Part 14

I must now say something upon the _arrangement_ of these spines. Though in a few instances so thickly set as entirely to conceal the body of the animal, as in the case of the Brazil one lately mentioned, yet generally speaking, even when they are most numerous, they permit the skin to be distinctly seen. Their arrangement is various, though always orderly: in the majority they are planted singly, but in some caterpillars in bundles. In that of _Saturnia Io_, on each segment there are six bundles of longish, quill-shaped, sharp, slender, diverging spines, which also appear to sheath aculei. Madame Merian has figured this larva, or one very near it, as the grub of a _Euglossa_[437], with which, though she affirms she traced it to the fly, it can have no connection. With regard to _number_, some larvæ have only four spines on each segment; others five, others again six, and others seven, or even eight: they are planted on the sides and back only, never on the belly. They are often more numerous on the intermediate than on the anterior and posterior segments; but sometimes the reverse of this takes place; in that of _Attacus Erythrinæ_ only the head and tail are armed with spines, the rest of the body being without any[438]; and in that of _Morpho Teucer_ there is only a single spine on the four intermediate segments[439]. They are usually all nearly of equal length; but in some cases those of the head and tail are much longer than the rest, and remarkably so in the caterpillar of _Urania Leilus_, also beautifully plumose, and gracefully waved[440]. Those in the second and third segments are much longer than any of the rest in that of _Bombyx regalis_; which circumstance gives it the terrific appearance lately alluded to. In the family to which _Argynnis Paphia_ belongs, the larva is adorned with two on the back of the first segment twice as long as the rest, and resembling at first sight two antennæ.

The spines, as well as the hairs of the new skin, are concealed under the old one, and not incased in its spines; but Bonnet ascertained, that if cut off very closely, the larva sometimes died in consequence, whilst no such result followed a similar operation on hairy larvæ. We learn from Reaumur[441], that some spinous larvæ of saw-flies (_Tenthredo_ L.) lose their spines at the last change of their skin; and from Madame Merian, that that of _Attacus Erythrinæ_ before mentioned loses also at the same period the six tremendous black spikes that arm its black and yellow larvæ. The grubs of ants that are destined to pass the winter in the larva state are hairy, but are not so in summer[442]. The spines found in the grubs of some gad-flies (_Œstrus_ L.) are of a different kind from those above described, being very minute triangular flat plates, arranged in different and contrary directions[443], and serving the insect merely to change its place and fix itself[444].

Two other kinds of clothing, if so they may be called, neither coming under the description of _hairs_ nor _spines_, are found in some other larvæ, not only amongst the _Lepidoptera_, but also in some of the other orders. _Nymphalis Populi_ and others of the same family have larvæ furnished on the back of each segment with cylindricoconical processes of a fleshy substance, obtuse at the apex and surrounded with capitate hairs. In that of _N. Sybilla_, which has on each segment two fleshy protuberances, they are bifurcate or trifurcate, and also encircled at the base with a hairy tuft[445]. Others, as those of _Melitæa Artemis_, _Cynthia_, &c. have each segment beset on the back with from seven to nine fleshy, pubescent, wedge-shaped protuberances; two larger ones projecting over the head. Under this head, too, may be noticed, the glutinous secretion which clothes the grub of _Cionus Scrophulariæ_, a little weevil; and of _Tenthredo Cerasi_ L. a saw-fly, and that waxy or powdery substance which transpires through the skin of the larvæ of several _Aphides_, _Chermes_, _Cocci_, _Hylotoma ovata_ F., &c. The _Aphis_, whose extensive ravages of our apple-trees (_A. lanata_) were before described to you[446], is covered and quite concealed by this kind of substance, so that the crevices in the bark which they inhabit look as if they were filled, not with animals, but with cotton. The insect, also, that forms those curious galls produced upon the spruce fir, and which imitate its cones (_Chermes Abietis_ L., _Aphis_ De Geer) secretes a similar substance. In these and other cases of the same kind, this matter seems to be, if I may so speak, wire-drawn through numerous pores in certain oval plates in the skin, more depressed than the rest of the back, arranged regularly upon the segments, and exhibiting minute tuberosities. When young, these animals have more of this secretion than when more advanced: it then hangs from their anal extremity in locks[447].

But the insects most remarkable for a covering of this nature are those _Coccidæ_ of which Bosc has made a genus under the name of _Dorthesia_. De Geer is the first author that notices them, and has given a description and figure of one species under the name of _Coccus floccosus_[448]. It was discovered by Modeer upon some sere fir-leaves in a thick bed of moss. Panzer has figured a second found upon _Geranium sanguineum_, which from the figure appears distinct from De Geer's, under the name of _Coccus dubius_[449]. Fabricius regards this as synonymous with the _Dorthesia characias_ of Bosc, inhabiting _Euphorbia characias_ in South Europe[450]. Olivier found a species upon the bramble[451]. I once took one, which appears to differ in some respects from the preceding species, upon _Melampyrum cristatum_, and our indefatigable friend Mr. Sheppard has sent me another, on what plant found I do not remember, which does not agree with any that I have mentioned. The body of the animals of this genus is covered by a number of cottony or waxy laminæ which partly cover each other, and are arranged usually in a triple series: in De Geer's figure the series appears quadruple, the lateral ones being placed obliquely. The anterior one in my specimen covered the head, and they are all canaliculate. Above the anus are four diverging ones: the whole are of the most dazzling whiteness. When these laminæ are removed, the body appears divided into segments.

With respect to those larvæ which imitate slugs by the viscid covering that besmears them and issues from their pores, we learn from Professor Peck that this exudation takes place as soon as they are hatched; that the animal retains its humidity although exposed to the fiercest heat of the sun, and that at the last moult the skin becomes quite clean, and free from all viscidity[452]. It is probable that the other limaciform larvæ are similarly circumstanced. Madame Merian has figured an _onisciform_ one, the legs of which, she says, are covered with a viscid skin: this produced a _Noctua_. Those of _Papilio Anchises_ also are slimy, and adhere to each other[453].

v. Amongst other qualities which attach to larvæ, we must not omit to say something concerning their _Colour_. For though those which live in darkness, in the earth, in wood, in fruits, &c. are, with few exceptions[454], of an uniform whitish colour, yet such as are exposed to the influence of the light are usually adorned with a vast variety of tints, sometimes the most vivid that can be imagined. That the white colour of the former may be attributed to the absence of light is proved by an experiment of M. Dorthes, who having forced some to live under glasses, exposed to the light, found that they gradually became brown[455]. To attempt any classification of coloured larvæ would be in vain, since they are tinged with almost every possible shade that can be conceived, of many of which it would be difficult to find examples elsewhere; and infinitely diversified as to the arrangement and figure of their multiform markings and spots. A few general remarks, therefore, are all that you will expect on this head. Many are of one uniform colour; while a variety of tints, very different, and very vivid and distinct, ornament others. Sometimes they are distributed in longitudinal rays or bands, at others in transverse ones. Sometimes they are waved or spotted, regularly or irregularly; at others they are sprinkled in dots, or minute streaks, in every possible way. Various larvæ are of the colour of the plant on which they feed, whence they are with difficulty discovered by their enemies. Thus, a large proportion of _Lepidoptera_ are green of different shades, sometimes beautifully contrasted with black bands; a circumstance which renders the caterpillars of two of our finest insects of this order as lovely as the fly: I mean that of _Papilio Machaon_ and _Saturnia Pavonia_. Very frequently the larvæ of quite different species resemble each other so exactly, in colour as well as shape, as scarcely to be distinguishable: this sometimes takes place even where they belong to different genera, as in those of _Bombyx versicolor_ a moth, and _Smerinthus Populi_ a hawk-moth. And it sometimes happens, very fortunately for distinguishing allied species, that where the perfect insects very nearly resemble each other, the larvæ are altogether dissimilar. Thus, the female of _Pieris Rapæ_ is so much like the same sex of _Pieris Brassicæ_, that it might be taken for a variety of it, did not the green caterpillar of the one, and the spotted one of the other, evince the complete distinction of these butterflies. _Noctua Lactuca_, _N. umbratica_, and several other species of the same tribe, which includes _N. Absinthii_, _Verbasci_, _Chamomillæ_, _Abrotani_, are so extremely alike, that the most practised eye can scarcely discover a shade of difference between them, though their larvæ in colour and markings are constantly distinct[456]. The markings of species belonging to the same family are usually different; but in some cases the latter may be prejudged from the former. The larvæ of many of the genus _Sphinx_ L., for example, have their sides marked by oblique streaks running from the back in a direction towards the head; and by this last circumstance they are distinguished from those of _Bombyx versicolor_, _Attacus Tau_, and others of the same tribe, which have also lateral oblique striæ, but running from the back towards the tail[457]. The colours of individual larvæ of the same species are usually alike, but in _Sphinx Elpenor_ and some others they vary exceedingly. Many, like those of _Lasiocampa Rubi_, _Saturnia minor_, &c., are of one colour when first disclosed, and assume others quite different in riper age. Just previously to _changing their skin_, the tints of most larvæ become as dull and obscure, as they are fresh and vivid when the change has fully taken place; and in some instances the new skin is quite differently marked from the old one. This is remarkably the case with the last skin of some of the larvæ of the genus _Tenthredo_ L., which is entirely different from all the preceding ones. As people when they advance far in years usually become more simple in their dress than when they were young, so the larvæ in question change an agreeably variegated skin for one of a uniform and less brilliant colour[458]. Madame Merian has observed with respect to _Attacus Erythrinæ_, that its caterpillar is at first yellowish, with nine black striæ on each side: when arrived at one third of its size, they become orange; the striæ are obliterated, and in their place a round black spot appears on each of the eight intermediate segments[459]. Mr. Sheppard has remarked to me, that the skin of that of _Sphinx Ligustri_, after being under ground four days, was changed from a vivid green to a dull red. Very rarely, however, it becomes of a more brilliant hue just before entering the pupa state: thus, that of another hawk-moth (_Smerinthus Tiliæ_) changes to a bright violet; and the yellow hairs of that of _Laria pudibunda_ then become of a lovely rose colour. And here I may observe, that the hairs and spines also, of larvæ, vary greatly in colour. They are to be met with brown, black, red, yellow, violet, white, &c. De Geer found, that in the larva of _Cimbex nitens_ the two sides of the body were of a different colour, the left being of a deep green, whilst the right side and the rest of the body were paler[460]; but as he saw only a single individual, this was probably an accidental circumstance. Though the caterpillars, as I lately said, of one of the most beautiful butterflies and moths that inhabit Britain contend with the perfect insect in loveliness, yet in general no judgement can be formed of the beauty of the future fly from the colour of the larva; and the young Aurelian must not flatter himself always with the hope, because the caterpillar excites admiration by its colours and their arrangement, that the butterfly or moth it is to produce will do the same; nor ought he to despise and overlook a sombre or plain-coloured individual of the former, under the idea that it will produce one equally plain of the latter, for it often happens that the splendid caterpillar gives a plain butterfly or moth, and _vice versâ_. De Geer, however, gives us two instances of conformity between the colours of the caterpillar and those of the future moth; the one is that of the common currant-moth (_Phalæna G. grossulariata_ L.), the caterpillar of which is white, ornamented with several black spots varying in size. At the two extremities it is yellowish, with a longitudinal ray of the same colour on each side, the head and legs being black. These colours are all to be found in the fly, the ground of its wings being white ornamented with many black spots of different sizes. Its upper wings are traversed by a yellowish band; and towards their base is a spot of the same colour. Its body is yellowish, with black spots; but the head and legs are black[461]. The other is that of a green caterpillar, which gives a green moth, figured by Reaumur (_Pyralis prasinaria_ Fab.)[462]. Sometimes, also, the sex of the future perfect insect may be predicted from the colour it exhibits in its first state: thus, the brown caterpillars of _Noctua Pronuba_ produce males, and the green ones females[463]. The sexes, also, of _N. exoleta_ and _Persicariæ_ differ in that state.

vi. To the full account of the _Food_ of insects given in a former letter[464], which had reference chiefly to their larva state, it is only necessary in this place to add a few particulars not there noticed. Many larvæ when first excluded, as those of _Pieris Cratægi_, &c. devour the shells of the eggs from which they have proceeded[465]; and others (_Cerura Vinula_, _Sphinx Euphorbiæ_, _Noctua Verbasci_), though their usual food is of a vegetable nature, eat with great apparent satisfaction the skins which they cast from time to time, not leaving even the horny legs. This strange repast seems even a stimulating dainty, which speedily restores them to vigour, after the painful operation by which they are supplied with it. Under this head it will not be out of place to mention, that some larvæ of insects, which feed only on the juices of animals, or the nectar and ambrosia of flowers, have no anal passage, and of course no feces. This is said to be the case with the grubs of bees, wasps, the larvæ of _Myrmeleon_, &c.[466]

vii. You will require no stimulus to induce you to attend to the subject I am next going to enter upon,--the _Moulting_, namely, of Larvæ; or their changes of skin. This, indeed, is a subject so replete with interest, and which so fully displays the power, wisdom, and goodness of the Creator, affording at the same time such large occasion for nice investigation, that a pious and inquisitive mind like yours cannot but be taken with it. In the higher orders of animals, though the _hairs_ of quadrupeds and the _feathers_ of birds are in many cases annually renewed, the change, or scaling and increment of the _skin_, is gradual and imperceptible; no simultaneous rejection of it, in which it is stripped off by the animal itself like a worn shirt, being observable, till you descend in the scale to the Serpent tribe[467], which at certain periods disengage themselves from their old integument, and start forth with that new and deadly beauty so finely described by the Mantuan bard:--

"So from his den, the winter slept away, Shoots forth the burnished snake in open day; Who, fed with every poison of the plain, Sheds his old spoils and shines in youth again: Proud of his golden scales rolls tow'ring on, And darts his forky _tongue_[468], and glitters in the sun." PITT.

In these the new skin, I imagine, is formed under the old from the _rete mucosum_; but in _insects_, as I formerly stated[469], since the time of Swammerdam it has generally been believed by entomologists, that the larva includes a series of cases or envelopes, one within the other, containing in the centre the germe of the future perfect insect, whose development and final exclusion take place only when these cases have been successively cast off. This hypothesis, as was explained to you on a former occasion[470], has been controverted by a late writer, Dr. Herold; who affirms that the skins of caterpillars are also successively produced out of the _rete mucosum_. I have however, I hope, satisfied you that the old system is most consonant to nature and probability: but as we are now to enter at large upon the _Moults_ of insects, it will not be without use if I add a few additional reasons which seem to me still further to prove the correctness of Swammerdam's system, as far as it relates to that subject. With regard to the mere formation of the skin from the _rete mucosum_, were this the whole question few would hesitate to adopt the sentiments of M. Herold; but when we come to consider further--that the number of moults of individuals of the _same_ species is always the same, and that it varies in _different_ species, and takes place at certain periods,--we begin to suspect that something more than the mere formation of a new skin upon an old one being cast is to be accounted for; and that the law which prescribes its own definite number of skins to each species, must begin to act in the primordial formation of the larva. Again, the _hairs_ observable in the higher animals do not take their origin from the epidermis solely, but are planted below it in the _rete mucosum_, or deeper[471]; so that the change of skin does not affect them; but in the larvæ of insects they are a continuation of that integument, since, when the moult takes place, they always remain on the rejected skin[472]: this is the case, also, even with spines. If you shave a caterpillar ready to change its skin, either partially or generally, you will find that the parts in the new skin that correspond with those that are denuded, are equally hairy with those that were not[473]; and if you pay attention to the new-clad animal, you will find further, that the hairs never grow after a moult. From hence it follows, that the _hairs_ have their place and take their whole growth between the new skin and the old[474]. Whether the _spines_, simple or compound, lately described to you, that arm some larvæ are similarly circumstanced, seems not as yet to have been ascertained; but as the spinous ones of certain _Tenthredines_ L. and _Lepidoptera_ at their last moult have no spines, the presumption is, that, whether incased or not, they are mere appendages of the skin on which they appear. A new set of hairs, therefore, and probably of spines in spinous larvæ, accompanying each skin, and these varying very much in size, composition, &c. though a new membrane may be admitted to be formed from an action in the _rete mucosum_ without a pre-existent germe of it, it seems not easy to conceive how these hairs and spines can spring up and grow there, each according to a certain law, without existing previously as a kind of _corculum_ or _punctum saliens_; and that the germes of the tubercles, in which the hairs are so generally planted, according to a certain arrangement and in a given number, should also pre-exist, seems most consonant to reason. These and the several skins may all co-exist in their primordial germes, and remain beyond the discovery of our highest powers of assisted vision, till a certain period when they may first enter the range of the microscope-aided eye. It does not therefore follow, because these _primordia semina rerum_ are not discoverable, that therefore they may not exist. Our faculties and organs are too limited and of too little power to enable us to see the essences of being.

Upon the supposition that the hypothesis of Swammerdam is the true one, we may imagine that the envelope that lies within all the rest is that which covers the insect in its pupa state. Above this are placed several others, which successively become external integuments. These changes or casting of the skin in larvæ, analogous, as before observed, to that of serpents, are familiar to every breeder of silk-worms, in which _four_ such changes occur: the first at the end of about _twelve_ days from its birth, and the three next each at the end of _half_ that time from the moulting which preceded it. With some exceptions[475], similar changes of the skin take place in all larvæ, not however in the same number and at the same periods. Most indeed undergo this operation only three or four times; but there are some that moult oftener, from five up to eight (_Arctia villica_), nine (_Callimorpha Dominula_), or even ten times; for so often, M. Cuvier informs us, the caterpillar of the tiger-moth (_Callimorpha Caja_) casts its exuviæ. It has been observed that the caterpillars of the day-flying _Lepidoptera_ (_Papilio_ L.) usually change only _three_ times, while those of the night-flying ones (_Phalæna_ L.) change _four_[476]. The periods that intervene between each change depend upon the length of the insect's existence in the larva state. In those which live only a few weeks or months, they are from eight to twenty days; while in those that live more than a year, as the cockchafer, &c. they are probably proportionably longer: though we know very little with regard to the moult of any insects besides the _Lepidoptera_.