In nothing do the sexes differ more materially than in the ramification of these organs, and their plumage. By attending to this, you may often detect the sexes in an instant; since the antennæ of the males in numerous instances are much more complex than those of the females. For what end the Creator has so distinguished them is not quite clear; but most probably this complex structure is for the purpose of receiving from the atmosphere information of the station of the female. A tendency to branching will be found in the antennæ of some males, in tribes where these organs are usually perfectly simple in both sexes. Thus, in the male of Chelostoma maxillosa,—mistaken for another species by Linné, which he names Apis florisomnis,—the intermediate joints on their inner side project into an angle[782]; and those of the same sex of the common hornet, by means of a central sinus, have two obtuse teeth on each. With regard to more direct ramifications, some male antennæ terminate in a fork, or two branches. This is the case with Hylotoma furcata Latr., a saw-fly[783]; and the peacock-louse (Nirmus Pavonis Herm.)[784]. Others, again, have three lateral branches, as in Eulophus Geoffr. a little parasite, the male antennæ of which send forth a hairy external and rather long branch, from the base of the fourth, fifth, and sixth joints[785]. In Elater flabellicornis L., the eight last joints are flabellate, or elongated and flat, resembling the sticks of a fan in the male[786]; in the female they are shorter, and more properly may take their denomination from the teeth of a comb. In Lampyris Latreillii Kirby, the antennæ of the former are flabellate on both sides, while those of the latter are little more than serrate[787]. These organs are extremely beautiful in the males of the Rhipiceræ of Latreille. In R. marginata K. they consist of thirty-two joints, from thirty of which issues a branch, the first very short, but the rest gradually increasing in length as they approach the middle of the antenna; then gradually decreasing to the end, so as to represent an expanded fan[788]. But in none are they altogether so remarkable as in those moths that Linné denominates Bombyces Attaci, and some others. In these, in the males, these organs in their contour are lanceolate, and every joint is furnished with a couple of parallel equal branches on each side[789]. In the females these branches are shorter on the whole, and alternately one long and one short; but in some, as Saturnia Pavonia, there is only one short branch or tooth on each joint in this sex[790]. In Bombyx regalis &c. only the first part of the antenna is so branched; and those of the female are setaceous and without branches. In B. versicolor, &c. there is only one branch from each side on every joint; those of the female being much shorter than those of the male. The latter sex of Pteronus Laricis Jur., a saw-fly, afford an example of a different structure, the antennæ on one side sending forth a branch from every joint but the two first; but on the other side, the nine or ten last joints also are without a branch. The female antenna is serrated[791]. In another of this tribe, Pterygopterus cinctus Klug, the male antenna resembles a single-toothed comb, being branched only on one side: that of the female, like the former instance, is serrated[792]. Whether the remarkable antennæ that distinguish the known individuals of the genus Phengodes (Lampyris plumosa F.) is a sexual character has not been ascertained; but it is not improbable that it may be, as in other Lampyridæ. A pair of delicate flexile and almost convolute plumose branches proceeds from the apex of each joint except the basal ones, which have something the air of cirri, and give a more than usual degree of lightness and elegance to these organs[793]. Other antennæ, especially in the Diptera order, assume an appearance of plumes—not from the branches that proceed from them, but from the fine long hairs that beset and adorn them. These are universally indications of the male sex, those of the females being generally comparatively naked. If you take the common gnat, you will find that the antennæ of one individual are thickly fringed on each side, and tufted at the end with fine long hairs, while in the other only four or five placed at intervals in a whorl are to be perceived[794]. In Chironomus Meig., a kind of Tipula L., resembling a gnat, the male antennæ are beset on all sides with the finest hairs, and resemble a beautiful plume[795], while the females to the unarmed eye appear naked. Even in some Hymenoptera, the antennæ of the males are thus feathered, in a less degree: for instance, in Hylotoma Latr.[796] Whether the tufts and fringes which ornament, in a remarkable manner, the antennæ of many Cerambycidæ[797], are sexual characters, is not certainly known.
We are now to consider other sexual differences in these organs, resulting from the size or configuration of one or more individual joints. To begin with the first joint, or scapus. In many of the Hymenoptera, particularly the Anthophila Latr., this is elongated, and the remaining joints form an angle with it in the females: while in the other sex it is much shorter, and in the same line with the rest of the antennæ; and in Hylæus dilatatus (Melitta dilatata Kirby) the first joint in the male is dilated and shaped something like a patella[798]. In Malachius bipustulatus, &c. the sex just mentioned is peculiarly distinguished by a white excrescence on the first four joints of the organs in question, most conspicuous in the second and fourth. The antennæ of male Cerocomæ are not very different[799]. Mr. Marsham has described a little Haltica under the name of Chrysomela nodicornis, from a peculiarity of the same sex not to be found in the other. The fourth joint is very large and obtriangular; in the female it is merely longer than the rest. In H. Brassicæ and quadripustulata the fifth joint is larger and longer than all but the first in the male, in their females it is only longer. In some moths (Herminia Latr., Crambus F.) there is also a knot in the middle of the male antennæ[800]. In Noterus, a water-beetle, the six intermediate joints are thicker than the rest, beginning from the fourth, and the last but one ends internally in a truncated tooth. The fifth and two following joints in the male antennæ of Meloe are larger than the rest, which distinguishes them, as well as a remarkable bend observable at that part[801].
Variations of the kind we are considering are also observable in the clava, or knob, in which antennæ often terminate. You have doubtless observed that the lamellated clava of the antennæ of the common cockchafer is much longer and more conspicuous in some individuals than in others—the long clava belongs to the male[802]. In another species, M. Fullo, that of this sex is nine or ten times the length of that of the other. In Colymbetes serricornis, a water-beetle, the male has a serrated clava of four joints. In Dorcatoma dresdensis[803], and also Enoplium damicorne, two beetles, it is nearly branched in the male, but much less so in the female. In a little destructive beetle, common in our houses (Attagenus Pellio), in the latter it is very short, but in the former it is very long, and nearly formed by a single joint. In Eurhinus Kirby, a New Holland genus of the weevil-tribe, in the male the last joint, also, is much longer than it is in the female[804]. These examples will give you some idea of the principal variations that take place in the antennæ of the sexes, and of the wonderful diversity of forms in this respect to which mere sexuality gives rise amongst insects.
In the eyes, or stemmata, this diversity is less remarkable. Latreille has described two ants, Formica contracta and coeca, in the neuter of which he could discover no eyes[805]: in the former, the female, however, had large ones. The male he appears not to have known, but it probably was not destitute of these organs; of the latter he was acquainted only with the workers. The neuter of Myrmica rubra, another ant, has no ocelli or stemmata, although the male and female are provided with them[806]. They are discoverable only in the former sex of that singular insect related to the ants, Mutilla europæa. Other insects differ in the size of the eyes of their sexes. In the hive-bee, and some Ephemeræ, the eyes of the drone or male are much larger than those of the worker and female, and also meet at the vertex, having their stemmata below the conflux; whereas in these latter they are widely distant[807]. In Stratyomis, Tabanus, and many other two-winged flies, the male eyes meet at some point below the stemmata, and above the antennæ. In the former they touch more at an angle; for the vertex forming a narrow isosceles triangle, and for the anterior part of the face one nearly equilateral: while those of the female are separated by a considerable interval. In Heptatoma and Hæmatopota in that sex, a similar interval obtains; while in the other, after forming a minute short triangle, they unite for a considerable space, and then diverging, form the face. This is also the case in Tabanus; but in the female, the space that intervenes between the posterior part of the eyes is much narrower than in these two cognate genera of the horse-flies. In some others of this order, as Musca Latr., the eyes of the male do not touch, but approach posteriorly much nearer to each other than those of the other sex. In a few instances the sexes vary even in the number of their eyes, as well as the size. This occurs in some species of Ephemera L. (E. diptera, &c.), in which the male, besides the common lateral ones, has two large and striking intermediate eyes, that sit upon vertical pillars or footstalks[808].
2. The Trunk. The thorax of many coleopterous males, especially of the Dynastidæ and Copridæ amongst the petalocerous tribes, exhibits very striking differences from that of the female. In many Lucani the lateral angle is more prominent. In Anthia it is bilobed posteriorly, while in the last-mentioned sex it is entire[809]. In Phanæus carnifex MacLeay (Copris F.) it is elevated into a plane triangular space, with the vertex of the triangle pointing to the head; but in the female it is convex, with an anterior abbreviated transverse ridge[810].
In a large proportion terrific horns, often hollow, like those of the head lately noticed, arm the thorax of the male, of which you will usually only discover the rudiments in the other sex. In the first place, some are unicorns, or armed only with a single thoracic horn, which frequently, in conjunction with the thorax itself, not a little resembles a tunnel reversed: of this description are Dynastes Hercules, Tityus, Gedeon, Enema, &c.[811] In the three first this horn is porrected, or nearly in the same line with the body; but in the last, and D. Pan, it forms an angle with it; and in D. Ægeon it is nearly vertical[812]. In D. Hercules it is very long; in D. Alcides[813] and Tityus very short; in the two last, and in Oxytelus tricornis which is similarly armed, it is undivided at the apex; but in D. Gedeon, Pan, bilobus, &c.[814] it is bifid or bilobed. It is usually rather slender, but in D. Chorinæus[815] and bilobus, it is very stout and wide. In D. claviger it is hastate at the apex[816]. In D. hastatus it is short and truncated[817]. Others, again, have two thoracic horns. In Copris nemestrinus these are discoidal, diverging, and inclining forwards[818]. In Phanæus floriger[819] they are lateral, triangular, and incline towards each other, with, as it were, a deep basin between them. In P. splendidulus they sink into two longitudinal ridges, most elevated posteriorly, with an intervening valley[820]. In P. bellicosus they are posterior, compressed, truncated, and emarginate at the apex, and include a basin[821]. In Copris Sabæus they are merely two acute prominences[822].—Three horns distinguish the thorax of many. In D. Aloeus[823] and its affinities, they are arranged in a triangle, whose vertex is towards the head. In D. Antæus[824] these horns are nearly equal in length, and undivided at the apex. In D. Titanus[825] the anterior horn is longer than the rest, and bifid at the apex; in D. Atlas and Endymion[826], both of which have a horn on the head, it is much shorter. In others, as in Megasoma Kirby, the vertex of the triangle is towards the anus. In M. Typhon[827] it is longer than the anterior ones, and bifid at the apex; in M. lanigerum they are equal in length[828]. In M. Elephas and Actæon[829] it is merely an elevation of the thorax; in the last almost obsolete. In Geotrupes Typhæus, common on our heaths, the anterior of this part is armed by three horizontal horns, the intermediate one being the shortest[830]. Copris lunaris also, another of our own beetles, has three short posterior thoracic horns, two lateral and triangular ones, and a transverse intermediate elevation, with a notch in the middle[831]. In Dynastes Neptunus the horns are porrected, the middle one being very long, and the lateral ones short[832]. In D. Geryon the point of the lateral horns is towards the anus, and the base of the intermediate one covers the scutellum[833]. Others have four of these singular arms: this is the case with one of our rarest beetles, Bolbocerus mobilicornis K., which has four dentiform horns, the intermediate pair being the shortest, arranged in a transverse line on the anterior part of the thorax[834]. In B. quadridens these are merely teeth. In Phanæus Faunus[835] it has two lateral, elongated, compressed, truncate, horizontal horns, and two intermediate teeth. Dynastes Milon has a still greater number of horns on the thorax of the male, there being two lateral anterior ones and three posterior ones—the intermediate being the longest[836]; and Copris Antenor Fabricius and Olivier describe as having a many-toothed thorax; and from the figure of the latter[837], the male appears to have seven prominences.
But the males of insects are not only occasionally distinguished by these dorsal arms—in a few instances they are also furnished with pectoral ones. The illustrious traveller Humboldt found in South America a species of weevil (Cryptorhynchus Spiculator Humb.), the breast of which was armed with a pair of long projecting horns; and I possess both sexes of four species, three at least from Brazil, that exhibit in one individual the same character. One, concerning the country of which I am uncertain, recedes somewhat from the type of form of the rest, and comes very near that of Rynchænus Strix F.[838] In the individual which I take to be C. Spiculator, the pectoral horns are very long, curving upwards at the apex, and nearly in a horizontal position; while in the three others they are much shorter, and inclined towards the horizon. The males of some species of Rynchites, as R. Bacchus and Populi[839], are also armed with a pair of lateral horns or spines, which may be termed pectoral rather than dorsal.
I shall now advert to the sexual characters that are to be found in the instruments of motion attached to the trunk—beginning with those for flight. In the female of the common glow-worm (Lampyris noctiluca) not the slightest vestige of elytra or wings is visible, and it resembles a larva rather than a perfect insect; yet its mate is a true beetle furnished with both. The same circumstance distinguishes the female cockroach (Blatta) and is more universally prevalent in that genus than in Lampyris, in which a large number of females have both elytra and wings. The males of Bombyx antiqua and Gonostigma, and of many other moths, have wings of the usual ample dimensions, while those of their females are merely rudiments. This is the case, also, with some of the Ichneumonidæ[840]. In the tribes of Ants, Termites, &c. the neuters or workers are without wings. Amongst the plant-lice (Aphides) there are individuals of both sexes, some of which have wings, and others not[841]. Amongst the Coleoptera, the female of Tenebrio Molitor, the common meal-worm, has elytra and no wings; while the male has both[842].—Sometimes these organs vary in size in the sexes: thus in Aradus Betulæ F., a kind of bug, the hemelytra and wings are narrower and shorter in the female than in the male[843]. In the genus Blaps F., the mucro that arms the apex of each elytrum is longer in the former sex than in the latter. In Ateuchus gibbosus F., a dung-beetle, the elytra have a basal gibbosity near the suture in one sex that does not obtain in the other. In the Orthoptera order, the sexes are often to be known, almost at first sight, by a difference in the veining and areolets of the wings; but upon this I enlarged so fully when I treated of the sounds produced by insects, that it is not necessary to repeat what I have said; which observation also applies to the drums which distinguish the male Cicadæ[844]. The wings of some butterflies, and of most moths and hawkmoths (Sphinx L.), are furnished with a singular apparatus for keeping them steady, and the under-wing from passing over the upper in flight. This appears to have been first noticed by Moses Harris, and was afterwards more fully explained by M. Esprit Giorna[845]. From the base of the under-wing proceeds a strong bristle, received by an annulus or socket, which springing between the two principal nervures of the upper-wing terminates in the disk of the wing: in this annulus the bristle moves to and fro, and prevents the displacement of the under-wing. This apparatus is perfect only in the males, which alone have occasion for long flights; the females, though they have often several bristles, having no annulus[846].
The other instruments of motion, the legs, also differ in the sexes. In some instances they are disproportionably long. This is particularly the case with the anterior pair of some beetles, as Macropus longimanus, Scarabæus longimanus L., in which they are so long as to make the males of these individuals rather inconvenient in a cabinet. Amongst British beetles Clytra longimana and Curculio longimanus Marsh. are also remarkable in this respect. In some other males the middle pair are the longest; as in Anthophora retusa Latr., a kind of wild-bee[847]. There are two known instances of remarkably long posterior legs in the Capricorn tribe, which I suspect belong to the present head. One is Saperda hirtipes Oliv.[848], in which the hind-legs are longer than the whole body, and adorned with a singular tuft of hairs; and the other a Clytus, I think, which Mr. MacLeay purchased from the late Mr. Marsham's collection, in which the hind-legs are not only very long, but have tarsi convolute, like some antennæ. From analogy I should affirm that these were the characters of male insects.
To come to the parts of legs. Sometimes the coxæ of the last mentioned sex are distinguished from those of the female by being armed by a mucro or spine. Thus the male of Megachile Willughbiella, and others of that tribe, have such a spine on the inner sides of the anterior coxa[849]. The Trochanter also of some differs sexually; and you will find that the posterior one of the male in Anthidium manicatum is of a different shape from what it is in the female[850]. In Sphodrus leucopththalmus, one of the beetles called black dors, in one sex the same trochanter terminates in a long mucro or spine[851], and in the other it is rounded at the apex.