During the winter, when the leaves are not on the trees and shrubs which are frequented by these larvae, a large number of cocoons may easily be collected. These should be kept out of doors during the winter, for if kept in a warm room they will emerge during the winter or so early in the spring that food cannot be secured for the larvae.

If one secure a number of old cocoons, from which the moths have failed to emerge, and cut them open longitudinally, he may learn many interesting facts. A dead and dry mummified looking larva or chrysalis may be found, or, what is even more interesting, no trace of the larva or chrysalis may be present, but only a mass of small white, paper-like cocoons. These have been left by a colony of little wasp-like parasites which may occur in such large numbers that there is scarcely room for all to spin their cocoons, so that on account of being so closely crowded together, they are moulded into a mass of cocoons, having the form of the cavity formerly occupied by the larva.

The cocoons of Cecropia are composed of two parchment-like layers of silk which are generally very dense and strong. The space between these two layers contains loosely spun threads of silk like a layer of packing material. The larvae seem normally to make three varieties of cocoons; one kind is very loosely constructed, much larger than the ordinary form and not attached to a twig, but found in the grass or in shrubs near the ground. The two other forms of cocoons are much smaller and more closely woven, but differ in size; female moths as a rule emerging from the larger cocoons, and males from the smaller ones.

Dead larvae are sometimes found in cocoons which are practically of a single thickness; there being no space between the outer and inner layers. The hollow skins of the larvae found in such cocoons clearly show that this unusual cocoon is due to the influence of parasites upon the larva.

In the upper open end of the cocoon, kernels of wheat, corn, beechnuts and even acorns have been found. How these get in this position seems to be quite a puzzle. In opening twenty or thirty cocoons, five or six kernels of corn have been found, thus showing that this occurrence is by no means rare. Chickadees and blue jays have been given the blame for this work, since these birds are thought to have the habit of hiding food. The inverted outer layer of the cocoon clearly shows, in some cases, that the kernel of corn has been thrust into the cocoon with some force.

The head of the pupa lies at the small end of the cocoon, where the texture is less dense, and thus, when it is ready to transform into the moth, the head is in the best position for easy escape from the cocoon. But this provision alone is not sufficient to make sure the escape. At the time of emergence, the pupa secretes a fluid which escapes from the mouth and by moistening the cocoon softens the glue-like material which binds together the threads, thus making it possible for the freshly emerging moth to crowd its way between the fibres, and thus secure its freedom. When the moth first crawls out of the cocoon, its heavy body and small folded wings show but little resemblance to the fully expanded moth. By degrees, however, the wings expand and become more rigid, the colors brighten, and finally the mature moth is developed.

The Promethea Moth is only about one-half the size of Cecropia, and the two sexes are very different in appearance; so much so that one would not at all think they were the same kind of moths, As in Cecropia the male moths are somewhat smaller than the females, and the antennae show the same kind of differences, i. e., the antennae of the males are much larger and feather-like. In color, the sexes of Cecropia are much alike, but in this moth the differences in color are very great, the dominant color in the female being a reddish brown, while that in the male is a very dark-brown or almost black. Thus these moths furnish an excellent illustration of what is called sexual dimorphism, a term used for those animals in which the sexes are very different in appearance, a subject to which Charles Darwin gave considerable attention, in his "Descent of Man."