The general rule is,—that each larva spins for itself a separate cocoon; but amongst those of Arctia chrysorhea and others which live in society, two or three sometimes begin their operations so close together that they are under the necessity of forming one common cocoon, which serves for a covering to the whole number. The same thing happens to silk-worms, the double or treble cocoons of which are called Dupions by the breeders. The larvæ of some Ichneumons, besides forming each its separate cocoon, spin a joint cottony covering for the whole[508], which is effected thus:—After they leave the caterpillar they have devoured, they fix themselves side by side at a little distance from it, and begin to spin each a cocoon; and in order to defend its end and side that is not covered by others, they spin further an envelope of loose silk, and thus this exterior covering is formed.
The size, figure, colour, substance, and texture of silken cocoons are extremely various. Their size indeed is usually proportioned to that of the included larva or pupa; yet it is by no means always so. Some large caterpillars spin cocoons so small, that the observer can hardly conceive how they can be contained in so narrow a compass: Eriogaster Catax is a moth of this description[509]. And others smaller in size lodge themselves in apartments apparently much more spacious than necessary. The transparent hammock-like cocoons of Hepialus Humuli and Arctia villica, two other moths, would contain several of their pupæ. I possess one in which the pupa is suspended in the centre, that is ten times its size, and not very short in dimensions of that of Attacus Paphia, a giant silk-moth. The largest cocoon I ever read or heard of, is that thus described by Mr. Hobhouse in his Travels: "Depending," says he, "from the boughs of the pines, near the Attic mountain Parnes, and stretching across from tree to tree so as to obstruct our passage, were the pods, thrice as big as a turkey's egg! and the thick webs of a chrysalis, whose moth must be far larger than any of those in our country."[510] If this statement is correct, and I am not aware that there is any reason for doubting it, the cocoon must be vastly larger than the pupa, or the moth it produced would far exceed in size any yet known. Perhaps, however, as this gentleman is probably no entomologist, what he took for a cocoon might be a nidus, in which many larvæ were associated, of the nature of those formerly described[511].
With regard to figure, the majority are like those of the silk-worm, of a shape more or less oval or elliptic: some, however, vary from this. That of Lasiocampa Rubi is oblong. I have one from New Holland somewhat resembling an acorn, fixed to the twigs of some tree or shrub, of a very close silk, and covered by a circular operculum, which the animal pushes off when it assumes the imago; this is ovate or conico-ovate; others again are globose[512]; others are conical[513], as that of Gastropacha quercifolia; others almost fusiform[514] (Odenesis potatoria). Reaumur received one from Arabia which was nearly cylindrical[515]. Those of T. prasinana before noticed, and many other Tortrices, are shaped like a reversed boat[516]; that of Saturnia Pavonia and others of the same tribe, like a Florence flask with a wide and short neck. The cocoon of Lygæna Filipendulæ resembles a grain of barley. Another cocoon in my cabinet, of very slight network, is shaped something like an air-balloon. But the most remarkable one for its form and characters, is one that I received from the rich cabinet above quoted. This, which is of an unusually close texture, is suspended by a thread more than two inches long from the point of a leaf; it then swells into a perfect cone, at the base about four-fifths of an inch in diameter and half an inch in length, and covered with scattered setiform appendages: from the centre of the base projects a large hemispherical protuberance, which terminates in a long stalk, much thicker than the thread that suspends the cocoon. There is commonly no difference between the shape of cocoons spun by larvæ which are to give birth to different sexes of the same species. The silk-worm cocoons, however, which will produce male moths, have more silk at the ends, and consequently are more round than those which are to produce females: but the difference is not striking.
The most usual colour of silken cocoons is white, yellow, or brown, or the intermediate shades. The whites are very pure in the general envelope of some species of Ichneumonidæ, and yellows often very brilliant. But besides these more general colours, some cocoons are black[517], some few blue or green, and others red[518]. Sometimes the same cocoon is of two different colours. Those of certain parasites of the tribe of Ichneumones minuti L. the motions of one of which I noticed on a former occasion[519], are alternately banded with black or brown and white, or have only a pale or white belt in the middle, which gives them a singular appearance. In both cases the difference in colour depends upon the different tints with which the silky gum is imbued in the reservoirs: the first portion of it is white, and with this the larva first sketches the outline of its cocoon, and then thickens the layers of silk considerably in those parts where the white bands appear: when these are finished, its stock of white silk is exhausted, and the remainder of the interior of the cocoon is composed of brown silk[520]. The circular operculum above mentioned as covering an acorn-shaped cocoon, is paler than the latter, and also ornamented by a zone within the margin of deep brown. The pale cocoon also of Attacus Paphia is veined with silk of a deep red.
I have very little to say on the substance of the silk of cocoons. Though that of the silk-worm is composed of such a slender thread, that of many others is still finer, scarcely yielding in tenuity to the spider's web. On the other hand, the silk of the cocoons of Saturnia Pavonia and of several foreign species is as thick as a hair.
With regard to the texture of their cocoons—in some, as in that of the silk-worm, the threads are so slightly glued to each other, as to separate with facility; but in that of the emperor-moth just mentioned they are intimately connected by a gummy matter, furnished, as Reaumur conjectures, from the anus[521], with which the whole interior of the cocoon is often plastered. Some, as that of the silk-worm, are composed of an exterior loose envelope, and an inner compact ball; others have no exterior covering, the whole cocoon being of an uniform and thick texture. The larva of Cossus Robiniæ Peck, in spinning its cocoon, makes the end next the opening to the air, by which the imago is to emerge, of a slighter texture than the rest of it[522]. The exterior case is sometimes, as in Laria pudibunda, very closely woven, so as to resemble a real cocoon[523]: its form is usually adapted to that of the inner one; but in some which fix them under flat surfaces (Laria fascelina, Callimorpha Caja,) it resembles a hammock[524]. Cocoons of a close texture have generally no orifice in any part; but that of Eriogaster lanestris is spun with openings, as if bored from without, the use of which, however, does not seem to have been ascertained[525].
Many silken cocoons are of so close a fabric, as, when finished, entirely to conceal the included insect; but a very considerable number are of a more open texture, composed of a much smaller quantity of silk, and that woven so loosely, that the larva or pupa may always be discovered through it. Of this description are the cocoons of Hypogymna dispar, Arctia Salicis, &c., which consist only of a few slight meshes. Those of some others resemble gauze or lace[526]. Of the first description is one in my cabinet before alluded to, shaped somewhat like an air-balloon; the meshes are large and perfectly square. The pupa hangs in the centre, fixed by some few slight threads which diverge from it to all parts of the cocoon—so that it looks as if it was suspended in the air, like Mahomet's coffin, without support. Of the second description is a black one with very fine and nearly circular meshes: the threads that form these are thick, and seem to be agglutinated. In our own country, the cocoons of some beetles, as of Hypera Arator, Galeruca Tanaceti, and of some little Tineæ, also resemble gauze. Many of the larvæ, however, which spin these cocoons, whose thinness is probably attributable to the smallness of their stock of silk, seem anxious for a more complete concealment; and therefore commonly either hide them between leaves tied together, in some with a certain regularity, in others without art[527]; or thicken their texture, and render it opaque, by the addition of grains of earth[528], or of other materials with which their bodies supply them. These are principally of two kinds. The larvæ of Lasiocampa Neustria, Arctia Salicis, &c. after spinning their cocoons, cast from their anus three or four masses of a soft and paste-like matter, which they apply with their head all round the inside of the cavity; and which, drying in a short time, becomes a powder that effectually renders it opake. This is not, as might be conjectured, an excrement, but a true secretion, evidently intended for this very purpose: and, according to Reaumur, a similar powder, but white, derived from the varicose intestines, is used by the caterpillars of Gastropacha quercifolia, &c.[529] The other material, which is still more frequently employed, and which is occasionally mixed with the former, is the hair which everyone has observed to cover so thickly the bodies of some caterpillars. This, after spinning a sufficient envelope, they tear, or in some instances cut off with their mandibles, and distribute all round them, pushing it with their head amongst the interstices of the silk, so as to make the whole of a regular thick texture. After this process, which leaves the body completely denuded, and often seems to give them great pain, they conclude by spinning another tissue of slight silk, in order to protect the forthcoming pupa from the surrounding prickly points. It should be observed, however, that though many hairy larvæ, as those of Noctua Aceris, Arctia Caja, and others, employ their hairs in the composition of their cocoons, the rule is not general, several never making any such use of them. Nor do all that do so employ them distribute them in the same manner as those above described, which rarely attempt to arrange them in any regular position. Reaumur has noticed a small hairy caterpillar that feeds on lichens, which is more methodical: this actually places its hairs upright, side by side, as regularly as the pales in a palisade, in an oval ring around its body, connecting them by a slight tissue of silk, which forces them to bend into a sort of roof at the top; and under this curiously-formed cocoon assumes its state of pupa[530]. Some larvæ make so much hair and so little silk enter into the composition of their cocoons, that on the first inspection they would be pronounced wholly composed of it[531]; others, thickening the interior of their cocoon with hair, line the whole with a viscid matter like varnish[532].
The larvæ of some saw-flies (Tenthredo L.) are remarkable for inclosing themselves in a double cocoon, in which the inner is not, as in the silk-worm &c., connected with the outer, but perfectly distinct from it. Some species, as T. Rosæ (Cryptus Jur.), which have but a small stock of silk, compose the outer cocoon of thick silken cords crossing at right angles, and forming an oval net; which at the same time that it protects them effectually from the ants, which are always ready to attack them, demands much less silk than a covering of a closer texture. But the tender nymph itself requires to be inclosed in a case of a softer and more delicate substance; and accordingly the inner cocoon is composed of fine silk, woven so closely that the threads are scarcely perceptible under a microscope[533]. Reaumur mentions a hymenopterous larva belonging to Latreille's Fossores (Sphex L.) which thickened its cocoon with the legs, wings, and other relics of the flies which it had devoured[534]: trophies—like the drinking-cups of some savages, made of the skulls of their enemies, or the skull pyramid near Ispahan—of its powers of devastation.
It is a general rule, that those larvæ which spin cocoons, never in ordinary circumstances become pupæ without having thus inclosed themselves. An exception, however, is met with in the larva of a species of ant noticed by De Geer (Formica fusca L.), some of the individuals of which inclose themselves in cocoons; while others neglect this precaution, and undergo their metamorphosis uncovered[535]. Rösel also made nearly the same observation on the larva of the flea[536].
I must say something with regard to the situation, often very remote from their place of feeding, in which larvæ fabricate their cocoons. A very considerable number, probably the majority, form them either partially (Arctia lubricipeda) or wholly under ground; others beneath dead leaves, moss, or in the chinks of the trees; others within the wood in substances on which they have fed; the larva of Cossus leaves in these a communication with the open air by which the imago emerges; and a large number attach them to the leaves and branches of trees and plants; the cocoon of Donacia fasciata(?) is fastened by one side to the roots or surculi of Typha latifolia. There is usually nothing very remarkable in the mode of fixing them, the exterior threads being merely gummed irregularly to different portions of the objects which support them. But some effect this with greater art. I have one from New Holland, very long, which is suspended from a twig by a long riband, as it were, which entirely girths the twig. The larva of the magnificent silk-moth, Attacus Paphia, actually forms a solid silken stalk to its cocoon, an inch and half in length and a line in diameter, fastened by the other extremity to a twig, which it closely surrounds as if with a ring, at first sight resembling a fruit of a very singular appearance[537]. I have specimens of this cocoon with both stalk and ring. A bell-shaped cocoon fastened by a foot-stalk, but of softer consistence, to a blade of grass, found by Mr. Sheppard, I can also show you; and my friend Mr. Wilkin had a similar one out of the late Mr. Hudson's collection. Most larvæ spin their cocoons in solitude: some of those, however, which live in society do it close together under their common tent.