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 pupæ 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 female 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 pairing 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 antennæ which obtain between male and female Moths, those of the male taking the form of huge feather-like structures, as in some Saturniidæ, 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 Psychidæ 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 well 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 pupæ, 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 in 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 antennæ 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, these 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.