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 th............