The under-lip (labium)—taken in a restricted sense for that central part from which emerge the labial palpi, and which is often considered as the mentum,—does not offer any striking variations in the sexes. One, however, is of importance, as it helps to prove which are the true female Lucani. In the male the labium is emarginate, in the female it is intire. This may be seen both in L. Cervus and femoratus, and probably in other species. The sculpture also is different, the lip being smooth in the former and covered with excavated puncta in the latter. The tongue (lingua or ligula) of the sexes is usually the same; except in the hive-bee, in which that of the neuters is longer than that of the male and female.
The upper-jaws (mandibulæ), however, often afford striking sexual characters. The enormous protended ones of the common stag-beetle (Lucanus Cervus) attract the attention of the most incurious observer; and these are now generally allowed to be of this description. Geoffroy and Mr. Marsham, indeed, have asserted that they have taken in coitu those with long mandibles: but as these males are pugnacious, and attack each other with great fury, as Mr. Sheppard informs me, it is not improbable that these gentlemen may have mistaken a battle for an amour: since not only have those with long mandibles been often taken united with those that have short ones[751], but the same difference obtains in the sexes of other species. This is particularly observable in Lucanus femoratus, of which I received from Brazil many specimens agreeing in every respect except in this, that one had short and the other very long mandibles. These organs vary in different specimens, as to the number of their teeth and branches. They are singularly robust in L. Alces[752]; but in none more threatening than in L. Elephas[753], in which they curve outwards and downwards. In Mr. W. MacLeay's genus Pholidotus, they are almost parallel to each other, and curve downwards; in Lucanus nebulosus Kirby, they assume a contrary direction[754]; as they do likewise in Lamprima Latr.[755] In Lucanus Capreolus the points close over each other[756]. In Lethrus F. in the female, but not the male, the mandible is armed below with a long incurved horn. In Lucanus serricornis they form a complete forceps[757]. In Siagonium quadricorne Kirby[758] the mandible is furnished at its base with an exterior horn, which is probably a sexual distinction. The male of Synagris cornuta, a kind of wasp, is still more conspicuous in this respect; for from the upper side of the base of its straight slender mandibles proceed a pair of crooked, decurved, tortuous, sharp horns, not only longer than the mandible, but than the head itself[759]. Many sexual differences are observable in the mandibles of the various tribes of bees (Anthophila Latr.). Thus, in Colletes Latr. the male mandible is more distinctly bidentate at the apex than the female[760]: in Sphecodes Latr. and others, the reverse of this takes place[761]. Where these organs in both sexes are toothed at the apex, they often vary in the number of teeth. Thus, the female of Megachile centuncularis Latr. has four teeth at the apex of its mandible, while the male has only two[762]. In M. Willughbiella, though the mandibles of both sexes have four teeth, yet those of the male are sharp, and the two external ones the longest; while those of his mate are obtuse, and all nearly equal in length[763]. In Anthidium manicatum Latr., the former has only three teeth, while the latter has five[764]. The differences in this respect in the hive-bee have been before noticed[765]; those of the humble-bees (Bombus Latr.) are strikingly distinguished from each other; the female mandible being very stout and wide, constricted in the middle, and furrowed on its outer surface; and the male, on the contrary, very slender at the apex, dilated at the base, and without furrows[766].
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.
With regard to their shape, variations are sometimes observable between the antennæ of the sexes; but this principally occurs in the Hymenoptera order. For instance, those of Chelostoma maxillosa, a small bee that deposits its eggs in little holes in posts and rails, are clavate in the female and filiform in the male[774]—a circumstance that distinguishes in some degree those of Sphecodes, Halictus, and Andrena of Latreille, three other genera of wild-bees[775]. In Dinetus Jur. the male antennæ are moniliform at the base, and filiform at the apex; the female, on the contrary, are entirely filiform[776].
The antennæ of the sexes also sometimes differ in magnitude and length. This is the case in the three genera of wild bees just mentioned; those of the female being thicker than those of the male, while these last are longer than the former. But in this tribe the males of the Fabrician genus Eucera are most remarkable for their long antennæ[777]. With regard to the different length of these organs in the sexes, no insects are more distinguished than some species of the capricorn-beetles (Cerambyx L.). In Lamia Sutor the male antennæ are twice the length of the female; and in another Brazilian species in my cabinet, related to L. annulata (Stenocorus F.), they are thrice their length. Some of the Anthribi F. approach the Cerambycidæ, not only in some other characters, but also in this circumstance:—thus the antennæ of A. albinus, a native of Britain, are vastly longer in the male than in the female; and in A. cinereus (Macrocephalus Oliv.)[778], which I suspect to be of the former sex, they are as long nearly as is usual in the tribe just named, called in France capricorn-beetles.
I may here observe, that sometimes in the sexes a difference is also to be found in the direction or flexure of their antennæ. Thus in Scolia F., Pepsis F. &c., in the males the antennæ are nearly straight, but in the females convolute or subspiral. The reverse of this takes place in Epipone spinipes, a kind of wasp, and its affinities; and Systropha Illig., a kind of bee: for in these the male antenna is convolute at the apex[779], and the female straight. In the various tribes of bees (Anthophila Latr.), these organs in the latter are what is denominated broken, the main body of the antenna forming an angle with the first joints: but in the former this does not take place.
The antennas of the sexes do not always agree in the number of joints. In the bees, and many other Hymenoptera, the male has one more joint than the female; as is the case also in Œdemera notata (Cantharis acuta Marsh.). In Pteronus Laricis, a kind of saw-fly, the latter has only sixteen joints in its antennæ, while the former has twenty-four[780]. In Rhipicera marginata, a beetle, the beautiful antennæ of the male consist of thirty-two joints, while the female has no more than eleven! In Chelonus Jur. the male, on the contrary, has the smallest number of joints, namely sixteen; while the female has twenty-five[781].