It is interesting to inquire into the probable origin of this fatal desire for close contact with a source of light, a desire so strong as to be entirely unchecked by the deadly heat accompanying the light. The May-flies or Ephemerids are delicate little creatures, having four net-veined wings rarely more than three-quarters of an inch across, with two or three long filaments hanging from the tail. Three hundred species are known from all parts of the world, of which forty occur in the British Islands. They live as wingless, six-legged larvæ in the water for a couple of years, feeding voraciously. Then one summer's evening they very rapidly escape from their larval skin and fly over the water in countless swarms. But only for a few hours. The eggs of the females are fertilized, and they all, both males and females, drop dead or dying into the water, where they are greedily devoured by fishes. The males are far more numerous than the females; in some species as many as 6000 males to one female have been counted. They are attracted to an extraordinary degree by lights (flames or electric lamps) set up for nocturnal illumination by civilized man, and in some districts they are collected by fishermen in this way for use as food for fish, or were so in Holland in the eighteenth century according to Swammerdam's statement in his "Biblia Naturæ." Why do they thus seek artificial lights? There is some indication of an explanation in the fact that two tropical species of May-flies are known which, like the glow-worms and fire-flies, produce light in their bodies. The May-flies, especially the males, have unusually large and prominent eyes, as is the case with phosphorescent fishes and some other light-producing animals, and it appears probable that in the now rare instances of self-luminous May-flies, the sexes are attracted to one another by the light they produce, as is the case in other luminous insects. It seems probable that the ancestral May-flies, of which many remarkable kinds have been discovered in the fossilized condition in strata as far back in time as those of the coal-measures, were all self-luminous, and acquired an overpowering instinct of seeking the light given out by other individuals as a necessary step towards sexual congress. In the course of ages other senses (probably smell and touch) have been called in to bring the fluttering insects into association. The power of producing light, being no longer needed, has disappeared from all but two rare species. But the urgent erotic instinct, the nervous mechanism, which drove the ancient May-flies towards the dancing lights of other May-flies, has remained unaltered in all the living species of the group. It is a "disharmony" which has not been of sufficient destructive importance to be "cleared away" or suppressed by natural selection. In pre-human times, nocturnal fires and lights were too uncommon to cause much disaster to the May-flies. But now that