Birds and Nature, Vol. 08, No. 5, December 1900 / Illustrated by Color Photography

by Various

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.

The leaves upon which the larvae feed may have long or short petioles. A singular account has been given of how these larvae have overcome the difficulties associated with feeding upon long-petioled leaves. There is considerable risk of falling and of the leaf breaking away when a large larva crawls out upon a slender petiole. The larva avoids these risks and yet reaches the blade of the leaf. This is accomplished as follows: The larva grasps firmly the branch with its posterior legs; reaches out a considerable distance along the petiole, and bites it through in several places. This causes the leaf to droop; the larva now reaches out, seizes the drooping leaf, and draws it within convenient reach, where it can be eaten at leisure. This is a wonderful display of instinct, yet it is not infallible, because at times the petioles are eaten too far through, and when they droop, break completely away and fall to the ground.

When ready to spin its cocoon, the adult larva is about two inches long; these cocoons are very different from those of Cecropia. As a rule, they are found suspended from a branch by a silken cord, the length of which depends upon the length of the petiole of the leaf in which the cocoon was spun. Thus if the leaf has only a short petiole, this cord is also short, but if the petiole is two or three inches long, the suspensory cord is correspondingly long. The larva in constructing its cocoon, first spins a strong band around a twig, and binds the petiole of the leaf to the stem; this band extends down the petiole to the cocoon, and thus anchors it. The cocoon proper, or the part occupied by the chrysalis, is spun in a folded leaf. When this leaf dies and rots away, the cocoon hangs freely suspended by the cord, but it is very evident that the cocoon has been moulded in a leaf by the prints of the veins which remain upon it. A valve-like opening occurs in the upper end, through which the moth emerges.

The wings of the chrysalis are very small as compared with those of the adult moth; are folded to the body on the under side, and covered by the pupal skin. During the winter they remain transparent since there are at this time none of the rich colors present which are later found in the moth. About ten days before the moth emerges the wings become white, a few days later definite colors begin to appear on the under side of the wings between the veins. While in the adult moths the colors in the two sexes are very distinct, at this time their wings are very similar. The wings do not long retain this similarity, but gradually become more and more unlike until maturity.

Breeders of moths have often noticed that there is considerable uniformity with regard to the time of day at which certain kinds of moths emerge. For Promethea this time seems to be in the forenoon.

From an extensive series of experiments, it has been learned that the male finds his mate by means of scent and that this is doubtless the explanation for the very large antennae of the male, since it is in these organs that the sense of smell is located.

Although it may be very interesting to read about the activities of insects, a much more fascinating side of the subject is to handle and study the insects themselves, and there are but few better insects with which to begin a personal acquaintance than these which we have been considering.

Charles Christopher Adams.