The Descent of Man and Selection in Relation to Sex Volume I Part 14

Although many dragon-flies are such large, powerful, and fierce insects, the males have not been observed by Mr. MacLachlan to fight together, except, as he believes, in the case of some of the smaller species of Agrion. In another very distinct group in this Order, namely in the Termites or white ants, both s.e.xes at the time of swarming may be seen running about, "the male after the female, sometimes two chasing one female, and contending with great eagerness who shall win the prize."[478]

Order, _Hymenoptera_.-That inimitable observer, M. Fabre,[479] in describing the habits of Cerceris, a wasp-like insect, remarks that "fights frequently ensue between the males for the possession of some particular female, who sits an apparently unconcerned beholder of the struggle for supremacy, and when the victory is decided, quietly flies away in company with the conqueror." Westwood[480] says that the males of one of the saw-flies (Tenthredinae) "have been found fighting together, with their mandibles locked." As M. Fabre speaks of the males of Cerceris striving to obtain a particular female, it may be well to bear in mind that insects belonging to this Order have the power of recognising each other after long intervals of time, and are deeply attached. For instance, Pierre Huber, whose accuracy no one doubts, separated some ants, and when after an interval of four months they met others which had formerly belonged to the same community, they mutually recognised and caressed each other with their antennae. Had they been strangers they would have fought together. Again, when two communities engage in a battle, the ants on the same side in the general confusion sometimes attack each other, but they soon perceive their mistake, and the one ant soothes the other.[481]

In this Order slight differences in colour, according to s.e.x, are common, but conspicuous differences are rare except in the family of Bees; yet both s.e.xes of certain groups are so brilliantly coloured-for instance in Chrysis, in which vermilion and metallic greens prevail-that we are tempted to attribute the result to s.e.xual selection. In the Ichneumonidae, according to Mr. Walsh,[482] the males are almost universally lighter coloured than the females. On the other hand, in the Tenthredinidae the males are generally darker than the females. In the Siricidae the s.e.xes frequently differ; thus the male of _Sirex juvencus_ is banded with orange, whilst the female is dark purple; but it is difficult to say which s.e.x is the most ornamented. In _Tremex columbae_ the female is much brighter coloured than the male.

With ants, as I am informed by Mr. F. Smith, the males of several species are black, the females being testaceous. In the family of Bees, especially in the solitary species, as I hear from the same distinguished entomologist, the s.e.xes often differ in colour. The males are generally the brightest, and in Bombus as well as in Apathus, much more variable in colour than the females. In _Anthophora retusa_ the male is of a rich fulvous-brown, whilst the female is quite black: so are the females of several species of Xylocopa, the males being bright yellow. In an Australian bee (_Lestis bombylans_), the female is of an extremely brilliant steel-blue, sometimes tinted with vivid green; the male being of a bright bra.s.sy colour clothed with rich fulvous p.u.b.escence. As in this group the females are provided with excellent defensive weapons in their stings, it is not probable that they have come to differ in colour from the males for the sake of protection.

_Mutilla Europaea_ emits a stridulating noise; and according to Goureau[483] both s.e.xes have this power. He attributes the sound to the friction of the third and preceding abdominal segments; and I find that these surfaces are marked with very fine concentric ridges, but so is the projecting thoracic collar, on which the head articulates; and this collar, when scratched with the point of a needle, emits the proper sound. It is rather surprising that both s.e.xes should have the power of stridulating, as the male is winged and the female wingless. It is notorious that Bees express certain emotions, as of anger, by the tone of their humming, as do some dipterous insects; but I have not referred to these sounds, as they are not known to be in any way connected with the act of courts.h.i.+p.

Order, _Coleoptera_ (Beetles).-Many beetles are coloured so as to resemble the surfaces which they habitually frequent. Other species are ornamented with gorgeous metallic tints,-for instance, many Carabidae, which live on the ground and have the power of defending themselves by an intensely acrid secretion,-the splendid diamond-beetles which are protected by an extremely hard covering,-many species of Chrysomela, such as _C. cerealis_, a large species beautifully striped with various colours, and in Britain confined to the bare summit of Snowdon,-and a host of other species. These splendid colours, which are often arranged in stripes, spots, crosses and other elegant patterns, can hardly be beneficial, as a protection, except in the case of some flower-feeding species; and we cannot believe that they are purposeless. Hence the suspicion arises, that they serve as a s.e.xual attraction; but we have no evidence on this head, for the s.e.xes rarely differ in colour. Blind beetles, which cannot of course behold each other's beauty, never exhibit, as I hear from Mr. Waterhouse, jun., bright colours, though they often have polished coats: but the explanation of their obscurity may be that blind insects inhabit caves and other obscure stations.

Some Longicorns, however, especially certain Prionidae, offer an exception to the common rule that the s.e.xes of beetles do not differ in colour. Most of these insects are large and splendidly coloured. The males in the genus Pyrodes,[484] as I saw in Mr. Bates' collection, are generally redder but rather duller than the females, the latter being coloured of a more or less splendid golden green. On the other hand, in one species the male is golden-green, the female being richly tinted with red and purple. In the genus Esmeralda the s.e.xes differ so greatly in colour that they have been ranked as distinct species: in one species both are of a beautiful s.h.i.+ning green, but the male has a red thorax. On the whole, as far as I could judge, the females of those Prionidae, in which the s.e.xes differ, are coloured more richly than the males; and this does not accord with the common rule in regard to colour when acquired through s.e.xual selection.

[Ill.u.s.tration: Fig. 15. Chalcosoma atlas. Upper figure, male (reduced); lower figure, female (nat. size).]

[Ill.u.s.tration: Fig. 16. Copris isidis. (Left-hand figures, males.)]

[Ill.u.s.tration: Fig. 17. Phanaeus faunus.]

[Ill.u.s.tration: Fig. 18. Dipelicus cantori.]

[Ill.u.s.tration: Fig. 19. Onthophagus rangifer, enlarged.]

A most remarkable distinction between the s.e.xes of many beetles is presented by the great horns which rise from the head, thorax, or clypeus of the males; and in some few cases from the under surface of the body. These horns, in the great family of the Lamellicorns, resemble those of various quadrupeds, such as stags, rhinoceroses, &c., and are wonderful both from their size and diversified shapes. Instead of describing them, I have given figures of the males and females of some of the more remarkable forms. (Figs. 15 to 19.) The females generally exhibit rudiments of the horns in the form of small k.n.o.bs or ridges; but some are dest.i.tute of even a rudiment. On the other hand, the horns are nearly as well developed in the female as in the male of _Phanaeus lancifer_; and only a little less well developed in the females of some other species of the same genus and of Copris. In the several subdivisions of the family, the differences in structure of the horns do not run parallel, as I am informed by Mr. Bates, with their more important and characteristic differences; thus within the same natural section of the genus Onthophagus, there are species which have either a single cephalic horn, or two distinct horns.

In almost all cases, the horns are remarkable from their excessive variability; so that a graduated series can be formed, from the most highly developed males to others so degenerate that they can barely be distinguished from the females. Mr. Walsh[485] found that in _Phanaeus carnifex_ the horns were thrice as long in some males as in others. Mr.

Bates, after examining above a hundred males of _Onthophagus rangifer_ (fig. 19), thought that he had at last discovered a species in which the horns did not vary; but further research proved the contrary.

The extraordinary size of the horns, and their widely different structure in closely-allied forms, indicate that they have been formed for some important purpose; but their excessive variability in the males of the same species leads to the inference that this purpose cannot be of a definite nature. The horns do not show marks of friction, as if used for any ordinary work. Some authors suppose[486] that as the males wander much more than the females, they require horns as a defence against their enemies; but in many cases the horns do not seem well adapted for defence, as they are not sharp. The most obvious conjecture is that they are used by the males for fighting together; but they have never been observed to fight; nor could Mr. Bates, after a careful examination of numerous species, find any sufficient evidence in their mutilated or broken condition of their having been thus used. If the males had been habitual fighters, their size would probably have been increased through s.e.xual selection, so as to have exceeded that of the female; but Mr. Bates, after comparing the two s.e.xes in above a hundred species of the Copridae, does not find in well-developed individuals any marked difference in this respect. There is, moreover, one beetle, belonging to the same great division of the Lamellicorns, namely Lethrus, the males of which are known to fight, but they are not provided with horns, though their mandibles are much larger than those of the female.

The conclusion, which best agrees with the fact of the horns having been so immensely yet not fixedly developed,-as shewn by their extreme variability in the same species and by their extreme diversity in closely-allied species-is that they have been acquired as ornaments.

This view will at first appear extremely improbable; but we shall hereafter find with many animals, standing much higher in the scale, namely fishes, amphibians, reptiles and birds, that various kinds of crests, k.n.o.bs, horns and combs have been developed apparently for this sole purpose.

[Ill.u.s.tration: Fig. 20. Onitis furcifer, male, viewed from beneath.]

[Ill.u.s.tration: Fig. 21. Left-hand figure, male of Onitis furcifer, viewed laterally. Right-hand figure, female. _a._ Rudiment of cephalic horn. _b._ Trace of thoracic horn or crest.]

The males of _Onitis furcifer_ (fig. 20) are furnished with singular projections on their anterior femora, and with a great fork or pair of horns on the lower surface of the thorax. This situation seems extremely ill adapted for the display of these projections, and they may be of some real service; but no use can at present be a.s.signed to them. It is a highly remarkable fact, that although the males do not exhibit even a trace of horns on the upper surface of the body, yet in the females a rudiment of a single horn on the head (fig. 21, _a_), and of a crest (_b_) on the thorax, are plainly visible. That the slight thoracic crest in the female is a rudiment of a projection proper to the male, though entirely absent in the male of this particular species, is clear: for the female of _Bubas bison_ (a form which comes next to _Onitis_) has a similar slight crest on the thorax, and the male has in the same situation a great projection. So again there can be no doubt that the little point (_a_) on the head of the female _Onitis furcifer_, as well of the females of two or three allied species, is a rudimentary representative of the cephalic horn, which is common to the males of so many lamellicorn beetles, as in Phanaeus, fig. 17. The males indeed of some unnamed beetles in the British Museum, which are believed actually to belong to the genus Onitis, are furnished with a similar horn. The remarkable nature of this case will be best perceived by an ill.u.s.tration: the Ruminant quadrupeds run parallel with the lamellicorn beetles, in some females possessing horns as large as those of the male, in others having them much smaller, or existing as mere rudiments (though this is as rare with ruminants as it is common with Lamellicorns), or in having none at all. Now if a new species of deer or sheep were discovered with the female bearing distinct rudiments of horns, whilst the head of the male was absolutely smooth, we should have a case like that of _Onitis furcifer_.

In this case the old belief of rudiments having been created to complete the scheme of nature is so far from holding good, that all ordinary rules are completely broken through. The view which seems the most probable is that some early progenitor of Onitis acquired, like other Lamellicorns, horns on the head and thorax, and then transferred them, in a rudimentary condition, as with so many existing species, to the female, by whom they have ever since been retained. The subsequent loss of the horns by the male may have resulted through the principle of compensation from the development of the projections on the lower surface, whilst the female has not been thus affected, as she is not furnished with these projections, and consequently has retained the rudiments of the horns on the upper surface. Although this view is supported by the case of Bledius immediately to be given, yet the projections on the lower surface differ greatly in structure and development in the males of the several species of Onitis, and are even rudimentary in some; nevertheless the upper surface in all these species is quite dest.i.tute of horns. As secondary s.e.xual characters are so eminently variable, it is possible that the projections on the lower surface may have been first acquired by some progenitor of Onitis and produced their effect through compensation, and then have been in certain cases almost completely lost.

[Ill.u.s.tration: Fig. 22. Bledius taurus, magnified. Left-hand figure, male; right-hand figure, female.]

All the cases. .h.i.therto given refer to the Lamellicorns, but the males of some few other beetles, belonging to two widely distinct groups, namely, the Curculionidae and Staphylinidae, are furnished with horns,-in the former on the lower surface of the body,[487] in the latter on the upper surface of the head and thorax. In the Staphylinidae the horns of the males in the same species are extraordinarily variable, just as we have seen with the Lamellicorns. In Siagonium we have a case of dimorphism, for the males can be divided into two sets, differing greatly in the size of their bodies, and in the development of their horns, without any intermediate gradations. In a species of Bledius (fig. 22), also belonging to the Staphylinidae, male specimens can be found in the same locality, as Professor Westwood states, "in which the central horn of the thorax is very large, but the horns of the head quite rudimental; and others, in which the thoracic horn is much shorter, whilst the protuberances on the head are long."[488] Here, then, we apparently have an instance of compensation of growth, which throws light on the curious case just given of the loss of the upper horns by the males of _Onitis furcifer_.

_Law of Battle._-Some male beetles, which seem ill fitted for fighting, nevertheless engage in conflicts for the possession of the females. Mr.

Wallace[489] saw two males of _Leptorhynchus angustatus_, a linear beetle with a much elongated rostrum, "fighting for a female, who stood close by busy at her boring. They pushed at each other with their rostra, and clawed and thumped, apparently in the greatest rage." The smaller male, however, "soon ran away, acknowledging himself vanquished." In some few cases the males are well adapted for fighting, by possessing great toothed mandibles, much larger than those of the females. This is the case with the common stag-beetle (_Luca.n.u.s cervus_), the males of which emerge from the pupal state about a week before the other s.e.x, so that several may often be seen pursuing the same female. At this period they engage in fierce conflicts. When Mr. A.

H. Davis[490] enclosed two males with one female in a box, the larger male severely pinched the smaller one, until he resigned his pretensions. A friend informs me that when a boy he often put the males together to see them fight, and he noticed that they were much bolder and fiercer than the females, as is well known to be the case with the higher animals. The males would seize hold of his finger, if held in front, but not so the females. With many of the Lucanidae, as well as with the above-mentioned Leptorhynchus, the males are larger and more powerful insects than the females. The two s.e.xes of _Lethrus cephalotes_ (one of the Lamellicorns) inhabit the same burrow; and the male has larger mandibles than the female. If, during the breeding-season, a strange male attempts to enter the burrow, he is attacked; the female does not remain pa.s.sive, but closes the mouth of the burrow, and encourages her mate by continually pus.h.i.+ng him on from behind. The action does not cease until the aggressor is killed or runs away.[491]

The two s.e.xes of another lamellicorn beetle, the _Ateuchus cicatricosus_ live in pairs, and seem much attached to each other; the male excites the female to roll the b.a.l.l.s of dung in which the ova are deposited; and if she is removed, he becomes much agitated. If the male is removed, the female ceases all work, and as M. Brulerie[492] believes, would remain on the spot until she died.

The great mandibles of the male Lucanidae are extremely variable both in size and structure, and in this respect resemble the horns on the head and thorax of many male Lamellicorns and Staphylinidae. A perfect series can be formed from the best-provided to the worst-provided or degenerate males. Although the mandibles of the common stag-beetle, and probably of many other species, are used as efficient weapons for fighting, it is doubtful whether their great size can thus be accounted for. We have seen that with the _Luca.n.u.s elaphus_ of N. America they are used for seizing the female. As they are so conspicuous and so elegantly branched, the suspicion has sometimes crossed my mind that they may be serviceable to the males as an ornament, in the same manner as the horns on the head and thorax of the various above described species. The male _Chiasognathus grantii_ of S. Chile-a splendid beetle belonging to the same family-has enormously-developed mandibles (fig. 23); he is bold and pugnacious; when threatened on any side he faces round, opening his great jaws, and at the same time stridulating loudly; but the mandibles were not strong enough to pinch my finger so as to cause actual pain.

[Ill.u.s.tration: Fig. 23. Chiasognathus grantii, reduced. Upper figure, male; lower figure, female.]

s.e.xual selection, which implies the possession of considerable perceptive powers and of strong pa.s.sions, seems to have been more effective with the Lamellicorns than with any other family of the Coleoptera or beetles. With some species the males are provided with weapons for fighting; some live in pairs and show mutual affection; many have the power of stridulating when excited; many are furnished with the most extraordinary horns, apparently for the sake of ornament; some which are diurnal in their habits are gorgeously coloured; and, lastly, several of the largest beetles in the world belong to this family, which was placed by Linnaeus and Fabricius at the head of the Order of the Coleoptera.[493]

_Stridulating organs._-Beetles belonging to many and widely distinct families possess these organs. The sound can sometimes be heard at the distance of several feet or even yards,[494] but is not comparable with that produced by the Orthoptera. The part which may be called the rasp generally consists of a narrow slightly-raised surface, crossed by very fine, parallel ribs, sometimes so fine as to cause iridescent colours, and having a very elegant appearance under the microscope. In some cases, for instance, with Typhaeus, it could be plainly seen that extremely minute, bristly, scale-like prominences, which cover the whole surrounding surface in approximately parallel lines, give rise to the ribs of the rasp by becoming confluent and straight, and at the same time more prominent and smooth. A hard ridge on any adjoining part of the body, which in some cases is specially modified for the purpose, serves as the sc.r.a.per for the rasp. The sc.r.a.per is rapidly moved across the rasp, or conversely the rasp across the sc.r.a.per.

[Ill.u.s.tration: Fig. 24. Necrophorus (from Landois). _r._ The two rasps.

Left-hand figure, part of the rasp highly magnified.]

These organs are situated in widely different positions. In the carrion-beetles (Necrophorus) two parallel rasps (_r_, fig. 24) stand on the dorsal surface of the fifth abdominal segment, each rasp being crossed, as described by Landois,[495] by from 126 to 140 fine ribs.

These ribs are sc.r.a.ped by the posterior margins of the elytra, a small portion of which projects beyond the general outline. In many Crioceridae, and in _Clythra 4-punctata_ (one of the Chrysomelidae), and in some Tenebrionidae, &c.,[496] the rasp is seated on the dorsal apex of the abdomen, on the pygidium or pro-pygidium, and is sc.r.a.ped as above by the elytra. In Heterocerus, which belongs to another family, the rasps are placed on the sides of the first abdominal segment, and are sc.r.a.ped by ridges on the femora.[497] In certain Curculionidae and Carabidae,[498]

the parts are completely reversed in position, for the rasps are seated on the inferior surface of the elytra, near their apices, or along their outer margins, and the edges of the abdominal segments serve as the sc.r.a.pers. In _Pelobius hermanni_ (one of Dytiscidae or water-beetles) a strong ridge runs parallel and near to the sutural margin of the elytra, and is crossed by ribs, coa.r.s.e in the middle part, but becoming gradually finer at both ends, especially at the upper end; when this insect is held under water or in the air, a stridulating noise is produced by sc.r.a.ping the extreme h.o.r.n.y margin of the abdomen against the rasp. In a great number of long-horned beetles (Longicornia) the organs are altogether differently situated, the rasp being on the meso-thorax, which is rubbed against the pro-thorax; Landois counted 238 very fine ribs on the rasp of _Cerambyx heros_.

[Ill.u.s.tration: Fig. 25. Hind-leg of Geotrupes stercorarius (from Landois). _r._ Rasp. _c._ c.o.xa. _f._ Femur. _t._ Tibia. _tr._ Tarsi.]

Many Lamellicorns have the power of stridulating, and the organs differ greatly in position. Some species stridulate very loudly, so that when Mr. F. Smith caught a _Trox sabulosus_, a gamekeeper who stood by thought that he had caught a mouse; but I failed to discover the proper organs in this beetle. In Geotrupes and Typhaeus a narrow ridge runs obliquely across (_r_, fig. 25) the c.o.xa of each hind-leg, having in _G. stercorarius_ 84 ribs, which are sc.r.a.ped by a specially-projecting part of one of the abdominal segments. In the nearly allied _Copris lunaris_, an excessively narrow fine rasp runs along the sutural margin of the elytra, with another short rasp near the basal outer margin; but in some other Coprini the rasp is seated, according to Leconte,[499] on the dorsal surface of the abdomen. In Oryctes it is seated on the pro-pygidium, and in some other Dynastini, according to the same entomologist, on the under surface of the elytra. Lastly, Westring states that in _Omaloplia brunnea_ the rasp is placed on the pro-sternum, and the sc.r.a.per on the meta-sternum, the parts thus occupying the under surface of the body, instead of the upper surface as in the Longicorns.

We thus see that the stridulating organs in the different coleopterous families are wonderfully diversified in position, but not much in structure. Within the same family some species are provided with these organs, and some are quite dest.i.tute of them. This diversity is intelligible, if we suppose that originally various species made a shuffling or hissing noise by the rubbing together of the hard and rough parts of their bodies which were in contact; and that from the noise thus produced being in some way useful, the rough surfaces were gradually developed into regular stridulating organs. Some beetles as they move, now produce, either intentionally or unintentionally, a shuffling noise, without possessing any proper organs for the purpose.

Mr. Wallace informs me that the _Euchirus longima.n.u.s_ (a Lamellicorn, with the anterior legs wonderfully elongated in the male) "makes, whilst moving, a low hissing sound by the protrusion and contraction of the abdomen; and when seized it produces a grating sound by rubbing its hind-legs against the edges of the elytra." The hissing sound is clearly due to a narrow rasp running along the sutural margin of each elytron; and I could likewise make the grating sound by rubbing the s.h.a.greened surface of the femur against the granulated margin of the corresponding elytron; but I could not here detect any proper rasp; nor is it likely that I could have overlooked it in so large an insect. After examining Cychrus and reading what Westring has written in his two papers about this beetle, it seems very doubtful whether it possesses any true rasp, though it has the power of emitting a sound.

From the a.n.a.logy of the Orthoptera and h.o.m.optera, I expected to find that the stridulating organs in the Coleoptera differed according to s.e.x; but Landois, who has carefully examined several species, observed no such difference; nor did Westring; nor did Mr. G. R. Crotch in preparing the numerous specimens which he had the kindness to send me for examination. Any slight s.e.xual difference, however, would be difficult to detect, on account of the great variability of these organs. Thus in the first pair of the _Necrophorus humator_ and of the _Pelobius_ which I examined, the rasp was considerably larger in the male than in the female; but not so with succeeding specimens. In _Geotrupes stercorarius_ the rasp appeared to me thicker, opaquer, and more prominent in three males than in the same number of females; consequently my son, Mr. F. Darwin, in order to discover whether the s.e.xes differed in their power of stridulating, collected 57 living specimens, which he separated into two lots, according as they made, when held in the same manner, a greater or lesser noise. He then examined their s.e.xes, but found that the males were very nearly in the same proportion to the females in both lots. Mr. F. Smith has kept alive numerous specimens of _Mononychus pseudacori_ (Curculionidae), and is satisfied that both s.e.xes stridulate, and apparently in an equal degree.

Nevertheless the power of stridulating is certainly a s.e.xual character in some few Coleoptera. Mr. Crotch has discovered that the males alone of two species of Heliopathes (Tenebrionidae) possess stridulating organs. I examined five males of _H. gibbus_, and in all these there was a well-developed rasp, partially divided into two, on the dorsal surface of the terminal abdominal segment; whilst in the same number of females there was not even a rudiment of the rasp, the membrane of this segment being transparent and much thinner than in the male. In _H.

cribratostriatus_ the male has a similar rasp, excepting that it is not partially divided into two portions, and the female is completely dest.i.tute of this organ; but in addition the male has on the apical margins of the elytra, on each side of the suture, three or four short longitudinal ridges, which are crossed by extremely fine ribs, parallel to and resembling those on the abdominal rasp; whether these ridges serve as an independent rasp, or as a sc.r.a.per for the abdominal rasp, I could not decide: the female exhibits no trace of this latter structure.

Again, in three species of the Lamellicorn genus Oryctes, we have a nearly parallel case. In the females of _O. gryphus_ and _nasicornis_ the ribs on the rasp of the pro-pygidium are less continuous and less distinct than in the males; but the chief difference is that the whole upper surface of this segment, when held in the proper light, is seen to be clothed with hairs, which are absent or are represented by excessively fine down in the males. It should be noticed that in all Coleoptera the effective part of the rasp is dest.i.tute of hairs. In _O.

senegalensis_ the difference between the s.e.xes is more strongly marked, and this is best seen when the proper segment is cleaned and viewed as a transparent object. In the female the whole surface is covered with little separate crests, bearing spines; whilst in the male these crests become, in proceeding towards the apex, more and more confluent, regular, and naked; so that three-fourths of the segment is covered with extremely fine parallel ribs, which are quite absent in the female. In the females, however, of all three species of Oryctes, when the abdomen of a softened specimen is pushed backwards and forwards, a slight grating or stridulating sound can be produced.

In the case of the Heliopathes and Oryctes there can hardly be a doubt that the males stridulate in order to call or to excite the females; but with most beetles the stridulation apparently serves both s.e.xes as a mutual call. This view is not rendered improbable from beetles stridulating under various emotions; we know that birds use their voices for many purposes besides singing to their mates. The great Chiasognathus stridulates in anger or defiance; many species do the same from distress or fear, when held so that they cannot escape; Messrs.

Wollaston and Crotch were able, by striking the hollow stems of trees in the Canary Islands, to discover the presence of beetles belonging to the genus Acalles by their stridulation. Lastly the male Ateuchus stridulates to encourage the female in her work, and from distress when she is removed.[500] Some naturalists believe that beetles make this noise to frighten away their enemies; but I cannot think that the quadrupeds and birds which are able to devour the larger beetles with their extremely hard coats, would be frightened by so slight a grating sound. The belief that the stridulation serves as a s.e.xual call is supported by the fact that death-ticks (_An.o.bium tessellatum_) are well known to answer each other's ticking, or, as I have myself observed, a tapping noise artificially made; and Mr. Doubleday informs me that he has twice or thrice observed a female ticking,[501] and in the course of an hour or two has found her united with a male, and on one occasion surrounded by several males. Finally, it seems probable that the two s.e.xes of many kinds of beetles were at first enabled to find each other by the slight shuffling noise produced by the rubbing together of the adjoining parts of their hard bodies; and that as the males or females which made the greatest noise succeeded best in finding partners, the rugosities on various parts of their bodies were gradually developed by means of s.e.xual selection into true stridulating organs.

CHAPTER XI.

INSECTS, _continued_.-ORDER LEPIDOPTERA.

Courts.h.i.+p of b.u.t.terflies-Battles-Ticking noise-Colours common to both s.e.xes, or more brilliant in the males-Examples-Not due to the direct action of the conditions of life-Colours adapted for protection-Colours of moths-Display-Perceptive powers of the Lepidoptera-Variability-Causes of the difference in colour between the males and females-Mimickry, female b.u.t.terflies more brilliantly coloured than the males-Bright colours of caterpillars-Summary and concluding remarks on the secondary s.e.xual characters of insects-Birds and insects compared.

In this great Order the most interesting point for us is the difference in colour between the s.e.xes of the same species, and between the distinct species of the same genus. Nearly the whole of the following chapter will be devoted to this subject; but I will first make a few remarks on one or two other points. Several males may often be seen pursuing and crowding round the same female. Their courts.h.i.+p appears to be a prolonged affair, for I have frequently watched one or more males pirouetting round a female until I became tired, without seeing the end of the courts.h.i.+p. Although b.u.t.terflies are such weak and fragile creatures, they are pugnacious, and an Emperor b.u.t.terfly[502] has been captured with the tips of its wings broken from a conflict with another male. Mr. Collingwood in speaking of the frequent battles between the b.u.t.terflies of Borneo says, "They whirl round each other with the greatest rapidity, and appear to be incited by the greatest ferocity."

One case is known of a b.u.t.terfly, namely the _Ageronia feronia_, which makes a noise like that produced by a toothed wheel pa.s.sing under a spring catch, and which could be heard at the distance of several yards.