Fig. 29.—The Cocoon of the Six-spotted Burnet (Filipendulæ).
We have seen that the Large White Butterfly makes itself secure by a silk band round its middle, while the 'Tortoiseshell' is fixed only by its tail. But the extra provision for the safety of the former is not so necessary in the case of the latter, as it never spends more than two or three weeks in the pupal state. Here it is the perfect insect that braves the winter, and not the chrysalis.
There is a great variety in the means taken by the caterpillars of moths to protect themselves during their metamorphoses, but we shall have space for only a few illustrations.
A clever cocoon is spun by the larva of the Emperor Moth (Pavonia). It is pear-shaped, and composed of a brownish silk; and is so constructed that the newly emerged moth can easily walk out of the small end without breaking a fibre, while the entry of an insect enemy from without is impossible.
This is managed as follows. A number of rather stiff threads are made to project from the small end of the cocoon, and these converge as they pass outward so that the ends are all near
together. The other portions of the cocoon are of compact silk, and any insect intruder that ventures to enter by what we may almost term the open end is met by a number of spikes, as it were, that play on it at every attempt. Many of these wonderful cocoons may be found during the winter months attached to the food plants of this insect.
Of the silken cocoons spun by various caterpillars some are so thin and light that the chrysalis can easily be seen through them, and others are so densely woven as to be quite opaque. A great difference is also to be observed in the adhesive power of the silk fibres. In some cases little threads of silk can be pulled off the cocoon; but some of them, that of the Oak Eggar ([page 229]) for example, look as if they had been constructed of paper rather than of silk, because, at the time of spinning, the moist silk fibres stuck so closely together.
An extreme case of this character is to be met with in the cocoon of the Puss Moth ([page 235]); for here the fluid from the spinneret of the caterpillar does not harden at once on exposure to air, and so the threads become thoroughly united together, thus forming a solid gluey cocoon.
When the Puss caterpillar is about to change, it descends the tree (poplar, willow, or sallow) till it is within a few feet of the ground. Then it commences gnawing away at the bark, at the same time cementing all the pieces together with the gluey substance from its spinning glands. In this way it surrounds itself with a very hard cocoon, which so closely resembles the surrounding bark in colour that detection is difficult indeed.