iv. In many cases, the structure of particularpartsandorgansof 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—theHead,Trunk, andAbdomenof insects, and their several appendages.
1. TheHead. This part in some females is considerably larger than it is in the male. This is the case with the ants, and several otherHymenoptera; while in someAndrenæ, asA. hæmorrhoidalis, andStaphylinidæ, asSt. 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 theApiongenus, the rostrum of the female is generally longer and slenderer than that of her mate; and inBrentus, 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 inB. disparandmaxillosus[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 areunicorns, and have their head armed with only asinglehorn; which in some, as inOryctesIllig.,Dynastes Endymion[728], &c. is very short; in others,very long, as inDynastes Enema,Pan,Elephas[729]. In one, again, it is thick and robust; as in the clumsyDynastes Actæon[730]: in another very slender, as inOnthophagus spinifer[731]. With respect to its direction inElephastomus proboscideusMacLeay, it is horizontal[732]and straight; inPhaleria cornutahorizontal and broken, or the apex turning outwards and forming an angle with the base[733]; inDynastes Herculeshorizontal, and recurved at the apex[734]; inD. Actæon,Elephas, andTyphon, recurving from the base. InGeotrupes disparit is recurved, so that its point exactly coincides with that of the porrected thoracic horn, with which it forms a kind of forceps[735]. InCopris lunarisF. andDiaperis horrida, the horn is nearly upright[736]. InOnthophagus Xiphiasit is dilated at the base, and reclining upon the thorax; and at the apex attenuated, and bending forwards, or nodding. InPassalus cornutusit rises a little, and then bends wholly forwards. InDynastes Milon, a most remarkable beetle, it slopes backwards in a waving line[737]; and inOnthophagus spiniferit 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. InDynastes Herculesit is armed with several teeth. InD. ElephasandActæonit has only one large one at its upper base[738]. InD. Milonit is serrated above. InD. Alcides,Tityus,Ægeon,Copris lunaris, &c. the horn is unarmed and simple at the apex. InD. Oromedon,Gedeon,Enema,Actæonand congeners, it is bifid. In some the horn is at first a broad lamina or ridge, which terminates in two branches, as inOnthophagus Vacca. In this the branches are straight; but in another undescribed species in my cabinet (O. AriesKirby, MS.) they are first bent inwards, and then at the apex a little recurved: and inD. dichotomusit 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 apairof horns on their head. InOnthophagus Taurus, these arms in their curvature exactly resemble those of the first of these animals[740]. InGoliathus 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 namedO. cervicornis. InO. Vacca,Camelus, &c. the horns are very short, and nearly perpendicular. In the male ofCopris 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 orBeelzebulthan a Midas[741], or thanPhanæus BeelzebulMacL. A similar cavity is between the occipital horns ofDiaperis hæmorrhoidalisPayk. Some species ofRynchænus, asR. 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. ThusDitomus calydoniusBonelli, belonging toCarabusL., hasthreeequal horns[743]. The same number distinguishesOnthophagus Bonasus; but the intermediate one is very short. InGoliathus Polyphemusthe 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 littleDiaperis(D. viridipennisF.), a native ofCarolina, 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 distinguishesOnthophagus quadricornis(CoprisF.). Thesituationalso of the horns varies: In some it is in themiddleof the head, asOryctes nasicornis,Copris lunaris, &c.: in others, as inOnthophagus nuchicornis,Xiphias, &c. it is a processof theocciputor hind-head; and inO. OryxF. the two horns proceed from theanteriorpart 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 ofCopris lunaris; or by transverse ridges, as in theOnthophagi: or else the head is without arms, and quite smooth, as inDiaperis,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 thecui bonoof 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 ofAcheta umbraculataF.[746]Whether the sharp curved horns which arm this part in anotherAchetafigured by Stoll[747], in an incumbent posture, with their point towards the mouth, are a sexual distinction, we are not informed,—probably they are.
Theorgansof the head also present many sexualdistinctions. The upper lip (labrum) inHalictusLatr., 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 inSphecodesLatr. differs in the sexes[749]. Perhaps the horn or tubercle observable on this part of some femaleNomadæF.[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 femaleLucani. In the male the labium is emarginate, in the female it is intire. This may be seen both inL. Cervusandfemoratus, and probably in other species. The sculpture also is different, the lip being smooth in the former and covered with excavatedpunctain the latter. The tongue (linguaorligula) 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 takenin coituthose 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 abattlefor anamour: 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 inLucanus 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 inL. Alces[752]; but in none more threatening than inL. Elephas[753], in which they curve outwards and downwards. In Mr. W. MacLeay's genusPholidotus, they are almost parallel to each other, and curve downwards; inLucanus nebulosusKirby, they assume a contrary direction[754]; as they do likewise inLamprimaLatr.[755]InLucanus Capreolusthe points close over each other[756]. InLethrusF. in the female, but not the male, the mandible is armed below with a long incurved horn. InLucanus serricornisthey form a complete forceps[757]. InSiagonium quadricorneKirby[758]the mandible is furnished at its base with an exterior horn, which is probably a sexual distinction. The male ofSynagris 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 thanthe mandible, but than the head itself[759]. Many sexual differences are observable in the mandibles of the various tribes of bees (AnthophilaLatr.). Thus, inColletesLatr. themalemandible is more distinctly bidentate at the apex than the female[760]: inSphecodesLatr. 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 ofMegachile centuncularisLatr. has four teeth at the apex of its mandible, while the male has only two[762]. InM. 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]. InAnthidium manicatumLatr., 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 (BombusLatr.) 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 tosexualvariation as the under-jaws (maxillæ)[767]. Ican bring forward only one striking instance of it, and some degree of doubt rests even upon that. In the genusNemognathaof 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.GnathiumKirby is similarly characterized[769].
Themaxillary palpioccasionally differ in the sexes. InCerocomathose 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]. InHylæcetusandLymexylon, 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 itslength, have only three joints, and are not tufted. Whether thelabial palpiin 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 theantennæ. 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 theirshape, variations are sometimes observable between the antennæ of the sexes; but this principally occurs in theHymenopteraorder. For instance, those ofChelostoma 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 ofSphecodes,Halictus, andAndrenaof Latreille, three other genera of wild-bees[775]. InDinetusJur. 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 inmagnitudeandlength. 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 genusEuceraare 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 (CerambyxL.). InLamia Sutorthe male antennæ are twice the length of the female; and in another Brazilian species in my cabinet, related toL. annulata(StenocorusF.), they are thrice their length. Some of theAnthribiF. approach theCerambycidæ, not only in some other characters, but also in this circumstance:—thus the antennæ ofA. albinus, a native of Britain, are vastly longer in the male than in the female; and inA. cinereus(MacrocephalusOliv.)[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 thedirectionor flexure of their antennæ. Thus inScoliaF.,PepsisF. &c., in the males the antennæ are nearly straight, but in the females convolute or subspiral. The reverse of this takes place inEpipone spinipes, a kind of wasp, and its affinities; andSystrophaIllig., 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 (AnthophilaLatr.), theseorgans in the latter are what is denominatedbroken, 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 thenumberof joints. In the bees, and many otherHymenoptera, the male has one more joint than the female; as is the case also inŒdemera notata(Cantharis acutaMarsh.). InPteronus Laricis, a kind of saw-fly, the latter has only sixteen joints in its antennæ, while the former has twenty-four[780]. InRhipicera marginata, a beetle, the beautiful antennæ of the male consist of thirty-two joints, while the female has no more than eleven! InChelonusJur. 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 theramificationof these organs, and theirplumage. 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 ofChelostoma maxillosa,—mistaken for another species by Linné, which he namesApis florisomnis,—the intermediate joints on their inner side project into an angle[782]; and those of the same sex of the commonhornet, 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 withHylotoma furcataLatr., a saw-fly[783]; and the peacock-louse (Nirmus PavonisHerm.)[784]. Others, again, have three lateral branches, as inEulophusGeoffr. 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]. InElater flabellicornisL., 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. InLampyris LatreilliiKirby, 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 theRhipiceræof Latreille. InR. marginataK. 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 denominatesBombyces 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 shorteron the whole, and alternately one long and one short; but in some, asSaturnia Pavonia, there is only one short branch or tooth on each joint in this sex[790]. InBombyx regalis&c. only the first part of the antenna is so branched; and those of the female are setaceous and without branches. InB. 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 ofPteronus LaricisJur., 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 cinctusKlug, 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 genusPhengodes(Lampyris plumosaF.) is a sexual character has not been ascertained; but it is not improbable that it may be, as in otherLampyridæ. 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 theDipteraorder, assume an appearance ofplumes—not from the branches that proceed from them, but from the fine long hairs that beset and adorn them. These are universally indications of themale 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]. InChironomusMeig., a kind ofTipulaL., 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 someHymenoptera, the antennæ of the males are thus feathered, in a less degree: for instance, inHylotomaLatr.[796]Whether the tufts and fringes which ornament, in a remarkable manner, the antennæ of manyCerambycidæ[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 moreindividual joints. To begin with the first joint, or scapus. In many of theHymenoptera, particularly theAnthophilaLatr., 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 inHylæus dilatatus(Melitta dilatataKirby) the first joint in the male is dilated and shaped something like a patella[798]. InMalachius bipustulatus, &c. the sex just mentioned is peculiarly distinguished by a white excrescence on the firstfour joints of the organs in question, most conspicuous in the second and fourth. The antennæ of maleCerocomæare not very different[799]. Mr. Marsham has described a littleHalticaunder the name ofChrysomela 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. InH. Brassicæandquadripustulatathe fifth joint is larger and longer than all but the first in the male, in their females it is only longer. In some moths (HerminiaLatr.,CrambusF.) there is also a knot in the middle of the male antennæ[800]. InNoterus, 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 ofMeloeare 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 theclava, 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. InColymbetes serricornis, a water-beetle, the male has a serrated clava of four joints. InDorcatoma dresdensis[803], and alsoEnoplium damicorne, two beetles, it is nearly branched in themale, 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. InEurhinusKirby, 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 theeyes, orstemmata, this diversity is less remarkable. Latreille has described two ants,Formica contractaandcoeca, 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 ofMyrmica 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 thesizeof the eyes of their sexes. In the hive-bee, and someEphemeræ, 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]. InStratyomis,Tabanus, and many othertwo-winged flies, themaleeyes 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 thefemaleare separated by a considerable interval. InHeptatomaandHæmatopotain 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 inTabanus; 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, asMuscaLatr., 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 ofEphemeraL. (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. TheTrunk. The thorax of many coleopterous males, especially of theDynastidæandCopridæamongst the petalocerous tribes, exhibits very striking differences from that of the female. In manyLucanithe lateral angle is more prominent. InAnthiait is bilobed posteriorly, while in the last-mentioned sex it is entire[809]. InPhanæus carnifexMacLeay (CoprisF.) it is elevated into a plane triangular space, with the vertex of the trianglepointing to the head; but in the female it is convex, with an anterior abbreviated transverse ridge[810].
In a large proportion terrifichorns, 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 areunicorns, or armed only with asinglethoracic horn, which frequently, in conjunction with the thorax itself, not a little resembles a tunnel reversed: of this description areDynastes 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, andD. Pan, it forms an angle with it; and inD. Ægeonit is nearly vertical[812]. InD. Herculesit is very long; inD. Alcides[813]andTityusvery short; in the two last, and inOxytelus tricorniswhich is similarly armed, it is undivided at the apex; but inD. Gedeon,Pan,bilobus, &c.[814]it is bifid or bilobed. It is usually rather slender, but inD. Chorinæus[815]andbilobus, it is very stout and wide. InD. clavigerit is hastate at the apex[816]. InD. hastatusit is short and truncated[817]. Others, again, havetwothoracic horns. InCopris nemestrinusthese are discoidal, diverging, and inclining forwards[818]. InPhanæus floriger[819]they are lateral, triangular, and incline towards each other, with, as it were, a deep basin between them. InP. splendidulusthey sink into two longitudinal ridges, most elevatedposteriorly, with an intervening valley[820]. InP. bellicosusthey are posterior, compressed, truncated, and emarginate at the apex, and include a basin[821]. InCopris Sabæusthey are merely two acute prominences[822].—Threehorns distinguish the thorax of many. InD. Aloeus[823]and its affinities, they are arranged in a triangle, whose vertex is towards the head. InD. Antæus[824]these horns are nearly equal in length, and undivided at the apex. InD. Titanus[825]the anterior horn is longer than the rest, and bifid at the apex; inD. AtlasandEndymion[826], both of which have a horn on the head, it is much shorter. In others, as inMegasomaKirby, the vertex of the triangle is towards the anus. InM. Typhon[827]it is longer than the anterior ones, and bifid at the apex; inM. lanigerumthey are equal in length[828]. InM. ElephasandActæon[829]it is merely an elevation of the thorax; in the last almost obsolete. InGeotrupes Typhæus, common on our heaths, the anterior of this part is armed by three horizontal horns, the intermediate one being the shortest[830].Copris lunarisalso, 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]. InDynastes Neptunusthe horns are porrected, the middle one being very long, and the lateral ones short[832]. InD. Geryonthe point of the lateralhorns is towards the anus, and the base of the intermediate one covers the scutellum[833]. Others havefourof these singular arms: this is the case with one of our rarest beetles,Bolbocerus mobilicornisK., which has four dentiform horns, the intermediate pair being the shortest, arranged in a transverse line on the anterior part of the thorax[834]. InB. quadridensthese are merely teeth. InPhanæus Faunus[835]it has two lateral, elongated, compressed, truncate, horizontal horns, and two intermediate teeth.Dynastes Milonhas 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]; andCopris AntenorFabricius 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 withpectoralones. The illustrious traveller Humboldt found in South America a species of weevil (Cryptorhynchus SpiculatorHumb.), 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 ofRynchænus StrixF.[838]In the individual which I take to beC. Spiculator, the pectoral horns are very long, curving upwards at theapex, 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 ofRynchites, asR. BacchusandPopuli[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 theinstruments of motionattached to the trunk—beginning with those forflight. 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 inLampyris, in which a large number of females have both elytra and wings. The males ofBombyx antiquaandGonostigma, 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 theIchneumonidæ[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 theColeoptera, the female ofTenebrio 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 inAradus BetulæF., a kind of bug, the hemelytra and wings are narrower and shorter in thefemale than in the male[843]. In the genusBlapsF., the mucro that arms the apex of each elytrum is longer in the former sex than in the latter. InAteuchus gibbosusF., a dung-beetle, the elytra have a basal gibbosity near the suture in one sex that does not obtain in the other. In theOrthopteraorder, 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 maleCicadæ[844]. The wings of some butterflies, and of most moths and hawkmoths (SphinxL.), 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 byMoses Harris, and was afterwards more fully explained byM. 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, thelegs, also differ in the sexes. In some instances they are disproportionably long. This is particularly the case with theanteriorpairof some beetles, asMacropus longimanus,Scarabæus longimanusL., in which they are so long as to make the males of these individuals rather inconvenient in a cabinet. Amongst British beetlesClytra longimanaandCurculio longimanusMarsh. are also remarkable in this respect. In some other males themiddlepair are the longest; as inAnthophora retusaLatr., a kind of wild-bee[847]. There are two known instances of remarkably longposteriorlegs in the Capricorn tribe, which I suspect belong to the present head. One isSaperda hirtipesOliv.[848], in which the hind-legs are longer than the whole body, and adorned with a singular tuft of hairs; and the other aClytus, 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 ofmaleinsects.
To come to thepartsof legs. Sometimes thecoxæof the last mentioned sex are distinguished from those of the female by being armed by amucroor spine. Thus the male ofMegachile Willughbiella, and others of that tribe, have such a spine on the inner sides of the anterior coxa[849]. TheTrochanteralso of some differs sexually; and you will find that the posterior one of the male inAnthidium manicatumis of a different shape from what it is in the female[850]. InSphodrus leucopththalmus, one of the beetles calledblack dors, in one sex the same trochanterterminates in a long mucro or spine[851], and in the other it is rounded at the apex.
Peculiar characters in theirthighsalso often indicate different sexes. InPrionus damicornisthere is a short spine at the apex of theanteriorones in the female that is not in the male; while inMacropus longimanus, at their base externally the male is armed with a mucro, which I cannot find in the female[852]. InScarabæus longimanusL. this thigh is furnished with two teeth[853].—Theintermediatethighs also sometimes differ. In anOnitisfrom China, a variety perhaps ofO. Sphinx,those in the male are dolabriform, and in the other sex of the ordinary shape. InOdynerus spinipesthey have on their lower side two sinuses, so as to give them the appearance of being toothed. Theposteriorthighs 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 ofCimbexF., a kind of saw-fly; and the same circumstance distinguishes the latter sex in many species ofLygæusF., a kind of bug: I discovered this fromL. cruciger, of which I have both the sexes; and from Stoll's figure ofL. Pharaonis[854]. In some of these the female thighs are enormously large. A remarkable variation in this respect is observable in the coleopterous genusŒdemera(NecydalisL.). InŒ. Podagrariæthese limbs are incrassated in one sex and not in the other[855]; inŒ. cæruleathey are so in both sexes; and inŒ. ceramboidesin neither. InPelecinus PolyceratorF., 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.
Theanterior tibiæinScarabæus longimanusL. 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 genusOnitisF. they are bent like a bow, and acute at the end; but in the females they are formed on the common type[857]. InHispa spinipesF. 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 ofCrabro cribariusF. 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 (AnthophilaLatr.) theposteriortibia of the working sex is generally bigger than the corresponding part in their more idle partners: this is particularlyconspicuous in the genusEuglossa, 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 themalesof someLepidopteraare remarkable in this respect. That ofHepialus Humuliis much more hairy; but inH. Hectusit 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 thecalcariaor spurs that arm the apex of the tibiæ of a large number of insects. Thus inAcanthopusKlug, 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 thetarsus; 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 ofCryptophagusHerbst, asC. fumatus, &c. in which the female ispentamerous, or havingfivejoints in all the tarsi; and the maleheteromerous, or havingfivejoints in the twoanteriorpairs, and onlyfourin theposterior[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 carnivorousColeoptera, you may often, without further examination, ascertain whether any individual is male or female. Even in the slender-footedCicindelidæ, the three first joints of the anterior tarsus of the male are more dilated than the two last, and covered underneath with a brushof stiffish hair; in the female all are equally slender, and not so hairy. InCarabus,Feronia, &c. Latr. thefourfirst joints of these tarsi in the males are dilated, and furnished with a brush or cushion: in theSilphidæ, also, the same circumstance takes place. InHarpalusLatr., andSilpha americana, thefouranterior 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 theDytiscidæ, &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]. InHydrophilus 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 theHymenoptera, has described some very singular appendages which he observed on the first joint of the four posterior tarsi ofXylocopa latipesF. 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 amled to conjecture that, like the supposedClavariæthat 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]. InBeris clavipesandEmpis 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 ofAnoplognathus Dytiscoidesof 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 genusOnitisF.; but I have a specimen ofOnitis Apellesof 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 inPhanæus, 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'sScarabæidæin his cabinet, must have noticed: namely, that in all, exceptCoprisandOnthophagus, the anterior tarsi are usually broken off. Out of seventeen individuals ofScarabæusMacLeay in my own, not a single one has a relic of an anterior tarsus; and scarcely one in a much greater number ofPhanæi. The tarsus in question in the nobler sex inCrabro, at least inC. cribrariusand 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]. Theclaws, indeed, occasionally vary in the sexes in otherHymenoptera: thus inMelectaLatr., 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]. InCœlioxys conicaand others, those of the latter sex are bifid at the apex, but those of the former acute[871]. InMegachile, 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. Theabdomen. 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 generalshapeit often differs in the sexes. Thus, the abdomen of femaleTipulæis lanceolate; that of the male cylindrical, and thickest at the extremity[874]. InMolorchusF. it is convex above in the former, and flat in the latter,—the female of this beetle not unaptly representing some femaleIchneumonsin this respect, and the male their males[875]. InAndrenait 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.
Thenumber of segments, also, is generally different in the two sexes—the male having one more than the female; but inDytiscus 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 (FormicaL.), the little vertical scale, at thebaseof 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 theVespidæ, and related toSynagris, (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 ofChelostoma maxillosahas 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 genusPneumorabefore 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.
InStaphylinus splendens, thepenultimateventral segment is very deeply cleft, and the antepenultimate emarginate in one sex, and intire in the other. InS. 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.
Theextremityof the abdomen or itsanalsegments and organs furnish a variety of sexual characters. Sometimes the last dorsal segment is emarginate in the male, and not in the female; as inMegachile 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 inChelostoma maxillosa[884]. InAnthidium manicatum, another bee, the anus terminates in five spines[885]. InCœlioxys conicaof 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 genusPanorpa, 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. Marshamhas considered them (the sexes both ofF. auriculariaandF. minor) as distinct species.