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." The female moth lays her cream-colored eggs, which are a little smaller than those of Cecropia, upon shrubs and trees in clusters of five or six. The small larva usually hatches in about ten days, and feeds upon the leaves of ash, sassafras, lilac, tulip tree, maple, cherry, and a number of other trees and shrubs, but it is much more select in the choice of its food than Cecropia. The larvae have voracious appetites, devour many leaves and grow at a correspondingly rapid rate. The differences between the very young and the adult larva, aside from that of size, are very great. On account of the very limited elasticity of the skin, this larva, like other insect larvae, only increases in size after shedding. This is periodically accomplished by throwing off the old skin, which prevented expansion, and by growing a new and larger one. Promethea has from three to five of these moults, the number being influenced apparently by climate, since southern larvae have more moults than northern ones. The time between these moults varies from two days to a week.