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.

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].

Of all the organs of the head, none seem so little subject to sexual variation as the under-jaws (maxillæ)[767]. I can bring forward only one striking instance of it, and some degree of doubt rests even upon that. In the genus Nemognatha of Illiger, the maxillæ of the male are elongated, narrow, setiform, and often involute or spiral, like those of a bee or a butterfly. But that this is peculiar to the males is at present only surmised[768]. I possess several species of the genus, all of which are distinguished by long maxillæ; though in some they are as long as the body, and in others scarcely half that length. Gnathium Kirby is similarly characterized[769].

The maxillary palpi occasionally differ in the sexes. In Cerocoma those of the female are filiform, while the two intermediate joints of those of the other sex are much thicker than the first and the last[770]. In Hylæcetus and Lymexylon, those of the male are still more remarkable: they are pendent, the last joint very large, and laciniated so as to form a tuft[771]. The female ones grow gradually larger towards the end, but are not at all divided there[772]. The palpi of male spiders are of a very different structure from those of the other sex, terminating in a very complex incrassated piece, which has been supposed to contain the organ of generation; but this, according to Treviranus, is a mistaken idea—that organ being, as usual, to be found in the abdomen[773]. In the common gnat the palpi of the male are as long as the proboscis, consist of five joints, and at the end are tufted with hairs; while those of the female are scarcely one-fourth of its length, have only three joints, and are not tufted. Whether the labial palpi in any genus differ in the sexes, I cannot affirm with certainty: I have not, however, observed any such variation in them.

I shall next mention some organs of the head, in which the difference between the sexes is often very striking and peculiar. You will readily conjecture that I am speaking of the antennæ. And here the advantage seems wholly on the side of the males: since in them these wonderful instruments of unknown sensations are not only more complex, but usually more elegant, than those of the other sex. You will pardon me, therefore, if I enlarge a little more than ordinary upon a subject so full of interest, and say something upon the differences observable between the sexes—in the shape, magnitude, and length, number of articulations, ramification and plumage, and individual joints of their antennæ.

With regard to their shape, variations are sometimes observable between the antennæ of the sexes; but this principally occurs in the Hymenoptera order. For instance, those of Chelostoma maxillosa, a small bee that deposits its eggs in little holes in posts and rails, are clavate in the female and filiform in the male[774]—a circumstance that distinguishes in some degree those of Sphecodes, Halictus, and Andrena of Latreille, three other genera of wild-bees[775]. In Dinetus Jur. the male antennæ are moniliform at the base, and filiform at the apex; the female, on the contrary, are entirely filiform[776].

The antennæ of the sexes also sometimes differ in magnitude and length. This is the case in the three genera of wild bees just mentioned; those of the female being thicker than those of the male, while these last are longer than the former. But in this tribe the males of the Fabrician genus Eucera are most remarkable for their long antennæ[777]. With regard to the different length of these organs in the sexes, no insects are more distinguished than some species of the capricorn-beetles (Cerambyx L.). In Lamia Sutor the male antennæ are twice the length of the female; and in another Brazilian species in my cabinet, related to L. annulata (Stenocorus F.), they are thrice their length. Some of the Anthribi F. approach the Cerambycidæ, not only in some other characters, but also in this circumstance:—thus the antennæ of A. albinus, a native of Britain, are vastly longer in the male than in the female; and in A. cinereus (Macrocephalus Oliv.)[778], which I suspect to be of the former sex, they are as long nearly as is usual in the tribe just named, called in France capricorn-beetles.

I may here observe, that sometimes in the sexes a difference is also to be found in the direction or flexure of their antennæ. Thus in Scolia F., Pepsis F. &c., in the males the antennæ are nearly straight, but in the females convolute or subspiral. The reverse of this takes place in Epipone spinipes, a kind of wasp, and its affinities; and Systropha Illig., a kind of bee: for in these the male antenna is convolute at the apex[779], and the female straight. In the various tribes of bees (Anthophila Latr.), these organs in the latter are what is denominated broken, the main body of the antenna forming an angle with the first joints: but in the former this does not take place.

The antennas of the sexes do not always agree in the number of joints. In the bees, and many other Hymenoptera, the male has one more joint than the female; as is the case also in Œdemera notata (Cantharis acuta Marsh.). In Pteronus Laricis, a kind of saw-fly, the latter has only sixteen joints in its antennæ, while the former has twenty-four[780]. In Rhipicera marginata, a beetle, the beautiful antennæ of the male consist of thirty-two joints, while the female has no more than eleven! In Chelonus Jur. the male, on the contrary, has the smallest number of joints, namely sixteen; while the female has twenty-five[781].

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.

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.