Volume 1 — LETTER XXX. (continued 3)

All these cocoons, however, must yield in point of singularity of construction, materials, and ingenuity, to one formed by a small caterpillar, described by the illustrious naturalist lately quoted, which feeds upon the oak. This cocoon is wholly composed of small rectangular strap-shaped pieces of the fine upper skin, or epidermis of the twig upon which it rests, regularly fastened to each other in a longitudinal direction with very slender silken cords. But the mode of its construction is even more remarkable than the substance of which it is fabricated. The caterpillar's first process is to form its slips of bark into two flat triangular wing-like pieces, projecting opposite to each other from each side of the twig, somewhat like the feathers of an arrow. It does not, perhaps, require any great degree of intelligence in a larva to give its cocoon the usual oval form, when it begins to arrange its materials in that shape from the very first, and round so good a mould as its own bent body; but we surely must admit that it is a task to which no stupid artist would be competent, to form first a multitude of strap-shaped laminæ into two triangular plates, and then to bend these plates into a case resembling the longitudinal section of a cone, with an elliptical and protuberant base,—the figure which the cocoon of this insect assumes. All the minutiæ of the manœuvres which it employs in this nice operation could not be comprehended without a more diffuse explanation than I have here room to give: suffice it to say, that the caterpillar fastens silken lines to each exterior opposite and longer side of the laminæ, and by applying all the weight of its body forces them to bend and approach each other, in which position it secures them by other shorter lines. It next repeats the same process with the upper and shorter sides of the plates; which when joined form the base of the cocoon. Both these tasks are accomplished in less than an hour, and the seams are so nicely joined as to be imperceptible. A fine inner tapestry of silk, covering all the asperities of the exterior walls, concludes its labours[544]. It is to be lamented that Reaumur was unacquainted with the moth that proceeds from the pupæ inclosed in these ingenious cocoons; which being small, and precisely of the same colour as the bark of the twig that supports them, are not to be discovered but by a very narrow inspection. It would seem, however, to be Noctua Strigula of Berkhausen, Pyralis strigulalis of Hubner[545]. The larva, he informs us, is found in May: its body is flatter than common, of a yellowish flesh-colour, clothed with tufts of red hair on each segment, and furnished with fourteen feet. Should this description enable you to detect it upon your oaks, a view of its ingenious procedures would amply repay you for the trouble of seeking for it. The larvæ of Cerura vinula, Stauropus Fagi, and several other moths, form their cocoons of grains of wood gnawed from the trees on which they feed. These grains they masticate, mixed with a glutinous fluid secreted from the mouth, into a paste, which forms a covering of an uniform smooth texture, and so hard as not readily to yield to a knife. Of a substance apparently nearly similar is composed the cocoon of a weevil related to Liparus Pini; which with its inhabitant was given me by the ingenious Mr. Bullock. A little moth, whose ravages have been before noticed[546], lines the interior of the grain of barley, of which it has devoured the contents, with silk; divides it into two apartments, into one of which it pushes the excrement it had voided, and in the other assumes the pupa[547].

These, and the other larvæ mentioned above, commonly form their cocoons of the substances I have indicated; but when by any cause they are prevented from access to them, they often substitute such other materials as are at hand. Reaumur fed a larva that formed its cocoon of minute fragments of paper, which with its mandibles it had cut from the piece that covered the glass vessel that contained it[548]: and the same circumstance happened to Bonnet.

Upon a former occasion I described to you the cases of various kinds formed and inhabited by the insects of the Trichoptera Order (Phryganea L.) commonly called case-worms[549]. As these serve for the pupa as well as the larva, they may be regarded as a kind of cocoon. I shall not repeat here what I then said; but having purchased from the collection of the late Mr. Francillon some that seem to belong to this or some cognate tribe, that are of a curious construction, I shall give you some account of two or three of them in this place. The first is not quite three inches long, of a sublanceolate shape, but rather widest towards one end. It consists of an internal tough and thick bag or cocoon, of a silk resembling fine wool of a dirty white colour, which is closely covered transversely by pieces of the stalk of a plant, about three-fourths of an inch in length, and crossing each other at an obtuse angle. The next is thicker and shorter: the internal bag is just covered with small fragments of wood like sawdust; over these are fastened irregularly, short stout pieces of a pithy stick or stalk, and the whole is clothed with a very close-woven ash-coloured web. It seems difficult to conceive how the inclosed animal could contrive to cover her habitation with this web without going wholly out of it. The third is the most curious and remarkable of all. It is nearly six inches long, and about four-fifths of an inch in diameter. It consists of a bag of thick cinereous silk web, to which are fastened, in a sextuple series, pieces of stick about an inch long, the end of one mostly resting upon the base of another: between each series a space of about three-tenths of an inch intervenes, but at the apex they all converge. This probably imitates the branch or stem of some tree or plant, in which the leaves are linear, and diverge but little from the stem. A label upon it states its country to be New Holland. I suspect the inhabitants of the two last cocoons to be terrestrial animals: the first is probably a true aquatic case-worm.

The same purpose for which the cocoons above described serve, is answered in the case of numerous Dipterous insects, by a humble and less artificial contrivance—the skin, namely, of the larva; which, as was before observed[550], is never cast, but, when the insect is about to enter into the pupa state, assumes a different form and colour; becomes of a thicker and more rigid texture; and defends the included pupa, which is separate from it, till its exclusion. In this case the mouth of the larva is constantly different from that of the perfect insect, or at least has not with it those relations as to number and kind of organs, which have been observed in the mouth of other larvæ compared with the insects that they produce. The animal, immediately after it is clothed with this skin, if it is opened, exhibits only a soft gelatinous pulp, in the surface of which the exterior organs of the adult insect cannot yet be detected. Nature requires more time for their elaboration, or at least for the appearance of their outline, and to consolidate them. This pulp first takes an oblong form (Boule allongée Reaum.), and afterwards that of the insect it is destined to give birth to[551]. The skin of the larva also serves for a cocoon to the pupæ of male Cocci[552]. The grub of the genus Anthrenus, so destructive to our cabinets of natural objects[553], when it assumes the pupa does not quit its skin, but only splits it open longitudinally on the back, and when it becomes an imago makes its exit through the orifice[554]. Some Lepidopterous larvæ even (Alucita pentadactyla, Callimorpha rosea, &c.) assume the pupa state within their last skin[555].

When a larva has finished its cocoon,—which with some species, that proceed so earnestly as though they had not a moment to lose, is the work of a few hours, of others about two or three days,—after a certain interval it casts its last skin, which is usually suffered to remain in the cocoon (but which one moth, Geometra lacertinaria, ejects through an opening purposely left in its bottom), and the pupa makes its appearance[556]. This interval is exceedingly various. Most larvæ assume the pupa state within a few days after they have formed their cocoons; but some not for several weeks, or even months. The caterpillar of Bombyx cæruleocephala, according to Rösel, lies three weeks in the cocoon before this change is effected; those of many Pupivora and Diplolepariæ Latr., according to Reaumur, six months[557]; that of Phalæna urticata nine months[558]; and that of Cimbex lutea, according to De Geer, sometimes eighteen months[559]. Brahm observes, that such larvæ of the double-brooded moth, Hepialus Testudo, as form their cocoons in autumn, do not become pupæ until the following spring; while those which form them in summer undergo this change in a few days[560]. From this fact it might be conjectured, that the degree of heat prevailing at the time the insect incloses itself determines the period of the pupa's appearance; but this supposition seems contradicted by what Reaumur observed of a brood of the larvæ of Phalæna urticata, just mentioned, which, though they formed themselves cocoons in September, did not become pupæ till the June following[561]. I am unable, therefore, to assign any plausible cause for these extraordinary variations. The difficulty of comprehending how animals before so voracious can live so long without food may be partly surmounted, by adverting to the circumstance of its having attained its full growth, and laid up a store of nutriment for the development of the perfect insect. It is consequently no more wonderful that it should not have need of any further supply without casting off its upper integument, than that it should not eat after having done so and become a pupa.