Where vast numbers of individuals set out simultaneously to achieve their nuptials there would seem to be no need for special devices on the part of either sex to call attention to their whereabouts. Nevertheless, it is highly probable that the female exhales some distinctive odour; otherwise, having regard to the fact that she is overwhelmingly outnumbered by suitors, her discovery in such a crowd would be impossible, and it is of vital importance that no time should be lost in effecting conjugation, for the time for its accomplishment is perilously short. But there is another possible means of discrimination—the males may distinguish the females by the very different appearance of the head in the latter. At any rate, this may be true of some species wherein the males have no less than seven eyes, and these of three different kinds! The compound eyes, characteristic of insects, are, in these, divided, one half being set upon the summit of a pillar raised high above the level of the head, the other part remaining in its normal place at the side of the head; and in front of these, on what may be called the forehead, are three separate simple eyes, or “ocelli.” A reference to Plate 32, Fig. 3, will make this clear.
That the history of the later life of the May-fly is remarkable no one will deny: in many respects it is unique. Yet for all its strangeness it enables us to set our compasses, so to speak, in regard to the phenomena of sex in other groups. The extraordinary disparity in the proportions between male and female, for example, is full of significance, for it shows, as has been suggested more than once in these pages, that, in the case of polygamous species, we are probably in error in supposing that the excess of females is due to the reduction in the number of males by reason of the elimination of males by fighting. The excess of males, or females, as the case may be, is due to an inherent quality in the germ-plasm. The May-fly might be regarded as an excessively polyandrous species if the number of males in relation to females alone be regarded: but actually it is monogamous. After a prosaic infancy they are suddenly transformed into gay lovers, dancing a marriage-dance. But for them is no marriage feast, nor any later sequence of domesticity. One in ten thousand may find a mate, and only in this is he more fortunate than his neighbours, for, like them, he too must die before the dawn. Theirs is not even a sleep and a forgetting, but “one splendid hour of Life, and then—oblivion.” It may be urged that even these which might seem to have been fooled, have not really lived in vain, for hosts of animals feast upon their bodies. Myriads, indeed, are snapped up by fishes even before they have opened their wings, while birds rudely invade the swarms as they dance in mid-air, feasting on these fasting ones. But this is, after all, an inglorious end, and leaves us still asking Cui bono?
Is this amazing life-history a thing of yesterday, a new phase, or an order of things as old as the origin of the species, dating back some millions of years?
So far as one can profitably speculate on such a theme it would seem more likely to be a relatively recent innovation. The nearly-related Alder-flies (Sialidæ), so well known to anglers, seem like to meet with a similar fate, for the female lives but for a few days only and the male has an even briefer existence as a winged insect. The family to which the Alder-flies belong contains a few species which attain gigantic proportions, as, for example, in the case of the North American members of the genus Corydalis, which are giants. The males thereof are remarkable for the fact that they are armed with enormous jaws, which may be likened to a pair of callipers whose limbs have been crossed. These weapons serve as claspers, enabling the males to seize and hold the females during the act of mating. But even here the same brief span of life has to suffice them, for death follows swiftly on the fulfilment of the nuptial rites.
The Perlidæ, or Stone-flies, which, like the Sialidæ, are aquatic Neuroptera, the larval stages being passed in streams, present very puzzling features in regard to the adult males which, so far, have baffled all attempts at solution; yet they seem to have a very important bearing on the all-important work of reproduction. They are among the earliest insects to appear in spring, and possess an extraordinary power of resisting cold. One species, Capina vernalis, common in the Albany River, in Canada, frequently comes up through cracks in the ice and casts its skin there! Another, Nemoura glacialis, which appears at about the same time, actually performs the nuptial rites in crevices in the dissolving ice! Happily reason is denied them, or they would find life a mockery indeed; for having attained their final development, when the joyous and exhilarating exercise of flight should be theirs, they are compelled forthwith to fulfil their reproductive functions and die—in an ice-chamber! The males have wings which are rarely or never unfurled; as a rule they present nothing more than a crumpled mass of gauzy tissue, as if glued together. Such species as attain to flight are most indifferent performers, travelling but slowly, with laboured movements and settling after a few yards have been traversed.
As a rule, among insects, where there is a difference in the power of flight, it is the male which is superior. The case of Nemoura, just referred to, affords an instance where the contrary is the case, and Mr. J. J. Lister records the case of one of these flies—Isogenus nubecula—taken at Loch Tanna in Arran, wherein the wings of the female were greatly reduced, while those of the male were so much so as to be mere useless vestiges. Similar facts have been recorded of more than one species in Scotland, but in all such cases the phenomenon seems to be associated with the appearance of the insect in very early spring. In another species—Nemoura trifasciata—only the front wings are reduced, the hind pair being large enough to cover the body. In male specimens of Perla maxima taken in Scotland, the wings are so short as to be useless for the purposes of flight, yet, in the same species taken in Central Europe, they are of ample proportions.
These facts are puzzling indeed, but they seem to show that flight is not essential to attain the ends of reproduction. As to whether these flies secure their mates by any kind of “courtship,” or how they find one another, seems not to be known. But the female is remarkable for the fact that she carries her eggs about with her, to the number of five or six thousand, attached to the end of the abdomen.
Having regard to the fact that three thousand species of Perlidæ are known, and that they have a wide distribution over the earth’s surface, one might have expected that more would be known of their singular life-history. They are, however, flies of very unattractive appearance and great frailty, hence, save to anglers, by whom they are esteemed as bait for trout, they attract but little attention.