Figs. 122, 123.—Cocoon of Pyralis corticalis (magnified, proper size 2/3 inch).
"When one sees," he says,[39] "an insect which, to construct a cocoon, begins by collecting together an infinite number of small plates of bark in order to compose of them two flat triangular blades; which, to gain its end, takes means that seem so roundabout, although they are the most suitable and the quickest it could adopt, one is very much tempted to consider such an insect, when one sees it thus acting, possessed of reason."
These two blades are at last transformed into a regular cocoon. The little animal, which is at the same time architect, cabinet-maker, and weaver, arranges it in such a way as to form a hollow cone, which it only remains for it to shut. Réaumur calls this sort of cocoon or shell, la coque en bateau, the boat-shaped cocoon. Some caterpillars weave cocoons of the same form with pure silk.
To bring this subject to an end, we will mention the industry of the Puss Moth (Dicranura vinula), and that of a small Tinea, which eats the barley stored away in our granaries.
The larva of the puss moth employs in the construction of its shell the wood of the tree on which it has lived. It bites it up, and mixing it with a glutinous fluid which it secretes from its mouth, reduces it to a sort of paste, which it then uses in the formation of an envelope, of such hardness that a knife can hardly cut into it.
The Tinea lines the interior of a grain, of which it has previously devoured the contents, with a coating of silk, and divides it thus into two different chambers. In one of these it is to change into a pupa; in the other it places its excrement. And so the little careful architect constructs its house in such a manner as to find in it tranquillity, cleanliness, and comfort.
When caterpillars have not within their reach the materials they are in the habit of employing, like good workmen, they content themselves with what they can get. Réaumur reared a caterpillar which formed its cocoon of pieces of the paper of which the box was made in which it was imprisoned.
What an extraordinary condition! what a strange phase of vitality does the chrysalis present to us—a being occupying the middle state between the caterpillar and the perfect insect! How little does it resemble that which it previously was, and what it will become! In appearance it is scarcely a living being; it takes no nourishment, and has no digestive organs; can neither walk nor drag itself along, and hardly bends the joints of its body. The outside skin of the chrysalis appears to be cartilaginous; it is generally smooth, although some species have hairs scattered over their bodies.
We distinguish in chrysalides two opposite sides. The one is the insect's back, the other its under side. On the upper part of the latter ([Fig. 124]) we perceive various raised portions, formed and arranged like the bands round the heads of mummies; the back is plain and rounded in a great number of pupæ; but a great many others have on the upper part, along the edges which separate the two sides, little humps, eminences broader than they are thick, ending in a sharp point ([Fig. 125]).
| Fig. 124. A conical pupa. | Fig. 125. Pupa having angular projections. | Fig. 126. Angular pupa of a Butterfly. |