In order to give a sufficient degree of strength to their cocoons, some larvæ adopt the plan of forming them partly of silken fibre and partly of other materials. Réaumur, being on an entomological excursion in the forest of Vincennes on a fine day in the month of May, fell in with a most curious cocoon, formed by a larva which feeds upon the oak-tree. At first he could scarcely conceive what the object he saw before him was, but on removing the branch it proved to be a very singular cocoon. Detaching a small branch, on which was a larva just commencing its cocoon, Réaumur, finding the evening draw on, set out on his return home. The larva did not seem to discover the fact of its being carried away a prisoner; and as the entomologist carried the branch with gentleness, it proceeded with its task as comfortably as if stationed on the tree it was now leaving behind. Réaumur beguiled the tedium of the way by carefully watching its mode of proceeding in the formation of its cocoon. He thus noticed the larva cutting very delicate layers of the fine upper skin or epidermis of the bark of the twig, which it fastened together by silken cords, and then formed into two wing-like pieces fastened on each side of the twig in the manner represented in the cut, something like the feathers of an arrow in appearance. And now, this being done, the difficulty was to draw the two sides together, and so to convert them into a covering for its body. The manner in which this was done will hardly, perhaps, be imagined. It will be best understood if the reader will cut out two pieces of paper of the shape of these two side portions, fasten them on with a little glue by one of their edges to a piece of twig, and then, by a fine needle and thread, draw them together by stitching from side to side all the way up. The larva does something very like this, for it fixes silk cords to each of the outer edges, and then pulls and hauls with all its might until it has forcibly bent over the layers until they meet, and then it ties the edges together by short cords, so as to produce a seam so beautifully close, even, and fine, as would put the "fine-drawing" of the most superlative tailor to shame; in fact, the join is frequently quite imperceptible. Having done this, it lines the inside with beautiful tapestry of silk. By the time Réaumur and his friends had got to the end of their walk, which occupied an hour and a little more, the larva had, after vast labour, joined together its seams, and far advanced toward the completion of its task.
Réaumur made also some most interesting observations upon a larva which constructs its cocoon of a sort of silken mesh, the interstices of which are filled with grains of earth. Having broken off the top of a cocoon he witnessed the singular spectacle of the larva proceeding to mend it again. Without leaving its cell, the patient labourer put its head out of the opening in the attitude represented in the cut, and, after looking about for a little time, picked up, one by one, a number of grains of earth, which it stored up in its case. After this, it filled up the sides of the opening with a net-work of silk, working the grains of earth into it as it proceeded. For three hours the larva worked incessantly at its task, and at the end of that time had materially reduced the size of the opening. Réaumur was now curious to observe how the opening still left would be filled up, as the insect could no longer put its head out, and he expected to see it filled up with a mesh of silk. But he had not given the larva due credit for ingenuity in making this supposition. It filled up the opening first, with a mesh of silk, but between the meshes, in a most curious manner, it thrust out some little grains of earth, which it had previously stored up, until they actually appeared on the outside, and any one would have supposed they had been laid on from without. It finally ended its labours by coating the inside of the opening also with a layer of earth. Réaumur afterwards cut the cocoon in half with a knife, and by that means was enabled to see how successfully the larva had repaired the injury.
We must now speak briefly of the Suspension of the Larva; and it may be mentioned, that the insects, whose larvæ perform this manœuvre, are almost exclusively the butterfly tribe. There are various ways of effecting this object. The spinning apparatus, by which the cords are to be formed to sustain the body of the insect in the air, is situated in the mouth of the larva, and it may well be imagined that the insect, which is about to hang itself up by the tail, has no easy task to perform in having to fasten its cords to the tail, and then to the branch above it. If a spider wished to hang itself up by the tail it would be a very easy thing so to do, for the spinning apparatus is placed there, and it has only to let itself fall from a branch after first glueing the end of the cord to it; but a larva is differently circumstanced, and it requires no slight display of ingenuity to accomplish its purpose.
Its first process is to select a suitable leaf or twig, upon which it weaves a sort of little mound of silk of the shape of a button. This done, it examines it carefully to ascertain its strength, and satisfied therewith, it then proceeds to thrust its two hind legs in amongst the threads of which the button is composed, and in so doing it causes the hook-like processes which fringe these legs to become securely entangled in them. It then is safe as to the issue, and now lets its body drop down, fearless of the most violent rockings to which even a tempest might expose it.
Another way of suspending themselves is almost equally singular with this. It may be witnessed by the reader if he will take the pains to collect the larvæ of the common white butterfly, found in abundance upon our cabbage rows; and by properly attending to, feeding and watching them, this curious proceeding may generally be observed, in a few at least of the number, supposing that the others may have hung themselves up without being seen in the very act. If we were to set a mountebank to the task this larva has to perform, it may well be doubted whether with the richest reward before him he could manage to effect it. First, he would have to tie both his feet fast to a branch, so that he would swing head downwards; then, in order to hang himself in the horizontal posture, he must bend his body up, fasten a cord round the branch in a proper place, so as to form a loop, and then put his head and body into it, so that it would support him comfortably just under the arms. Probably the most supple-jointed gentleman would find himself discomfited in the attempt. The caterpillar in question, however, does all this, and more; for it has to spin the ropes with which it is to be suspended. Fixing its hinder part, in the manner just described, to the little button of silk, which it first forms, it then spins the girth, in which it intends to trust its safety, by the apparatus of the mouth. Sometimes larvæ tie themselves in an upright position, as martyrs to a post; but the most common method is the horizontal one, giving the insect the resemblance of a sailor swinging in his hammock.
Horizontal Suspension.
Perpendicular Suspension.