Peculiar characters in their thighs also often indicate different sexes. In Prionus damicornis there is a short spine at the apex of the anterior ones in the female that is not in the male; while in Macropus longimanus, at their base externally the male is armed with a mucro, which I cannot find in the female[852]. In Scarabæus longimanus L. this thigh is furnished with two teeth[853].—The intermediate thighs also sometimes differ. In an Onitis from China, a variety perhaps of O. Sphinx, those in the male are dolabriform, and in the other sex of the ordinary shape. In Odynerus spinipes they have on their lower side two sinuses, so as to give them the appearance of being toothed. The posterior thighs are sometimes incrassated in the male, and not in the female. This you will see in a weevil, not uncommon, Apoderus Betulæ, and also in many species of Cimbex F., a kind of saw-fly; and the same circumstance distinguishes the latter sex in many species of Lygæus F., a kind of bug: I discovered this from L. cruciger, of which I have both the sexes; and from Stoll's figure of L. Pharaonis[854]. In some of these the female thighs are enormously large. A remarkable variation in this respect is observable in the coleopterous genus Œdemera (Necydalis L.). In Œ. Podagrariæ these limbs are incrassated in one sex and not in the other[855]; in Œ. cærulea they are so in both sexes; and in Œ. ceramboides in neither. In Pelecinus Polycerator F., one of the Ichneumon tribe, or an insect very near it from Brazil, these thighs in the female are armed with two spines underneath, which are not in the male.
The anterior tibiæ in Scarabæus longimanus L. differ remarkably in the sexes. In the female they are of the ordinary shape, and serrated externally; but in the male they are very long, incurved, and without teeth or serratures[856]. In the males of the genus Onitis F. they are bent like a bow, and acute at the end; but in the females they are formed on the common type[857]. In Hispa spinipes F. they are armed internally with a crooked spine[858]. But the most extraordinary sexual variation of this joint of the leg may be seen in the male of Crabro cribarius F. and several other species of the same family, in which these tibiæ are dilated externally into a concavo-convex plate, or rather have one fixed to them and part of the thigh, of an irregular and somewhat angular shape[859], with numerous transparent dots, so as not badly to resemble a sieve: whence the trivial name of the species. Rolander, who first described it, fancied that this plate was really perforated, and that by means of it the animal actually sifted the pollen; but it is most probably for sexual purposes. In another species, the plate is ornamented with transparent converging streaks. In the bee-tribes (Anthophila Latr.) the posterior tibia of the working sex is generally bigger than the corresponding part in their more idle partners: this is particularly conspicuous in the genus Euglossa, in the females of which this part is triangular, very broad towards the apex, and fitted for carrying a large mass of pollen paste. The tibiæ of the males of some Lepidoptera are remarkable in this respect. That of Hepialus Humuli is much more hairy; but in H. Hectus it is a dilated mis-shapen mass, without a tarsus, and with long scales pendent from the disk[860]. Differences of this kind also occur in the calcaria or spurs that arm the apex of the tibiæ of a large number of insects. Thus in Acanthopus Klug, a singular bee, in the male the spur of the intermediate leg is dilated at the apex, and armed with six strong spines, the inner one larger than the rest[861].
But the part of the leg in which the sexes most vary is the tarsus; and this variation takes place both in the number of the joints, and their form and circumstances. The first case has been observed only with regard to certain species of Cryptophagus Herbst, as C. fumatus, &c. in which the female is pentamerous, or having five joints in all the tarsi; and the male heteromerous, or having five joints in the two anterior pairs, and only four in the posterior[862]. With respect to the form of the tarsal joints, the sexes more frequently differ; and by inspecting this part, especially in the predaceous and carnivorous Coleoptera, you may often, without further examination, ascertain whether any individual is male or female. Even in the slender-footed Cicindelidæ, the three first joints of the anterior tarsus of the male are more dilated than the two last, and covered underneath with a brush of stiffish hair; in the female all are equally slender, and not so hairy. In Carabus, Feronia, &c. Latr. the four first joints of these tarsi in the males are dilated, and furnished with a brush or cushion: in the Silphidæ, also, the same circumstance takes place. In Harpalus Latr., and Silpha americana, the four anterior ones are similarly formed in this respect. But one of the most remarkable sexual characters, in this tribe of insects, that distinguish the males, are those orbicular patellæ, furnished below with suckers of various sizes, and formed by the three first joints of the tarsus, which are to be met with in the Dytiscidæ, &c.; but as I shall have occasion to treat of these more fully in another Letter, I shall only allude to them now. The second pair of tarsi have in these also the three first joints dilated and cushioned[863]. In Hydrophilus piceus, another water-beetle, the fifth joint of the tarsus is dilated externally, so as to form nearly an equilateral triangle[864]. Christian, a German writer on the Hymenoptera, has described some very singular appendages which he observed on the first joint of the four posterior tarsi of Xylocopa latipes F. These were battledore-shaped membranaceous laminæ, with a reticulated surface, of a pale colour; which were fixed in pairs by the intervention of a footstalk to the above joint, on which they sometimes amounted to more than a hundred: the use of which, he conjectures, is the collection of pollen[865]. I possess two specimens of this bee; one has none of these appendages, and on the other I can discover them only in one of the tarsi—from which circumstance I am led to conjecture that, like the supposed Clavariæ that were imagined to grow on some humble-bees, but which are now ascertained to be the anthers of flowers—these also belong to the kingdom of Flora, and are spoils which the bee in question has filched from the blossom of some plant. The individuals that have been thus circumstanced are males; whether the female is guilty of similar spoliations is not known. In my specimen there are no traces of them. In many bees, the first joint of the posterior tarsi is much larger in the females and workers than in the males; but in the hive-bee this joint is largest in the latter[866]. In Beris clavipes and Empis nigra, two flies, the joint in question is large and thick in the male, but slender in the female. The penultimate tarsal joint in the posterior legs is dilated internally, and terminates in a mucro in one sex of Anoplognathus Dytiscoides of Mr. W. MacLeay[867]. In some insects the anterior tarsus of the males has been supposed to be altogether wanting: I allude to the petalocerous genus Onitis F.; but I have a specimen of Onitis Apelles of this sex, or a species nearly related to it, in which one of these tarsi is to be found[868]; which, though very slender, consists of five joints, and is armed with a double claw: from which circumstance it may, I think, be concluded, that although, as in Phanæus, these tarsi are very minute, they are not wanting. What renders this more probable is, a circumstance which every collector of insects, who has many specimens of Mr. W. MacLeay's Scarabæidæ in his cabinet, must have noticed: namely, that in all, except Copris and Onthophagus, the anterior tarsi are usually broken off. Out of seventeen individuals of Scarabæus MacLeay in my own, not a single one has a relic of an anterior tarsus; and scarcely one in a much greater number of Phanæi. The tarsus in question in the nobler sex in Crabro, at least in C. cribrarius and its affinities, is also very short, especially the three intermediate joints; but at the same time very broad and flat. In the species just named, the external claw forms a kind of hook; and in the rest it is considerably longer than the other[869]. The claws, indeed, occasionally vary in the sexes in other Hymenoptera: thus in Melecta Latr., a kind of bee, in the female they are intire, but in the male they are furnished with an internal submembranaceous tooth or process[870]. In Cœlioxys conica and others, those of the latter sex are bifid at the apex, but those of the former acute[871]. In Megachile, the male claw is as in the instance just mentioned, while the female has a lateral tooth[872]; and a similar character distinguishes the sexes in the hive-bee[873].
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.
In Staphylinus splendens, the penultimate ventral segment is very deeply cleft, and the antepenultimate emarginate in one sex, and intire in the other. In S. laminatus, an allied species, the penultimate segment is cleft, less deeply, however; but the antepenultimate is very short and intire; while the fourth is extremely long, and rounded at the margin, appearing as if it was only an elevated part of the last-mentioned segment; for which it was mistaken by Gravenhorst[881], while it is of the usual form in the other sex.
The extremity of the abdomen or its anal segments and organs furnish a variety of sexual characters. Sometimes the last dorsal segment is emarginate in the male, and not in the female; as in Megachile ligniseca, one of the leaf-cutter bees, Cimex hæmorrhoidalis, &c.[882] At other times little lateral teeth are added to this notch, as in another of the same tribe, M. Willughbiella[883]. Again, in other males, both the ventral and dorsal anal segment are armed each with a pair of teeth or mucros, as in Chelostoma maxillosa[884]. In Anthidium manicatum, another bee, the anus terminates in five spines[885]. In Cœlioxys conica of the same tribe, in which this part in the female is very acute, that of the male is armed with six points[886]. In that singular Neuropterous genus Panorpa, while the abdomen of the female is of the ordinary form, with a pair of biarticulate palpiform organs attached to the last retractile joint, or ovipositor, that of the male terminates in a jointed tail, not unlike a scorpion's, at the end of which is an incrassated joint armed with a forceps[887]. In the common earwig (Forficula auricularia) the two sexes differ considerably in their anal forceps: in one it is armed with internal teeth at the base, and suddenly dilated, above which dilatation it is bent like a bow: in the other it is smaller, without teeth, grows gradually narrower, is very minutely crenulate from the base to the end, and is straight, except at the very summit, where it curves inwards. Misled by these and similar differences, Mr. Marsham has considered them (the sexes both of F. auricularia and F. minor) as distinct species.