The _organs_ of the head also present many sexual distinctions. The upper lip (_labrum_) in _Halictus_ Latr., a tribe of wild bees, in the female is furnished with an inflexed appendage, which is not discoverable in that of the male[748]; and the shape of this lip in _Sphecodes_ Latr. differs in the sexes[749]. Perhaps the horn or tubercle observable on this part of some female _Nomadæ_ F.[750] may be wanting in the male.
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].