In the Diptera, Aptera, Arachnida, &c., I am not aware of any striking difference in the colours of the sexes.
ii. The sexes of insects vary (but more rarely than in colour) in their sculpture also, and pubescence. Thus the elytra of the females of many of the larger water-beetles (Dytiscus) are deeply furrowed, while those of the males are quite smooth and level[713]. The thorax of the female in several species of Colymbetes of the same tribe, as C. Hybneri and transversalis, on each side has several tortuous impressed lines or scratches, like net-work, which are not to be discovered in the male. Hyphydrus gibbus Latr., which differs solely from H. ovalis (Dytiscus ovalis Illig.) in being thickly covered with minute impressed puncta, is, from the observation of the Rev. R. Sheppard, the other sex of this last, with which he has taken it coupled; and it is by no means improbable that Hydroporus picipes (Dytiscus punctatus Marsh.) and H. lineatus,—between which, as Gyllenhal has justly observed, the same difference only exists,—are in like manner sexual varieties. With respect to pubescence, I have not much to say. Another aquatic beetle, Acilius sulcatus Leach, has not only its elytra sulcated, but the furrows of these, and a transverse one of the thorax, are thickly set with hair; while the male is smooth, and quite naked. Particular care seems to have been taken by the Creator, that when all the above inhabitants of the water are paired, the male should be able to fix himself so firmly, by means of his remarkable anterior tarsi, (which I shall afterwards describe,) and these asperities, &c. in the upper surface of his mate, as not to be displaced by the fluctuations of that element, the reluctance of the coy female, or any other slighter cause.
In a moth called the ghost (Hepialus Humuli), the posterior tibia of the male is densely bearded, but not of the female[714].—Some Hymenoptera, as Ammophila Kirb. and Stigmus Jurine, have the upper lip of the male clothed with silver pile, while that of the female is not so ornamented. The legs of some bees are distinguished in the sexes by a difference in their clothing. That observable in those of the hive-bee has been before noticed[715]. In Andrena of Latreille[716] the posterior tibia of the female is covered externally with a dense brush of hairs, for collecting the pollen; and the posterior legs at their base have a curled lock of hair—which are not to be found in the male[717]. In Dasypoda, Melecta, Anthophora, Centris, Epicharis, &c. of the same author, the first joint of the tarsus of the female, and in Xylocopa almost the whole tarsus, is also similarly signalized from that of the other sex. In Bombus, as in the hive-bee, the posterior tibiæ of the females and neuters are furnished with a basket of hairs for carrying their pollen paste, which you will in vain look for in the male[718]. The latter, however, in some species of this tribe are distinguished from the former by the longer hairs of their legs, but not in the posterior ones. Thus, in Anthophora retusa the first joints of the intermediate tarsus are bearded internally with a thin fringe of long hairs, and the first externally with a triangular one of short ones at the apex: but what is most remarkable, the last or unguicular joint, which in almost every other bee is naked, is on both sides fringed with long hairs[719]. In that remarkable genus Acanthopus Illig., of which the male only is known, the first and last joint of the intermediate tarsus have a dense external brush of stiff hairs, which probably is also a sexual character[720]. Another sexual kind of clothing is exhibited by the females of those bees that have their labrum or upper-lip inflexed (Megachile Latr.)[721]. Their abdomen is covered underneath with a brush of stiff hairs, involved in which they carry the pollen they collect. In the males of some of this tribe, as of M. Willughbiella, the first four joints of the anterior tarsus on their inner side have a long dense fringe of incurved hairs[722]: a circumstance also to be found in the same sex of Xylocopa latipes, in which the claw-joint also is bearded[723]. In Andrena Latr. the last dorsal segment of the abdomen of the same sex is fringed, while that of the male is naked[724]. In the humble-bees (Bombus), the mandibles of the male are bearded with curled hairs, while those of the females and neuters are without them. Some bees, as Andrena and Halictus Latr., have the anus of the female bearded, and that of the male naked: in some Bombyces the reverse takes place.
iii. With regard to the general shape of their body, the male and female usually resemble each other: but there are some exceptions to this rule. The male of the hive-bee is much thicker and more clumsy than either the female or the worker; but in Halictus Latr. the males are nearly cylindrical in shape, and very narrow; while the other sex are oblong or ovate, especially their abdomen: and in Andrena Latr. the former are much slenderer than the females, and of a more lanceolate shape. But a still more striking difference in this respect between the sexes is exhibited by some species of the genus Ptinus F., in which the male is long and slender, and the female short and thick. This, in more than one instance, has occasioned them to be mistaken for distinct insects: thus, P. Lichenum and P. similis, P. ovatus and P. testaceus, of Mr. Marsham, are mere sexual varieties. But the most entire abalienation of shape at present known, is that which distinguishes the male from the female Coccus; these are so completely dissimilar as scarcely to have any part in common. In Bombyx vestita F., and others of the same family, while the males are of the ordinary conformation of the order, the females are without even the slightest rudiments of wings; they have no antennæ, the legs are extremely short, not longer than those of the caterpillar; and the body is entirely destitute of scales, so that they altogether assume the exact appearance of hexapod larvæ[725]. A conformation nearly similar takes place in the female of Tinea Lichenella; but in this the feet are longer, and the anus is furnished with a long retractile ovipositor[726].
iv. In many cases, the structure of particular parts and organs of the body differs in the sexes. As the facts connected with this part of our present subject are extremely numerous and various, it will be convenient to subdivide it, and consider the sexual characters that distinguish—the Head, Trunk, and Abdomen of insects, and their several appendages.
1. The Head. This part in some females is considerably larger than it is in the male. This is the case with the ants, and several other Hymenoptera; while in some Andrenæ, as A. hæmorrhoidalis, and Staphylinidæ, as St. olens, that of the male is the largest. But in none is the difference more conspicuous than in the stag-beetle (Lucanus); in which genus the male not only exceeds the female in the length of his mandibles, but also greatly in the size and dimensions of his head. In the Apion genus, the rostrum of the female is generally longer and slenderer than that of her mate; and in Brentus, the rostrum of one sex (probably the male) is long and filiform, while in the other it is thick and short. This is particularly visible in B. dispar and maxillosus[727], &c.
One of the most striking distinctions of the males in this part of their body, are those threatening horns, usually hollow, with which the heads of many of the male lamellicorn insects and some others are armed, and which give them some resemblance to many of the larger quadrupeds. Many are unicorns, and have their head armed with only a single horn; which in some, as in Oryctes Illig., Dynastes Endymion[728], &c. is very short; in others, very long, as in Dynastes Enema, Pan, Elephas[729]. In one, again, it is thick and robust; as in the clumsy Dynastes Actæon[730]: in another very slender, as in Onthophagus spinifer[731]. With respect to its direction in Elephastomus proboscideus MacLeay, it is horizontal[732] and straight; in Phaleria cornuta horizontal and broken, or the apex turning outwards and forming an angle with the base[733]; in Dynastes Hercules horizontal, and recurved at the apex[734]; in D. Actæon, Elephas, and Typhon, recurving from the base. In Geotrupes dispar it is recurved, so that its point exactly coincides with that of the porrected thoracic horn, with which it forms a kind of forceps[735]. In Copris lunaris F. and Diaperis horrida, the horn is nearly upright[736]. In Onthophagus Xiphias it is dilated at the base, and reclining upon the thorax; and at the apex attenuated, and bending forwards, or nodding. In Passalus cornutus it rises a little, and then bends wholly forwards. In Dynastes Milon, a most remarkable beetle, it slopes backwards in a waving line[737]; and in Onthophagus spinifer it is recurved and reclining.—In speaking of the direction of the horn, you must recollect that it will vary in proportion as the head varies from a horizontal position: so that an upright horn will become inclined or reclined, as the head bends forwards or backwards; but I speak of it as it appears when the head is horizontal. Again, it varies in its teeth or branches. In Dynastes Hercules it is armed with several teeth. In D. Elephas and Actæon it has only one large one at its upper base[738]. In D. Milon it is serrated above. In D. Alcides, Tityus, Ægeon, Copris lunaris, &c. the horn is unarmed and simple at the apex. In D. Oromedon, Gedeon, Enema, Actæon and congeners, it is bifid. In some the horn is at first a broad lamina or ridge, which terminates in two branches, as in Onthophagus Vacca. In this the branches are straight; but in another undescribed species in my cabinet (O. Aries Kirby, MS.) they are first bent inwards, and then at the apex a little recurved: and in D. dichotomus it is divided into two short branches, each of which is bifid[739]. Other males emulate the bull, the he-goat, or the stag, in having a pair of horns on their head. In Onthophagus Taurus, these arms in their curvature exactly resemble those of the first of these animals[740]. In Goliathus pulverulentus, the straight, robust, diverging, sharp horns are not unlike those of some of the goat or gazel tribe. I have a beautiful little specimen in my cabinet, (I believe collected by Mr. Abbott of Georgia,) in which the horns have a lateral tooth, or short branch, like those of a stag; and which I have therefore named O. cervicornis. In O. Vacca, Camelus, &c. the horns are very short, and nearly perpendicular. In the male of Copris Midas, the two longer perpendicular horns have a deep cavity between them, which, together with its black colour, give it a most demoniac aspect; so that you would think it more aptly representative of a Beelzebub or Beelzebul than a Midas[741], or than Phanæus Beelzebul MacL. A similar cavity is between the occipital horns of Diaperis hæmorrhoidalis Payk. Some species of Rynchænus, as R. Taurus, have a pair of long horns upon the rostrum of the male, the rudiments only of which are to be traced in the female[742]. Other species go beyond any known quadrupeds in the number of horns that arm their heads. Thus Ditomus calydonius Bonelli, belonging to Carabus L., has three equal horns[743]. The same number distinguishes Onthophagus Bonasus; but the intermediate one is very short. In Goliathus Polyphemus the middle horn, on the contrary, is much longer and thicker than the lateral ones, and forked at the apex; so that it looks as if it had four of these weapons[744]. A little Diaperis (D. viridipennis F.), a native of Carolina, has four horns upon the head of the male; namely, two long ones on the occiput, and two short dentiform ones on the nose. In a species nearly related to this, sent me by Professor Peck from New England, there is a cavity between the two occipital horns. The same number distinguishes Onthophagus quadricornis (Copris F.). The situation also of the horns varies: In some it is in the middle of the head, as Oryctes nasicornis, Copris lunaris, &c.: in others, as in Onthophagus nuchicornis, Xiphias, &c. it is a process of the occiput or hind-head; and in O. Oryx F. the two horns proceed from the anterior part of the head. In the other sex, in insects the head of whose males is armed with horns, they are usually replaced by mere tubercles, or very short elevations, as you may see in the female of Copris lunaris; or by transverse ridges, as in the Onthophagi: or else the head is without arms, and quite smooth, as in Diaperis, Phaleria, &c. What may be the use of these extraordinary appendages, as well as those on the thorax, and in some cases on the abdomen, (which I shall mention afterwards), to the males, has not yet been ascertained. Whether the individuals of this sex are more exposed to the attack of birds and other enemies, in consequence of being more on the wing than the females, and are therefore thus provided with numerous projecting points for defence, is a question worth considering[745]. It is the only probable conjecture on the cui bono of these arms that I can at present make. Under this head I ought to notice the remarkable membranous process of an obovate shape, which like an umbrella covers the head of Acheta umbraculata F.[746] Whether the sharp curved horns which arm this part in another Acheta figured by Stoll[747], in an incumbent posture, with their point towards the mouth, are a sexual distinction, we are not informed,—probably they are.
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.