ii. 638.

[156] _Bibl. Nat._ i. 132. b.

[157] Gould 36.

[158] Reaum. v. 477.

[159] Ibid. iii. 579. v. 121.

[160] _Fourmis_, 69--.

[161] Rösel iii. 152.

[162] De Geer vii. 145.

[163] Ibid. 123--. See above, VOL. I. p. 393.

[164] Raii _Hist. Ins._ 265.

[165] Eggs of various shapes are given PLATE XX. FIG. 3-23. See also Brunnich. _Entomologia_ 4. _N. Dict. d'Hist. Nat._ xvi. 245. Reaum. ii. _t._ iii. iv. xiv. xxvi. xxvii. &c.

[166] PLATE XX. FIG. 18.

[167] PLATE XX. FIG. 23. Swamm. _Bibl. Nat. t._ iii. _f._ 7, 8. In a specimen I opened of this insect the bristles converged so as to form a kind of tail to the egg.

[168] Darwin _Phytolog._ 512.

[169] Geoffr. _Ins. Par._ i. 480. _t._ x. _f._ 1. _b. c._

[170] See above, VOL. I. p. 261.

[171] Reaum. iii. 386--. _t._ xxxii. _f._ 1. _t._ xxxiii. _f._ 5.

[172] I allude to _Ophion luteum_ F. (_Ichneumon_ L.) Vol. i. Ed. 3. p. 269, figured PLATE XX. FIG. 22; and the _Hydrachnæ_ or _Trombidia_. See above, and De Geer vii. 145.

[173] From this circumstance called πολυποικιλος σοφια by the Apostle, _Ephes._ iii. 10.

[174] _Nat. Theol._ 11th Ed. 375.

[175] PLATE XX. FIG. 19. _a a._

[176] Reaum. iv. 376--. _t._ xxvii. _f._ 9, 10.

[177] _Hist. Nat. gen. et partic. des Crust. et Ins._ xii. 282.

[178] Reaum. iv. 381. _t._ xxvi. _f._ 19, 20.

[179] Roxburgh in _Linn. Trans._ vii. 34.

[180] Some of the _Noctuæ_ have similar eggs, as _N. Lappa._ Sepp iv. _t._ iii. _f._ 1. _c._

[181] Reaum. _ubi supr. f._ 22, 23.

[182] PLATE XX. FIG. 6. 8.

[183] PLATE XX. FIG. 5.

[184] Ibid. FIG. 3. 4. 7. 9. &c.

[185] Ibid. FIG. 15.

[186] Bonnet _Œuvr._ ii. 9.

[187] _Naturf._ xiii. 229.

[188] _N. Dict. d'Hist. Nat._ xvi. 245.

[189] Reaum. ii. 286.

[190] PLATE XX. FIG. 11. Sepp _t._ iv. _f._ 2.

[191] Reaum. iv. 617.

[192] _Philos. Ent._ 76.

[193] See above, VOL. I. 358--.

[194] See above, VOL. I. Ibid.

[195] Young's _France_, ii. 34. This author asserts, that no art will hatch the eggs of the common silk-worms the first year, or that in which they are laid; but that there is a sort brought from Persia, which are hatched three times a year, and which will hatch in fifteen days in the proper heat. In 1765, it is said, the common sort hatched in the first year. _Ibid._ 226--.

[196] In the _N. Dict. d'Hist. Nat._ xii. 564. the eggs of the flesh-fly are said to hatch in two hours. This is true I believe in very warm weather.

[197] Brahm. 310.

[198] Rimrod _Naturf._ xvi. 131.

[199] _Fourmis._ 69.

[200] De Geer vii. 195.

[201] Ibid. 196.

[202] Reaum. ii. 167.

[203] Brahm. 249. Rösel. iv. 130. Swamm. _Bibl. Nat. t._ i. _f._ 2.

[204] By Mr. White, jun. cordwainer at Ipswich.

[205] PLATE XX. FIG. 16. _a._

LETTER XXX.

_STATES OF INSECTS._

LARVA STATE.

The _Larva_ state is that in which insects exist immediately after their exclusion from the egg (or from the mother in ovo-viviparous species), in which they usually eat voraciously, change their skin several times, and have the power of locomotion, but do not propagate.

Almost all larvæ, at their birth, are for a time in a very feeble and languid state, the duration of which differs in different species. In most it continues for a very short time, a few minutes or perhaps hours, after which they revive and betake themselves to their appropriate food. In others, as in the generality of spiders, this debility lasts for seven or eight days, and in some species even a month, during which the young ones remain inactive in the egg-pouch[206], and it is not till they have cast their first skin that their active state of existence commences.

All larvæ may be divided into two great divisions:--

I. Those which in general form more or less resemble the perfect insect.

II. Those which are wholly unlike the perfect insect.

I shall begin by calling your attention to the characters of the _first_ of these divisions: the second, which is by far the most numerous, will be afterwards considered.

I. The _first_ division includes the larvæ of _Scorpions_, _Spiders_, _Cockroaches_, _Grasshoppers_, _Lanthorn-flies_, _Bugs_, &c.; or generally, with the exception of the _Flea_ and _Crustacea_, the whole of the _Linnean_ Orders _Aptera_ and _Hemiptera_. All these larvæ, however remotely allied in other respects, agree in the general similarity which they bear to the perfect insects which proceed from them. The most acute entomologist, untaught by experience, could not even guess what would be the form of the perfect insects to be produced from larvæ of the _second_ division, while they can recognise the form of the spider, the cricket, the cockroach, the bug, and the frog-hopper, in that of the larvæ. There are, however, differences in the degrees of this resemblance, according to which we may, perhaps, divide this tribe in their second state as follows:--

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].