It may be thought that the strongly falcate shape of the anterior wings, in many of the Saturniæ, would afford a good mark of distinction; and in certain instances such may be the case. But this is a character that requires to be used with caution, for in many cases it is only a sexual distinction. A careful comparison of the two sexes in all the species we can find either figured or described, has convinced us that there are few genera in which the male and female present such marked differences. The male, as is usual among insects, is much smaller than the female, and the whole outline of his form is, so to speak, comparatively contracted. Hence there is almost always a strong tendency in the exterior line of the upper wings to be curved inwards, or assume a falcate shape. The outline of the female, on the contrary, is comparatively full, and the tendency of the line in question is rather in an opposite direction. The species, accordingly, in which the fore wings of the male are most decidedly falcate, have this form much less strongly marked in the female; where the former are not very strongly falcate, in the female they become subfalcate (H. Promethea may serve as an example); while the females of subfalcate winged males have the exterior outline of their fore wings either straight or slightly curved outwards. Nay, if our views regarding one of the species be correct, there exists an example of the fore wings of the male being strongly falcate, while those of the female are as obviously rounded laterally, describing a convex curve.
