3. The _abdomen_. This part affords many external sexual characters, whether we consider its general shape; the number of segments that compose it; its base, middle, or extremity.
In general _shape_ it often differs in the sexes. Thus, the abdomen of female _Tipulæ_ is lanceolate; that of the male cylindrical, and thickest at the extremity[874]. In _Molorchus_ F. it is convex above in the former, and flat in the latter,--the female of this beetle not unaptly representing some female _Ichneumons_ in this respect, and the male their males[875]. In _Andrena_ it is oblong in the one, and lanceolate in the other. In the hive-bee the drones have a thick, obtuse, and rather long abdomen; in the females it is long, and nearly represents an inverted cone; and in the workers a three-sided figure, or prism.
The _number of segments_, also, is generally different in the two sexes--the male having one more than the female; but in _Dytiscus marginalis_, &c. the reverse of this takes place: the female, if you reckon the bipartite half-concealed anal segment as one, having seven ventral segments, and the male only six. She has also eight dorsal, and the male seven.--In the ant tribes (_Formica_ L.), the little vertical scale, at the _base_ of the abdomen in one description of them, or the double knot in another, is less in the male than in the female. In a very singular male insect belonging to the _Vespidæ_, and related to _Synagris_, (which I purchased from the late Mr. Drury's cabinet,) the second ventral segment sends forth from its disk two remarkable parallel very acute and rather long spines. The same sex of _Chelostoma maxillosa_ has likewise on the same segment a concave elevation, opposite to which on the fifth is a cavity which receives it, when the animal rolls itself up to take its repose[876]. In another species, _C. Campanularum_, the segment in question has only a tubercle[877].
On the second segment of the abdomen of some specimens, probably males, of the remarkable African genus _Pneumora_ before alluded to[878], there are thirteen little elevated ridges, placed rather obliquely in an oblique series; and gradually, though slightly, diminishing in size towards the belly: on their upper side they are flat, forming nearly a horizontal ledge, but on the lower they slope to the abdomen. The posterior thigh in its natural position covers the three first of them, and, if moved downwards, would strike them all[879]. I conjecture, therefore, that these are the animal's instruments of sound, imitating the harp or violin rather than the drum; and that the thigh acts the part of the hand or bow. The abdomen of these insects being blown out like a bladder, and almost empty[880], must emit a considerable sound when the thigh of the animal passes briskly over these ridges; and their different length would produce a modulation in the sound. When struck with a pin, they emit a grating noise.