Of all the organs of the head, none seem so little subject to _sexual_ variation as the under-jaws (_maxillæ_)[767]. I can bring forward only one striking instance of it, and some degree of doubt rests even upon that. In the genus _Nemognatha_ of Illiger, the maxillæ of the male are elongated, narrow, setiform, and often involute or spiral, like those of a bee or a butterfly. But that this is peculiar to the males is at present only surmised[768]. I possess several species of the genus, all of which are distinguished by long maxillæ; though in some they are as long as the body, and in others scarcely half that length. _Gnathium_ Kirby is similarly characterized[769].
The _maxillary palpi_ occasionally differ in the sexes. In _Cerocoma_ those of the female are filiform, while the two intermediate joints of those of the other sex are much thicker than the first and the last[770]. In _Hylæcetus_ and _Lymexylon_, those of the male are still more remarkable: they are pendent, the last joint very large, and laciniated so as to form a tuft[771]. The female ones grow gradually larger towards the end, but are not at all divided there[772]. The palpi of male spiders are of a very different structure from those of the other sex, terminating in a very complex incrassated piece, which has been supposed to contain the organ of generation; but this, according to Treviranus, is a mistaken idea--that organ being, as usual, to be found in the abdomen[773]. In the common gnat the palpi of the male are as long as the proboscis, consist of five joints, and at the end are tufted with hairs; while those of the female are scarcely one-fourth of its length, have only three joints, and are not tufted. Whether the _labial palpi_ in any genus differ in the sexes, I cannot affirm with certainty: I have not, however, observed any such variation in them.
I shall next mention some organs of the head, in which the difference between the sexes is often very striking and peculiar. You will readily conjecture that I am speaking of the _antennæ_. And here the advantage seems wholly on the side of the males: since in them these wonderful instruments of unknown sensations are not only more complex, but usually more elegant, than those of the other sex. You will pardon me, therefore, if I enlarge a little more than ordinary upon a subject so full of interest, and say something upon the differences observable between the sexes--in the shape, magnitude, and length, number of articulations, ramification and plumage, and individual joints of their antennæ.