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.