The Descent of Man and Selection in Relation to Sex Volume I Part 13

Diversified structures possessed by the males for seizing the females-Differences between the s.e.xes, of which the meaning is not understood-Difference in size between the s.e.xes-Thysanura-Diptera-Hemiptera-h.o.m.optera, musical powers possessed by the males alone-Orthoptera, musical instruments of the males, much diversified in structure; pugnacity; colours-Neuroptera, s.e.xual differences in colour-Hymenoptera, pugnacity and colours-Coleoptera, colours; furnished with great horns, apparently as an ornament; battles; stridulating organs generally common to both s.e.xes.

In the immense cla.s.s of insects the s.e.xes sometimes differ in their organs for locomotion, and often in their sense-organs, as in the pectinated and beautifully plumose antennae of the males of many species.

In one of the Ephemerae, namely Chloeon, the male has great pillared eyes, of which the female is entirely dest.i.tute.[427] The ocelli are absent in the females of certain other insects, as in the Mutillidae, which are likewise dest.i.tute of wings. But we are chiefly concerned with structures by which one male is enabled to conquer another, either in battle or courts.h.i.+p, through his strength, pugnacity, ornaments, or music. The innumerable contrivances, therefore, by which the male is able to seize the female, may be briefly pa.s.sed over. Besides the complex structures at the apex of the abdomen, which ought perhaps to be ranked as primary organs,[428] "it is astonis.h.i.+ng," as Mr. B. D.

Walsh[429] has remarked, "how many different organs are worked in by nature, for the seemingly insignificant object of enabling the male to grasp the female firmly." The mandibles or jaws are sometimes used for this purpose; thus the male _Corydalis cornutus_ (a neuropterous insect in some degree allied to the Dragon-flies, &c.) has immense curved jaws, many times longer than those of the female; and they are smooth instead of being toothed, by which means he is enabled to seize her without injury.[430] One of the stag-beetles of North America (_Luca.n.u.s elaphus_) uses his jaws, which are much larger than those of the female, for the same purpose, but probably likewise for fighting. In one of the sand-wasps (_Ammophila_) the jaws in the two s.e.xes are closely alike, but are used for widely different purposes; the males, as Professor Westwood observes, "are exceedingly ardent, seizing their partners round the neck with their sickle-shaped jaws;"[431] whilst the females use these organs for burrowing in sand-banks and making their nests.

[Ill.u.s.tration: Fig. 8. Crabro cribrarius. Upper figure, male: lower figure, female.]

The tarsi of the front-legs are dilated in many male beetles, or are furnished with broad cus.h.i.+ons of hairs; and in many genera of water-beetles they are armed with a round flat sucker, so that the male may adhere to the slippery body of the female. It is a much more unusual circ.u.mstance that the females of some water-beetles (Dytiscus) have their elytra deeply grooved, and in _Acilius sulcatus_ thickly set with hairs, as an aid to the male. The females of some other water-beetles (Hydroporus) have their elytra punctured for the same object.[432] In the male of _Crabro cribrarius_ (fig. 8.), it is the tibia which is dilated into a broad h.o.r.n.y plate, with minute membraneous dots, giving to it a singular appearance like that of a riddle.[433] In the male of Penthe (a genus of beetles) a few of the middle joints of the antennae are dilated and furnished on the inferior surface with cus.h.i.+ons of hair, exactly like those on the tarsi of the Carabidae, "and obviously for the same end." In male dragon-flies, "the appendages at the tip of the tail are modified in an almost infinite variety of curious patterns to enable them to embrace the neck of the female." Lastly in the males of many insects, the legs are furnished with peculiar spines, k.n.o.bs or spurs; or the whole leg is bowed or thickened, but this is by no means invariably a s.e.xual character; or one pair, or all three pairs are elongated, sometimes to an extravagant length.[434]

[Ill.u.s.tration: Fig. 9. Taphroderes distortus (much enlarged). Upper figure, male; lower figure, female.]

In all the orders, the s.e.xes of many species present differences, of which the meaning is not understood. One curious case is that of a beetle (fig. 9), the male of which has the left mandible much enlarged; so that the mouth is greatly distorted. In another Carabidous beetle, the Eurygnathus,[435] we have the unique case, as far as known to Mr.

Wollaston, of the head of the female being much broader and larger, though in a variable degree, than that of the male. Any number of such cases could be given. They abound in the Lepidoptera: one of the most extraordinary is that certain male b.u.t.terflies have their fore-legs more or less atrophied, with the tibiae and tarsi reduced to mere rudimentary k.n.o.bs. The wings, also, in the two s.e.xes often differ in neuration,[436]

and sometimes considerably in outline, as in the _Aricoris epitus_, which was shown to me in the British Museum by Mr. A. Butler. The males of certain South American b.u.t.terflies have tufts of hair on the margins of the wings, and h.o.r.n.y excrescences on the discs of the posterior pair.[437] In several British b.u.t.terflies, the males alone, as shewn by Mr. Wonfor, are in parts clothed with peculiar scales.

The purpose of the luminosity in the female glow-worm is likewise not understood; for it is very doubtful whether the primary use of the light is to guide the male to the female. It is no serious objection to this latter belief that the males emit a feeble light; for secondary s.e.xual characters proper to one s.e.x are often developed in a slight degree in the other s.e.x. It is a more valid objection that the larvae s.h.i.+ne, and in some species brilliantly: Fritz Muller informs me that the most luminous insect which he ever beheld in Brazil, was the larva of some beetle.

Both s.e.xes of certain luminous species of Elater emit light. Kirby and Spence suspect that the phosph.o.r.escence serves to frighten and drive away enemies.

_Difference in Size between the s.e.xes._-With insects of all kinds the males are commonly smaller than the females;[438] and this difference can often be detected even in the larval state. So considerable is the difference between the male and female coc.o.o.ns of the silk-moth (_Bombyx mori_), that in France they are separated by a particular mode of weighing.[439] In the lower cla.s.ses of the animal kingdom, the greater size of the females seems generally to depend on their developing an enormous number of ova; and this may to a certain extent hold good with insects. But Dr. Wallace has suggested a much more probable explanation. He finds, after carefully attending to the development of the caterpillars of _Bombyx cynthia_ and _yamamai_, and especially of some dwarfed caterpillars reared from a second brood on unnatural food, "that in proportion as the individual moth is finer, so is the time required for its metamorphosis longer; and for this reason the female, which is the larger and heavier insect, from having to carry her numerous eggs, will be preceded by the male, which is smaller and has less to mature."[440] Now as most insects are short-lived, and as they are exposed to many dangers, it would manifestly be advantageous to the female to be impregnated as soon as possible. This end would be gained by the males being first matured in large numbers ready for the advent of the females; and this again would naturally follow, as Mr. A.

E. Wallace has remarked,[441] through natural selection; for the smaller males would be first matured, and thus would procreate a large number of offspring which would inherit the reduced size of their male parents, whilst the larger males from being matured later would leave fewer offspring.

There are, however, exceptions to the rule of male insects being smaller than the females; and some of these exceptions are intelligible. Size and strength would be an advantage to the males, which fight for the possession of the female; and in these cases the males, as with the stag-beetle (Luca.n.u.s), are larger than the females. There are, however, other beetles which are not known to fight together, of which the males exceed the females in size; and the meaning of this fact is not known; but in some of these cases, as with the huge Dynastes and Megasoma, we can at least see that there would be no necessity for the males to be smaller than the females, in order to be matured before them, for these beetles are not short-lived, and there would be ample time for the pairing of the s.e.xes. So, again, male dragon-flies (Libellulidae) are sometimes sensibly larger, and never smaller, than the females;[442] and they do not, as Mr. MacLachlan believes, generally pair with the females, until a week or fortnight has elapsed, and until they have a.s.sumed their proper masculine colours. But the most curious case, shewing on what complex and easily-overlooked relations, so trifling a character as a difference in size between the s.e.xes may depend, is that of the aculeate Hymenoptera; for Mr. F. Smith informs me that throughout nearly the whole of this large group the males, in accordance with the general rule, are smaller than the females and emerge about a week before them; but amongst the Bees, the males of _Apis mellifica_, _Anthidium manicatum_ and _Anthophora acervorum_, and amongst the Fossores, the males of the _Methoca ichneumonides_, are larger than the females. The explanation of this anomaly is that a marriage-flight is absolutely necessary with these species, and the males require great strength and size in order to carry the females through the air.

Increased size has here been acquired in opposition to the usual relation between size and the period of development, for the males, though larger, emerge before the smaller females.

We will now review the several Orders, selecting such facts as more particularly concern us. The Lepidoptera (b.u.t.terflies and Moths) will be retained for a separate chapter.

Order, _Thysanura_.-The members of this Order are lowly organised for their cla.s.s. They are wingless, dull-coloured, minute insects, with ugly, almost misshapen heads and bodies. The s.e.xes do not differ; but they offer one interesting fact, by showing that the males pay sedulous court to their females even low down in the animal scale. Sir J.

Lubbock[443] in describing the _Smynthurus luteus_, says: "it is very amusing to see these little creatures coquetting together. The male, which is much smaller than the female, runs round her, and they b.u.t.t one another, standing face to face, and moving backward and forward like two playful lambs. Then the female pretends to run away and the male runs after her with a queer appearance of anger, gets in front and stands facing her again; then she turns coyly round, but he, quicker and more active, scuttles round too, and seems to whip her with his antennae; then for a bit they stand face to face, play with their antennae, and seem to be all in all to one another."

Order, _Diptera_ (Flies).-The s.e.xes differ little in colour. The greatest difference, known to Mr. F. Walker, is in the genus Bibio, in which the males are blackish or quite black, and the females obscure brownish-orange. The genus Elaphomyia, discovered by Mr. Wallace[444] in New Guinea, is highly remarkable, as the males are furnished with horns, of which the females are quite dest.i.tute. The horns spring from beneath the eyes, and curiously resemble those of stags, being either branched or palmated. They equal in length the whole of the body in one of the species. They might be thought to serve for fighting, but as in one species they are of a beautiful pink colour, edged with black, with a pale central stripe, and as these insects have altogether a very elegant appearance, it is perhaps more probable that the horns serve as ornaments. That the males of some Diptera fight together is certain; for Prof. Westwood[445] has several times seen this with some species of Tipula or Harry-long-legs. Many observers believe that when gnats (Culicidae) dance in the air in a body, alternately rising and falling, the males are courting the females. The mental faculties of the Diptera are probably fairly well developed, for their nervous system is more highly developed than in most other Orders of insects.[446]

Order, _Hemiptera_ (Field-Bugs).-Mr. J. W. Douglas, who has particularly attended to the British species, has kindly given me an account of their s.e.xual differences. The males of some species are furnished with wings, whilst the females are wingless; the s.e.xes differ in the form of the body and elytra; in the second joints of their antennae and in their tarsi; but as the signification of these differences is quite unknown, they may be here pa.s.sed over. The females are generally larger and more robust than the males. With British, and, as far as Mr. Douglas knows, with exotic species, the s.e.xes do not commonly differ much in colour; but in about six British species the male is considerably darker than the female, and in about four other species the female is darker than the male. Both s.e.xes of some species are beautifully marked with vermilion and black. It is doubtful whether these colours serve as a protection. If in any species the males had differed from the females in an a.n.a.logous manner, we might have been justified in attributing such conspicuous colours to s.e.xual selection with transference to both s.e.xes.

Some species of Reduvidae make a stridulating noise; and, in the case of _Pirates stridulus_, this is said[447] to be effected by the movement of the neck within the pro-thoracic cavity. According to Westring, _Reduvius personatus_ also stridulates. But I have not been able to learn any particulars about these insects; nor have I any reason to suppose that they differ s.e.xually in this respect.

Order, _h.o.m.optera._-Every one who has wandered in a tropical forest must have been astonished at the din made by the male Cicadae. The females are mute; as the Grecian poet Xenarchus says, "Happy the Cicadas live, since they all have voiceless wives." The noise thus made could be plainly heard on board the "Beagle," when anch.o.r.ed at a quarter of a mile from the sh.o.r.e of Brazil; and Captain Hanc.o.c.k says it can be heard at the distance of a mile. The Greeks formerly kept, and the Chinese now keep, these insects in cages for the sake of their song, so that it must be pleasing to the ears of some men.[448] The Cicadidae usually sing during the day; whilst the Fulgoridae appear to be night-songsters. The sound, according to Landois,[449] who has recently studied the subject, is produced by the vibration of the lips of the spiracles, which are set into motion by a current of air emitted from the tracheae. It is increased by a wonderfully complex resounding apparatus, consisting of two cavities covered by scales. Hence the sound may truly be called a voice. In the female the musical apparatus is present, but very much less developed than in the male, and is never used for producing sound.

With respect to the object of the music, Dr. Hartman in speaking of the _Cicada septemdecim_ of the United States, says,[450] "the drums are now (June 6th and 7th, 1851) heard in all directions. This I believe to be the marital summons from the males. Standing in thick chestnut sprouts about as high as my head, where hundreds were around me, I observed the females coming around the drumming males." He adds, "this season (Aug.

1868) a dwarf pear-tree in my garden produced about fifty larvae of _Cic.

pruinosa_; and I several times noticed the females to alight near a male while he was uttering his clanging notes." Fritz Muller writes to me from S. Brazil that he has often listened to a musical contest between two or three males of a Cicada, having a particularly loud voice, and seated at a considerable distance from each other. As soon as the first had finished his song, a second immediately began; and after he had concluded, another began, and so on. As there is so much rivalry between the males, it is probable that the females not only discover them by the sounds emitted, but that, like female birds, they are excited or allured by the male with the most attractive voice.

I have not found any well-marked cases of ornamental differences between the s.e.xes of the h.o.m.optera. Mr. Douglas informs me that there are three British species, in which the male is black or marked with black bands, whilst the females are pale-coloured or obscure.

Order, _Orthoptera_.-The males in the three saltatorial families belonging to this Order are remarkable for their musical powers, namely the Achetidae or crickets, the Locustidae for which there is no exact equivalent name in English, and the Acridiidae or gra.s.shoppers. The stridulation produced by some of the Locustidae is so loud that it can be heard during the night at the distance of a mile;[451] and that made by certain species is not unmusical even to the human ear, so that the Indians on the Amazons keep them in wicker cages. All observers agree that the sounds serve either to call or excite the mute females. But it has been noticed[452] that the male migratory locust of Russia (one of the Acridiidae) whilst coupled with the female, stridulates from anger or jealousy when approached by another male. The house-cricket when surprised at night uses its voice to warn its fellows.[453] In North America the Katy-did (_Platyphyllum concavum_, one of the Locustidae) is described[454] as mounting on the upper branches of a tree, and in the evening beginning "his noisy babble, while rival notes issue from the neighbouring trees, and the groves resound with the call of _Katy-did-she-did_, the live-long night." Mr. Bates, in speaking of the European field-cricket (one of the Achetidae), says, "the male has been observed to place itself in the evening at the entrance of its burrow, and stridulate until a female approaches, when the louder notes are succeeded by a more subdued tone, whilst the successful musician caresses with his antennae the mate he has won."[455] Dr. Scudder was able to excite one of these insects to answer him, by rubbing on a file with a quill.[456] In both s.e.xes a remarkable auditory apparatus has been discovered by Von Siebold, situated in the front legs.[457]

[Ill.u.s.tration: Fig. 10. Gryllus campestris (from Landois).

Right-hand figure, under side of part of the wing-nervure, much magnified, showing the teeth, _st_.

Left-hand figure, upper surface of wing-cover, with the projecting, smooth nervure, _r._, across which the teeth (_st_) are sc.r.a.ped.]

In the three Families the sounds are differently produced. In the males Of the Achetidae both wing-covers have the same structure; and this in the field-cricket (_Gryllus campestris_, fig. 10) consists, as described by Landois,[458] of from 131 to 138 sharp, transverse ridges or teeth (_st_) on the under side of one of the nervures of the wing-cover. This toothed nervure is rapidly sc.r.a.ped across a projecting, smooth, hard nervure (_r_) on the upper surface of the opposite wing.

First one wing is rubbed over the other, and then the movement is reversed. Both wings are raised a little at the same time, so as to increase the resonance. In some species the wing-covers of the males are furnished at the base with a talc-like plate.[459] I have here given a drawing (fig. 11) of the teeth on the under side of the nervure of another species of Gryllus, viz. _G. domesticus_.

[Ill.u.s.tration: Fig. 11. Teeth of Nervure of Gryllus domesticus (from Landois).]

In the Locustidae the opposite wing-covers differ in structure (fig. 12), and cannot, as in the last family, be indifferently used in a reversed manner. The left wing, which acts as the bow of the fiddle, lies over the right wing which serves as the fiddle itself. One of the nervures (_a_) on the under surface of the former is finely serrated, and is sc.r.a.ped across the prominent nervures on the upper surface of the opposite or right wing. In our British _Phasgonura viridissima_ it appeared to me that the serrated nervure is rubbed against the rounded hind corner of the opposite wing, the edge of which is thickened, coloured brown, and very sharp. In the right wing, but not in the left, there is a little plate, as transparent as talc, surrounded by nervures, and called the speculum. In _Ephippiger vitium_, a member of this same family, we have a curious subordinate modification; for the wing-covers are greatly reduced in size, but "the posterior part of the pro-thorax is elevated into a kind of dome over the wing-covers, and which has probably the effect of increasing the sound."[460]

[Ill.u.s.tration: Fig. 12. Chloroclus Tanana (from Bates). _a_, _b_.

Lobes of opposite wing-covers.]

We thus see that the musical apparatus is more differentiated or specialised in the Locustidae, which includes I believe the most powerful performers in the Order, than in the Achetidae, in which both wing-covers have the same structure and the same function.[461] Landois, however, detected in one of the Locustidae, namely in Decticus, a short and narrow row of small teeth, mere rudiments, on the inferior surface of the right wing-cover, which underlies the other and is never used as the bow. I observed the same rudimentary structure on the under side of the right wing-cover in _Phasgonura viridissima_. Hence we may with confidence infer that the Locustidae are descended from a form, in which, as in the existing Achetidae, both wing-covers had serrated nervures on the under surface, and could be indifferently used as the bow; but that in the Locustidae the two wing-covers gradually became differentiated and perfected, on the principle of the division of labour, the one to act exclusively as the bow and the other as the fiddle. By what steps the more simple apparatus in the Achetidae originated, we do not know, but it is probable that the basal portions of the wing-covers overlapped each other formerly as at present, and that the friction of the nervures produced a grating sound, as I find is now the case with the wing-covers of the females.[462] A grating sound thus occasionally and accidentally made by the males, if it served them ever so little as a love-call to the females, might readily have been intensified through s.e.xual selection by fitting variations in the roughness of the nervures having been continually preserved.

[Ill.u.s.tration: Fig. 13, Hind-leg of Sten.o.bothrus pratorum: _r_, the stridulating ridge; lower figure, the teeth, forming the ridge, much magnified (from Landois).]

In the last and third Family, namely the Acridiidae or gra.s.shoppers, the stridulation is produced in a very different manner, and is not so shrill, according to Dr. Scudder, as in the preceding Families. The inner surface of the femur (fig. 13, _r_) is furnished with a longitudinal row of minute, elegant, lancet-shaped, elastic teeth, from 85 to 93 in number;[463] and these are sc.r.a.ped across the sharp, projecting nervures on the wing-covers, which, are thus made to vibrate and resound. Harris[464] says that when one of the males begins to play, he first "bends the shank of the hind-leg beneath, the thigh, where it is lodged in a furrow designed to receive it, and then draws the leg briskly up and down. He does not play both fiddles together, but alternately first upon one and then on the other." In many species, the base of the abdomen is hollowed out into a great cavity which is believed to act as a resounding board. In Pneumora (fig. 14), a S.

African genus belonging to this same family, we meet with a new and remarkable modification: in the males a small notched ridge projects obliquely from each side of the abdomen, against which the hind femora are rubbed.[465] As the male is furnished with wings, the female being wingless, it is remarkable that the thighs are not rubbed in the usual manner against the wing-covers; but this may perhaps be accounted for by the unusually small size of the hind-legs. I have not been able to examine the inner surface of the thighs, which, judging from a.n.a.logy, would be finely serrated. The species of Pneumora have been more profoundly modified for the sake of stridulation than any other orthopterous insect; for in the male the whole body has been converted into a musical instrument, being distended with air, like a great pellucid bladder, so as to increase the resonance. Mr. Trimen informs me that at the Cape of Good Hope these insects make a wonderful noise during the night There is one exception to the rule that the females in these three Families are dest.i.tute of an efficient musical apparatus; for both s.e.xes of Ephippiger (Locustidae) are said[466] to be thus provided. This case may be compared with that of the reindeer, in which species alone both s.e.xes possess horns. Although the female orthoptera are thus almost invariably mute, yet Landois[467] found rudiments of the stridulating organs on the femora of the female Acridiidae, and similar rudiments on the under surface of the wing-covers of the female Achetidae; but he failed to find any rudiments in the females of Decticus, one of the Locustidae. In the h.o.m.optera the mute females of Cicada, have the proper musical apparatus in an undeveloped state; and we shall hereafter meet in other divisions of the animal kingdom with innumerable instances of structures proper to the male being present in a rudimentary condition in the female. Such cases appear at first sight to indicate that both s.e.xes were primordially constructed in the same manner, but that certain organs were subsequently lost by the females.

It is, however, a more probable view, as previously explained, that the organs in question were acquired by the males and partially transferred to the females.

[Ill.u.s.tration: Fig. 14. Pneumora (from specimens in the British Museum).

Upper figure, male; lower figure, female.]

Landois has observed another interesting fact, namely that in the females of the Acridiidae, the stridulating teeth on the femora remain throughout life in the same condition in which they first appear in both s.e.xes during the larval state. In the males, on the other hand, they become fully developed and acquire their perfect structure at the last moult, when the insect is mature and ready to breed.

From the facts now given, we see that the means by which the males produce their sounds are extremely diversified in the Orthoptera, and are altogether different from those employed by the h.o.m.optera. But throughout the animal kingdom we incessantly find the same object gained by the most diversified means; this being due to the whole organisation undergoing in the course of ages multifarious changes; and as part after part varies, different variations are taken advantage of for the same general purpose. The diversification of the means for producing sound in the three families of the Orthoptera and in the h.o.m.optera, impresses the mind with the high importance of these structures to the males, for the sake of calling or alluring the females. We need feel no surprise at the amount of modification which the Orthoptera have undergone in this respect, as we now know, from Dr.

Scudder's remarkable discovery,[468] that there has been more than ample time. This naturalist has lately found a fossil insect in the Devonian formation of New Brunswick, which is furnished with "the well-known tympanum or stridulating apparatus of the male Locustidae." This insect, though in most respects related to the Neuroptera, appears to connect, as is so often the case with very ancient forms, the two Orders of the Neuroptera and Orthoptera which are now generally ranked as quite distinct.

I have but little more to say on the Orthoptera. Some of the species are very pugnacious: when two male field-crickets (_Gryllus campestris_) are confined together, they fight till one kills the other; and the species of Mantis are described as manuvring with their sword-like front-limbs, like hussars with their sabres. The Chinese keep these insects in little bamboo cages and match them like game-c.o.c.ks.[469] With respect to colour, some exotic locusts are beautifully ornamented; the posterior wings being marked with red, blue, and black; but as throughout the Order the two s.e.xes rarely differ much in colour, it is doubtful whether they owe these bright tints to s.e.xual selection.

Conspicuous colours may be of use to these insects as a protection, on the principle to be explained in the next chapter, by giving notice to their enemies that they are unpalatable. Thus it has been observed[470]

that an Indian brightly-coloured locust was invariably rejected when offered to birds and lizards. Some cases, however, of s.e.xual differences in colour in this Order are known. The male of an American cricket[471]

is described as being as white as ivory, whilst the female varies from almost white to greenish-yellow or dusky. Mr. Walsh informs me that the adult male of _Spectrum femoratum_ (one of the Phasmidae) "is of a s.h.i.+ning brownish-yellow colour; the adult female being of a dull, opaque, cinereous-brown; the young of both s.e.xes being green." Lastly, I may mention that the male of one curious kind of cricket[472] is furnished with "a long membranous appendage, which falls over the face like a veil;" but whether this serves as an ornament is not known.

Order, _Neuroptera_.-Little need here be said, except in regard to colour. In the Ephemeridae the s.e.xes often differ slightly in their obscure tints;[473] but it is not probable that the males are thus rendered attractive to the females. The Libellulidae or dragon-flies are ornamented with splendid green, blue, yellow, and vermilion metallic tints; and the s.e.xes often differ. Thus, the males of some of the Agrionidae, as Prof. Westwood remarks[474] "are of a rich blue with black wings, whilst the females are fine green with colourless wings." But in _Agrion Ramburii_ these colours are exactly reversed in the two s.e.xes.[475] In the extensive N. American genus of Hetaerina, the males alone have a beautiful carmine spot at the base of each wing. In _Anax junius_ the basal part of the abdomen in the male is a vivid ultra-marine blue, and in the female gra.s.s-green. In the allied genus Gomphus, on the other hand, and in some other genera, the s.e.xes differ but little in colour. Throughout the animal kingdom, similar cases of the s.e.xes of closely-allied forms either differing greatly, or very little, or not at all, are of frequent occurrence. Although with many Libellulidae there is so wide a difference in colour between the s.e.xes, it is often difficult to say which is the most brilliant; and the ordinary coloration of the two s.e.xes is exactly reversed, as we have just seen, in one species of Agrion. It is not probable that their colours in any case have been gained as a protection. As Mr. MacLachlan, who has closely attended to this family, writes to me, dragon-flies-the tyrants of the insect-world-are the least liable of any insect to be attacked by birds or other enemies. He believes that their bright colours serve as a s.e.xual attraction. It deserves notice, as bearing on this subject, that certain dragon-flies appear to be attracted by particular colours: Mr. Patterson observed[476] that the species of Agrionidae, of which the males are blue, settled in numbers on the blue float of a fis.h.i.+ng line; whilst two other species were attracted by s.h.i.+ning white colours.

It is an interesting fact, first observed by Schelver, that the males, in several genera belonging to two sub-families, when they first emerge from the pupal state are coloured exactly like the females; but that their bodies in a short time a.s.sume a conspicuous milky-blue tint, owing to the exudation of a kind of oil, soluble in ether and alcohol. Mr.

MacLachlan believes that in the male of _Libellula depressa_ this change of colour does not occur until nearly a fortnight after the metamorphosis, when the s.e.xes are ready to pair.

Certain species of Neurothemis present, according to Brauer[477] a curious case of dimorphism, some of the females having their wings netted in the usual manner; whilst other females have them "very richly netted as in the males of the same species." Brauer "explains the phenomenon on Darwinian principles by the supposition that the close netting of the veins is a secondary s.e.xual character in the males." This latter character is generally developed in the males alone, but being, like every other masculine character, latent in the female, is occasionally developed in them. We have here an ill.u.s.tration of the manner in which the two s.e.xes of many animals have probably come to resemble each other, namely by variations first appearing in the males, being preserved in them, and then transmitted to and developed in the females; but in this particular genus a complete transference is occasionally and abruptly effected. Mr. MacLachlan informs me of another case of dimorphism occurring in several species of Agrion in which a certain number of individuals are found of an orange colour, and these are invariably females. This is probably a case of reversion, for in the true Libellulae, when the s.e.xes differ in colour, the females are always orange or yellow, so that supposing Agrion to be descended from some primordial form having the characteristic s.e.xual colours of the typical Libellulae, it would not be surprising that a tendency to vary in this manner should occur in the females alone.