But there is a phenomenon in the life of caterpillars which we ought to point out, and which has attracted the attention of the most illustrious observers. All caterpillars change their skins many times during their life. It is not, indeed, enough to say that they change their skins; the skins or cases they cast are so complete that they might be taken for entire caterpillars. The hairs, the case of the legs, the claws with which the legs are provided, the hard and solid parts which cover the head, the jaws--all these are found in the skin which the insect abandons. What an operation for the poor little animal! This labour is so enormous, so troublesome, that one cannot form a just idea of it. One or two days before this grand crisis, the caterpillar leaves off eating, loses its usual activity, and becomes motionless and languid. Their colour fades, their skin dries little by little, they bow their backs, swell out their segments. At last this dried-up skin splits below the back, on the second or third ring, and lets us have a glimpse of a small portion of the new skin, easily to be recognised by the freshness and brightness of its colours. "When once the split has been begun," says Réaumur, "it is easy for the insect to extend it; it continues to swell out that part of its body which is opposite the slit. Very soon this part raises itself above the sides of the split; it does the work of a wedge, which elongates it; thus the split soon extends from the end or the commencement of the first ring as far as the other side of the end of the fourth. The upper portion of the body which corresponds to these four rings is then laid bare, and the caterpillar has an opening sufficiently large to serve it as an egress through which it can entirely leave its old skin. It curves its fore part, and draws it backwards; by this movement it disengages its head from under its old envelope, and brings it up to the beginning of the slit; at once it raises it, and puts it out through this slit. The moment afterwards it stretches out its fore part and lowers its head. There now remains for the caterpillar nothing but to draw its hinder part from the old case." This excessively laborious operation is finished in less than a minute. The new livery which the caterpillar has just put on is fresh and bright in colour. But the animal is exhausted by its fast, and the efforts which it has made. It requires a few hours in which to regain its equilibrium, and at the same time its former activity and voracity. THE CHRYSALIS, OR PUPA. Having attained its full development, the caterpillar ceases to eat, as at the approach of a moult, it empties its intestinal canal by copious ejections; it loses its colours, and becomes dull and livid, and thus prepares itself to enter a new phase of its existence.