butterflies and moths, and the coloration of their wings—female choice and “fine feathers”—when male butterflies are dominant—sexual selection among butterflies—abortive experiments—wallace and the sexual selection theory—the sense of smell in butterflies and moths—fragrant butterflies—wingless moths and their lures to lovers—methods of pairing among butterflies and moths—more experiments.
not the least impressive feature met with in the study of animal behaviour under the spell of the sexual instincts is its uniformity. this fact becomes the more apparent as one turns to the lower grades of life. whether one starts with the vertebrates and works downwards, or vice versa, the same problems arise and the same interpretation is demanded. that is to say, the theory of “sexual selection” leads one to the same conclusions whether it be tested by the evidence afforded by the butterflies and moths, or that furnished by birds or mammals.
the accessory phenomena, the vehicles which give expression to these internal fires, are in like manner curiously similar. these “vehicles” are the “secondary sexual characters”—colour, and armature, and scent. these very tangible signs are the phenomena in the 186mystery play of sex which first catch the attention of the investigator. to account for these the theory of “sexual selection” was first devised.
after the birds, probably the group most conspicuous for its splendour is that which contains the scale-winged insects or lepidoptera, and it has always been allowed that any explanation of the one must apply also to the other. it seems impossible to avoid this conclusion. but before going further it would be well to take note of one or two interesting features in regard to coloration that have so far not been touched upon in these pages.
the coloration of animals is generally regarded as a by no means fortuitous feature, but one, on the contrary, controlled and determined by various factors. hence are recognized various kinds of coloration: obliterative or protective-resemblance coloration; warning coloration; mimetic coloration; and epigamic coloration, or the colours associated with courtship. these various types have been subdivided and accorded technical labels by professor e. b. poulton, in his “colours of animals,” but these need not be enlarged upon here. suffice it to say that it is generally held that all forms of coloration can be explained, and all can be labelled, as to their origin, with more or less certainty. there are those who doubt the warranty for this classification. commonly, it must be admitted, the arguments of these sceptics are not impressive; they are sometimes even stupid. that such coloration, however it be labelled, is subjected to some control seems to be shown in the case of the lepidoptera, for, generally, in the butterflies, the upper surface of the wings is much more vividly coloured than the under surface, and this, apparently, because when the 187creature is at rest the wings are brought up over the back like the leaves of a book, so that the brightly-tinted, and therefore conspicuous, area is concealed, as, for example, in the “red admiral.” with the moths the wings, while the creature is at rest, are held horizontally, and it is the upper instead of the under surface which is exposed, but the hind-wing is covered by the fore-wing. the coloration is here very different; for while the exposed surfaces of the fore-wings are commonly soberly tinted, the hind-wings may be quite glaringly coloured. these bright colours are exposed only during flight, or during moments of unusual excitement, as in the case of the eyed hawk-moth. according to weismann, this insect when alarmed raises the fore-wings so as to expose the “eye-spots “on the hind-wings, which, with the increased area of the wings, impart a terrifying appearance to the body to would-be assailants. this is as it may be, but for the moment the feature to be insisted upon is that the bright colours are almost invariably hidden when the insect is at rest, and by quite different means, determined, apparently, by the different carriage of the wings. now, according to some, bright colours are begotten by strong light, but in the moth and butterfly the surface area of the wing which is most exposed is the surface turned to the light during rest, and this is the least coloured. the curious relation between this coloration and the resting position is strikingly illustrated by the case of one of the “small blues” (lyccen?), cited by weismann. herein the male, which has the upper surface of the wings of a bright blue, rests in the position common to butterflies—with the wings raised and concealing the bright colour—while the female, which has the upper surface of a dull brown, rests with the wings expanded. 188as, however, the concealed under surface is not brightly coloured, it is difficult to believe that these different postures and conspicuously different colours can have been brought into existence solely by the action of natural selection, which, it is generally contended, has brought about the extinction of those individuals which neglected, when resting, and therefore liable to be “caught napping,” to conceal their arresting colours. there is, indeed, no apparent reason why the female, which has nothing to conceal, should depart from the custom common to butterflies, of resting with the wings closed and raised, this position effectively protecting the male. the facts seem to show that the coloration of the exposed surfaces of the wings is determined primarily by some physiological factor rather than by the incidence of natural selection directly through external agencies. thus, for example, the action of light on the surface of the wings when in the resting posture may well inhibit the production of vivid pigment owing to some inherent physiological idiosyncrasy. but any individuals which lack this inhibiting factor—as some species which, though resting, are brightly coloured, appear to do—will be eliminated, if they live in an environment harbouring eliminating factors, which the exceptions to the rule we must suppose do not. but on this interpretation the fundamental factor in the determination of the coloration is the action of light. selection imposes a bar only to certain types of coloration.
some butterflies and moths, it has just been hinted, when resting exhibit bright colours. our “swallow-tail” the under surface of the wings is as brightly tinted as the upper. among the moths may be cited many of the 189gorgeous atlas moths, the hawk moths, the beautiful indian dysphania militaris—wherein the whole of the exposed surface is of a beautiful and vivid violet and yellow—and the tropical members of the burnet moths, belonging to the family syntomid?. in all these cases it is not the under but the upper surface of the fore-wings which has thus departed from the usual rule of the tribe. not the least remarkable feature of these insects is the fact that while the atlas and hawk moths are crepuscular in habits, the dysphanias and syntomids and burnet moths are diurnal, and revel in the sunlight.
to revert for a moment to the factors to which these and other bright and often conspicuous hues are due. that all highly-coloured animals are descendants of dull-coloured ancestors there can be no room for doubt. the vivid tints they now display are to be regarded as due to some change in the metabolism, some clarifying process of the organism whereby the various pigments became segregated, concentrated and intensified. but many of the most vivid hues are not due to pigment at all, but to changes in the surface structure of the coloured areas. such are the wonderful metallic colours which all kinds of animals display. the iridescence is due to the breaking up of the light by reflection from finely-grooved surfaces.
whatever their nature, one still asks what is their origin, what brought them into being. they cannot be regarded simply as adaptations which have arisen to meet the demands of the environment, as are the structural peculiarities of the skeleton for example; for in this case both sexes, and all stages of growth, should display the same hues, and this is rarely the case. furthermore, we should not in this case be left with a vast assemblage 190of forms which certainly cannot be “pigeon-holed” as to the nature of their coloration. such, for example, as the marine types of birds.
the metallic and iridescent tints to which reference has just been made, occur among animals to which they can be of but doubtful value, as in the golden mole, for example, or the inside of the oyster shell. their existence in such places well illustrates what we may call the fortuitous, or apparently fortuitous, beginning of colour of whatever kind, regarded from an analytical point of view. that is to say, we are not concerned with the fact that animals are coloured—that is inseparable from their existence; but with why this coloration should, in some cases, assume so conspicuous a brilliancy and vividness—a coloration varying in its character with every species, but apparently unchanging among the individuals of that species.
no answer to this, likely to find general acceptance, seems to be forthcoming at present. but it is significant to remark that all coloration of the kind now under consideration has its origin, as have most other structural characters, in the male. it is as true of coloration as of, say, skeletal characters. one turns to the male for what is new in the history of a species, to the female and young for indications of past history. it is equally true that in their coloration one finds the same sequence of development—the male first, then the female, then the young, till both sexes, and all stages, are once more alike in hue. and this rule seems to apply to coloration of all kinds—protective—warning—epigamic.
the tendency to develop brilliant colours is associated with some physiological diathesis with which we are not yet acquainted. but once having started, this tendency gathers force with each succeeding generation and continues 191to exhibit an almost kaleidoscopic capacity for change, unless, and until, checked by natural selection, whereby its further progress in any given direction may be barred, or some other element or aspect of the coloration may be introduced.
given this controlling factor, all the various types of coloration would seem to be interpretable. by almost common consent, however, the resplendent coloration of the males among many species of birds, a coloration often apparent only during the reproductive period, and the more conspicuous ornamentation of the males of many other groups, higher and lower in the scale of organization, are supposed to be governed by an entirely different factor—female choice, or preference. the exercise of this, it is contended, has gone on for countless generations, and the tendency has ever been to heighten the intensity of the ornament by the rejection of the less favoured suitors in favour of their more resplendent rivals. birds and butterflies alike are supposed to be swayed by the same irresistible desire to mate, and mate only with what we may call the smartest and best—groomed of their many suitors; and these, of course, being the most vigorous, most virile, sustain the stamina of the race and so attain nature’s end.
so long as attention was focused alone, or mainly, on birds conspicuous for the highly ornamental character of their plumage, this theory seemed reasonable and probable enough, for one may admit in their courtships an element, at least, of intelligence and keenness of perception. but it has now been abundantly demonstrated that the animated displays so characteristic of these gaily-bedecked gallants, are enacted with no less persistence and vim by species which exhibit a quaker-like soberness 192of dress. thus, then, the champions of the sexual selection theory have been dazzled by the tinsel, and have missed the essential elements—the physical and psychological side of the display—the contortions, prancings, and so on, and they have missed the even more important element, the preliminary struggle for territory.
in this new light, the gaily-bedizened individuals of the insect world may be surveyed afresh. the explanation of such of their features as are commonly attributed to sexual selection in terms of female choice, whereby only the most favoured from among a crowd of suitors could hope to succeed, may now be replaced by that which obtains also in the case of the higher animals. it seems to fit the facts better. one cannot understand, for example, how, on the interpretation of sexual selection, the extraordinary disparity in numbers between the sexes of some species of butterflies came about. thus in that marvellously beautiful genus ornithoptera there is one species (o. brookiana) in which the females are excessively rare; so much so that the collector kunstler could only obtain fifteen females to one thousand males. though the males, among the butterflies, are commonly much more numerous than the females, the disparity is rarely so great as with this species; but there are many in which the proportion of males to females is as fifty to one. as with the higher vertebrates selection affords no explanation of this curious disproportion. though according to weismann it fulfills “the first postulate in ‘sexual selection’ namely, that there be an unequal number of individuals in the two sexes.” but sexual selection here has a little over-reached itself, for surely one hundred suitors seems an embarrassing number for 193an inexperienced female to have to choose from! to say nothing of the ninety-nine males doomed to perish without leaving offspring.
that the beauty of colour and form which the lepidoptera, and especially the diurnal lepidoptera, or butterflies, exhibit is due to the choice by the females—albeit an unconscious choice—of the most resplendent of her suitors, that is, in other words, that she yields at last to the most ravishing member of the crowd—there is no evidence to show. there would seem to be no possibility of a differential selection from among a number of males, for there is no “display” comparable to that, say, of birds. and what is more, it is unlikely that, if there were, she would find anything to choose between them, for the range of variation in, say, one hundred males of any given species is very slight. finally we have no trustworthy evidence to show that the eyes of butterflies and moths are sufficiently good to enable them to make nice distinctions between slightly different males. we have no evidence that the eyes of insects are capable of discriminating the details of the often intricate patterns which their own wings, and those of their suitors, exhibit.
in the matter of “secondary sexual characters,” indeed, the lepidoptera exhibit very little difference between the sexes. as a rule the females are larger, often strikingly so, but in the matter of coloration they show far less disparity. but there are exceptions to every rule. a striking illustration of this is afforded by the genus ornithoptera. the butterflies of this superb group are of huge size, and the females are larger than their consorts, and commonly are extremely different therefrom both in coloration and habits. in ornithoptera paradisea this disparity attains its maximum. the female, remarks 194mr. david sharp, “is a large, sombre creature of black, white and grey colours, but the male is brilliant with gold and green, and is made additionally remarkable by a long tail of unusual form on each wing.” but a glance at the two sexes will show that the female, though less gorgeously arrayed, still disports a livery which is of a highly specialized or elaborated character. how are we to account for her differences in shape, size and coloration on the older interpretation of sexual selection? the perceptual powers, the mentality, of a butterfly are surely of a far lower grade than those of a bird, or even a fish. here, therefore, we cannot attribute the same possibilities of response to form and colour which we can ascribe with tolerable safety to the vertebrates. yet the sexual selection theory as generally understood demands this.
so far so good. and now as to the part played by sexual selection among the lepidoptera. darwin, in formulating this, found its application to the lepidoptera a very disconcerting problem, being naturally disposed to regard the extraordinary wealth of colour which these insects exhibit as the outcome of a process of female selection, in every way comparable to that which he held to obtain among the birds. he did not postulate a conscious, deliberate, selection; but a final abandonment on the part of the female to the male which, by his beauty and demonstrativeness, pleased her most. he assumed that at this critical time she would always be surrounded by rival suitors, offering varying if slight degrees of difference: and, indeed, in many cases she is thus surrounded. he remarks, in discussing the case of butterflies: “the males sometimes fight together in rivalry, and 195many may be seen pursuing or crowding round the same female.” but in the case of the silk-moths—and here is another illustration of the merciless criticism to which he submitted his own theories—he remarks: “the females appear not to evince the least choice in regard to their partners.” this fact, which is certainly true in the case of both butterflies and moths, and these gorgeous hues, disconcerted him, as is shown in the passage: “unless the female prefer one male to another, the pairing must be left to mere chance, and this does not appear probable.” the facts which have come to light in regard to the “courtship” of butterflies since darwin wrote are meagre enough, but such as have been recorded give no support to the supposition that the females are really influenced by, or even perceive the colours of, their mates. just on five-and-twenty years ago the naturalist skertchly published some observations on the courtship of that magnificent bornean butterfly ornithoptera brookiana. he one day came on a male sipping honey from the flowers of a tree, vibrating its wings with the rapidity of a hawk-moth, and the vivid green of the wings flashing in the sunlight, though the crimson areas thereof were invisible. the female came “and did all the wooing.” they circled about in flight with the female above and somewhat behind, so that she could see, we are told, the emerald markings; but there was no real evidence here that she was really influenced by his coloration, and if this really were the case then the coloration of the female equally demands an explanation, for this, though less gorgeous than that of the male, is far from a primitive type; on the contrary, it is of a highly differentiated character. furthermore, in this genus, as has already been remarked, the males outnumber the females by, roughly, one hundred to one. again, moseley, the naturalist on the memorable voyage of the challenger 196in 1872, when in the aru islands, was once “lucky enough to find a flock of about a dozen males fluttering round and mobbing a single female. they were then hovering slowly, quite close to the ground, and were easily caught.” but he was by no means convinced that any choice was exerted. and he suggests “a series of experiments, in which, in the case of highly-coloured and decorated butterflies, the colours should be rubbed off the wings of a few among a number of males, or painted over of a black or brown colour. it might be tested whether the females would always prefer the highly-coloured ones.” such experiments are foredoomed to failure, for the removal of the scales would remove the only source of communication between the sexes.
wallace, always a strenuous opponent of the sexual selection theory, found in the behaviour of butterflies and moths when mate-hunting a particularly powerful countervailing weapon. he assumes that darwin postulated a conscious selection on the part of the female, and with some show of reason, though it is probable that wallace was mistaken in this. “the weakness of the evidence for conscious selection among these insects,” he remarks, “is so palpable, that mr. darwin is obliged to supplement it by the singularly inconclusive argument, ‘unless the female prefer one male to another the pairing must be left to mere chance, and this does not appear probable’ but he has just said, ‘the males sometimes fight together in rivalry, and many may be seen pursuing or crowding round the same female’ while in the case of the silk-moths—‘the females appear not to evince the least choice in regard to their partners.’ surely the plain inference from all this is, that the males fight 197and struggle for the almost passive female, and that the most vigorous and energetic, the strongest-winged or the most persevering wins her. how can there be chance in this? natural selection would here act, as in birds, in perpetuating the strongest and most vigorous males; and as these would usually be the more highly coloured of their race, the same results would be produced as regards the intensification and variation of colour in the one case as the other.”
commenting on darwin’s interpretation of those cases wherein the females are more brilliantly coloured than the males, he insists that on his (darwin’s) theory “throughout the whole animal kingdom the males are usually so ardent that they will accept any female, while the females are coy, and choose the handsomest males, whence it is believed the general brilliancy of males as compared with females has arisen.”
“mr. darwin admits,” he continues, “that these bright colours have been acquired for protection [because they resemble those of species which from their disagreeable taste are avoided by birds and other insect-eating enemies]; but as there is no apparent cause for the strict limitation of the colour to the female, he believes that it has been kept down in the male by its being unattractive to her. this appears to me to be a supposition opposed to the whole theory of sexual selection itself. for this theory is, that minute variations of colour in the male are attractive to the female, have always been selected, and that thus the brilliant male colours have been produced. but in this case he thinks that the female butterfly had a constant aversion to every trace of colour, even when we must suppose it was constantly recurring during the successive variations which resulted in such a marvellous 198change in herself. but the case admits of a much more simple interpretation. for if we consider the fact that the females frequent the forests where the heliconid? abound [the distasteful species already referred to] while the males fly much in the open and assemble in great numbers with other white and yellow butterflies on the banks of rivers, may it not be possible that the appearance of orange-stripes or patches would be as injurious to the male as it was useful to the female, by making him a more easy mark for insectivorous birds among his white companions? this seems a more probable supposition than the altogether hypothetical choice of the female, sometimes exercised in favour of, and sometimes against, every new variety of colour in her partner.”
wallace’s arguments are not so crushing as he supposed them to be, and they contribute nothing towards the solution of the problem to be faced. but if colour played the part which darwin believed, and colour alone be concerned, it is curious that the males should recognize their mates in a guise so unlike their own. how is it that they do not pass them by as members of the totally different distasteful species? whenever, indeed, the female is more or less brightly liveried than the male, how do the sexes recognize one another, and how, when they live in environments so different as those referred to by wallace, do they find one another when possessed by the insistent demands of the “sex-hunger” which is the all-essential stimulant to secure the continuation of the race?
the factors which assure the satisfaction of this hunger differ in some important features from those which obtain among the higher animals—birds, for example. in the first place there is no necessity to find and hold territory, 199which is an imperative necessity where there are eggs to be brooded and young to be fed. in the second, the males, as has just been remarked, must search for the females, often indeed, in the case of many moths, because they are wingless.
this search is conducted by the sense of smell. this fact, familiar enough to-day to the entomologist and the student of evolution, was unknown to the earlier naturalists. neither darwin nor wallace suspected it. it would have been wonderful if they had, for there is nothing in the general appearance of these insects which suggests an organ of smell, nor is there anything in the structure of the nervous system which would indicate this subtle sense. during recent years, however, the number of workers engaged on the investigation of the senses of animals has increased immensely, and great strides have been made in perfecting instruments of research. to the efforts of these workers we owe the discovery of the seat of the scent-detecting organs and the source of the scent. the former are furnished by the antenn?, which lodge also the senses of taste and touch.
among the lepidoptera these constitute important secondary sexual characters, the antenn?, among the moths at any rate, presenting striking differences in male and female. the scent-producing organs are very elusive structures, and so far have been definitely traced, among butterflies, only in the males, where they are formed by certain peculiarly modified scales known as “androconia.” they may be either irregularly scattered over the wing, or may form complex structures. sometimes they are arranged in the form of brightly-coloured, bristle-like tufts on the hind-wings, sometimes in a fringe 200along the edge of the hind-wing. in some of the moths they are arranged to form a thick, glistening white felt, which fills a folded-over portion of the edge of the hind-wing, and in many cases “the perfume can be retained,” weismann remarks, “and then, by a sudden turning out of the wing-fold, be allowed to stream forth.” in the ghost-moth (hepialus humuli), the hind-legs of the male have become pressed into service and have become transformed into scent-bottles, since they are swollen and filled with glands for the manufacture of odorous matter.
the naturalist fritz müller discovered the fact that some of the butterflies which haunted his brazilian garden exhaled a flower-like fragrance. anyone can test this curious trait for himself who will take the trouble to brush his finger over the wing of a newly-caught male garden-white butterfly (pieris napi). the white powder which will adhere to the finger will be found to be made up of the wing-scales, which will exhale a delicate perfume of lemon or balsam! among the moths the strong odour of musk is exhaled by the convolvulus hawk-moth (sphinx convolvuli).
it is, however, only in the males that these odours can be detected, and, though palpable enough to human nostrils, their power of diffusion is apparently extremely limited. they would seem to serve as aphrodisiacs for the stimulation of the female, and, as a consequence, there is no need that they should start into activity until the male has arrived at the immediate neighbourhood of his prospective mate.
plate 31.
bright colours which cannot uk attributed to “sexual selection.”
1. eyed hawk-moth, under the influence of excitement.
2, a butterfly, zeuxidia horsfieldi, feld, showing tufts of scent-diffusing scales on the hind-wings.
[face page 200.
with the females of the moths, however, matters are otherwise. for the most part moths are nocturnal, and hence could not distinguish one another when on the search for mates, and in many species the females are wingless, and consequently are unable to move 201from the immediate neighbourhood in which they emerged from the pupal stage. in either case some means of informing the males of the presence of females is an imperative necessity for the continuation of the race. this is provided by means of a subtle odour exhaled by the females which, though imperceptible to human nostrils, must possess an extraordinarily penetrating power. weismann gives an instance of this in the case of the nocturnal eyed hawk-moth (smerinthus ocellatus). he placed some females, without any special intention, in a covered vessel near an open window. “the very next morning several males had gathered, and were sitting on the window-sill, or on the wall of the room close to the vessel, and by continuing the experiment i caught, in the course of nine nights, no fewer than forty-two males of this species, which i had never believed to be so numerous in the gardens of the town....” to this power of exhaling odours we may attribute the wingless condition of many moths, for otherwise the loss of flight would have brought about extinction long before any perceptible reduction in the wings had taken place. the odour which such prisoners emit seems to possess an irresistible attractiveness, and this fact is commonly taken advantage of by entomologists. the common vapourer moth (orgyia antiqua) affords a good illustration of this. the female is wingless, and little more than a pouch for eggs, but in certain seasons it is very abundant, even in the midst of london. that experienced entomologist prof. selwyn image, in a letter to my friend mr. john cooke, remarks, on this theme, that the caterpillars may be seen crawling by hundreds in and around the squares, while the males may be seen flying up and 202down new oxford street or tottenham court road. if a virgin female be put in a box placed outside the window, within a very short space of time, often not more than a few minutes, several males will be fluttering round her. this device for attracting males is commonly known as “assembling.”
more striking is the case of the oak-eggar moth (lasiocampa quercus). mr. richard south, in his most useful “moths of the british isles,” relates that on one occasion he had a number of pup? in a cage in a cottage on the edge of a moor near lynton, north devon, and these attracted quite a number of males into the room containing the precious casket, and he was enabled to capture several. the next day he placed a female which had meanwhile emerged, in a “roomy chip-box, and carried it, in a satchel, to the moor, where it was placed on the ground; the males began to arrive soon afterwards, and some fine examples were secured.” but the sequel is even more remarkable; for, he remarks: “although the female was taken on the moor only on one occasion, that satchel continued to be an object of interest to the male eggars for several days afterwards.” that this scent is capable of being transferred to foreign objects, and of retaining its power for several days, is a striking proof of its pungency, yet it is quite impalpable to human nostrils! the kentish glory moth (endromis versicolor) affords yet another instance of this curious attraction by scent, the effectiveness of which is not even lessened by exhalations of the human body, for if a virgin female be placed in a box, and this be placed in one’s pocket, the males will often swarm round one and even endeavour to gain access to the box. in all such cases the females, even when capable of flight—the 203female vapourer is wingless—never fly until after impregnation has taken place. hence males with defective scent—detecting powers inevitably fail to leave offspring.
selection, then, here lies between males of the most active scent-detecting powers, and not between those of the most brilliant colours. nevertheless, both males and females—where the females are winged—exhibit a remarkably beautiful coloration, and this is especially true of the kentish glory, wherein both sexes wear a resplendent dress. that of the male—which is much smaller than the female—differs in that the fore-wings are darker, but bear the same pattern as in the female, while the hind-wings are chestnut-red instead of cream colour as in the female. if this scent-factor has replaced colour as an inciting agent to pairing, then these moths should be of sombre hues. that such is not the case seems sufficient to show that the colour is not due to sexual selection, for it is highly improbable that scent and colour are both of equal importance, and this being so, one would expect to find the negligible factor eliminated.
the existence, then, of bright colours in this and other species in like case, seems to show that it has nothing to do with sexual selection, directly at any rate. the males having assembled, their presence is probably communicated to the female by the characteristic male odour, which is never of the same penetrating quality as that of the female. there is no need that it should possess this, for the females never seek their mates. the successful male, where several rivals are competing, is probably not simply the strongest, but he who also disperses the right odour necessary to provoke the 204pairing response. these illustrations furnished by the scent-hunting, scent-dispersing males and females are of the highest importance to students of the sexual selection theory, for they seem to show conclusively that coloration plays at any rate but a minor part therein. the importance of the scent-detecting organs is shown in the very different types of antenn? which obtain between male and female moths, those of the male taking the form of huge feather-like structures, as in some saturniid?, and far exceeding those of the female in size.
the methods of pairing which obtain among butterflies and moths, it is not surprising to find, are very different; for whereas in the former it takes place on the wing, in the latter the female is always in a resting position. where the females are winged, long flights are often taken for the purpose of depositing and distributing the eggs: the flightless forms make no such excursions. a few, as in the case of some of the psychid? are not only wingless, but limbless and maggot-like. they never leave the chrysalis case, but deposit their eggs inside it. though there is undoubtedly much that is wonderful about the mating of these scent-distributing species, the history of the moths of the genus acentrophus is more wonderful and more mysterious still. for the females are aquatic. the males may sometimes be found in crowds fluttering over the surface of large but shallow sheets of water. the females, which are wingless, come to the surface and, like sirens, draw the males under water, where coupling takes place; after which they probably immediately die. but how do they discover their submerged mates? the escape from the water of any odour which the females may possess seems 205well nigh impossible.
whether display, such as birds appear to delight in ever takes place among the lepidoptera seems doubtful nevertheless, something closely akin thereto seems to have been found in the case of certain species of butterflies (heliconius melpomene and h. rhea), which have been seen dancing in the air like gnats, and when some of them withdrew others took their places. again, having regard to the fact that birds, when alarmed or excited, will perform the display which is more or less characteristic of periods of sexual excitement, it is possible that the position of alarm assumed by some of the hawk moths may also be used in courtship (fig. 1, plate 31). but we have no evidence on this point, and from the part played by scent in the mating of butterflies it seems improbable that such displays take place.
a serious attempt to test the sexual selection theory by experiment—to test the extent, if any, of female choice in mating—was made some years ago by mayer, an american naturalist, on the large bombycid moth (callosamia promethea). this species exhibits striking dissimilarity between the sexes in regard to colour and pattern. “the females,” remarks professor kellog, “are reddish brown in ground colour, while the males are blackish, and in the two sexes the pattern is distinctly different....” mayer took four hundred and forty-nine pup?, in cocoons, of this moth and endeavoured to discover, first of all, whether the males found the females by sight or smell. enclosing females in jars, some of which were covered and some of which were uncovered, he found that males paid no attention to females enclosed in transparent jars so closed as to prevent the escape of odours, while such as were enclosed in boxes or wrapped 206in cotton-wool, so as to be invisible, but yet capable of exhaling odour, were besieged by males. to locate the organs of scent in the female he cut off the abdomen of several and placed the abdomens and their late owners at some distance apart. males came to the abdomens and not to the thorax and wings. males whose antenn? were covered with shellac, photographic paste, glue, paraffin, etc., showed no response to the female exhalations, until the covering medium was removed.
mayer next tested the selective action of the females. he began by removing their wings and affixing to the stumps the wings of males. the males mated with the females quite as readily as under normal conditions, though the most conspicuous female characters had been exchanged for those of the male. after this he affixed female wings upon the males, but mating took place as usual. the females did not seem to detect anything unusual in their suitors, nor did normal males attempt to pair with males bearing female wings. later he tried the experiment of dyeing the wings of three hundred males scarlet or green, and matched these against three hundred which were left untouched. the disguised, dyed males succeeded in pairing as easily as their normally-coloured brethren. the females exhibited no choice whatever. hence, then, we have further reason to believe that with the lepidoptera scent, not sight, is the channel by which mates are found. so far as the evidence goes, it seems to show conclusively that in all that concerns sexual relationships, scent is the guiding and determining factor. by scent the females attract the males, and by scent of another kind the males sharpen the procreative appetites of the females.
if the interpretation adopted in these pages is correct, 207these manifestations and emanations of colour and scent are readily accounted for; for they are manifestations of inherent growth changes which, having started, are free to go on increasing in amplitude unless, and until, checked by natural selection. there is nothing unreasonable or improbable in this interpretation; on the contrary, it embraces also many other features hitherto ignored, but no less demanding an explanation. such, for example, as the infinite variety of form and sculpture which the scales of the wings and the eggs display. these are details visible only by the aid of the microscope, but they demand explanation as much as the more obvious characters. moreover they have the advantage of belonging to a set of characters which cannot in any way influence the choice, if choice there be, in the selection of mates, nor are they of a nature likely to affect the results of the struggle for existence. of these characters, then—the sculpturing of the egg-shell and of the scales, the “nervation” of the wings, and coloration—we can say no more than that they are idiosyncrasies of growth, free to develop in any direction unless, and until, checked by natural selection, which will speedily eliminate disharmonies with the environment.
