In the case of the Aphides, winged males normally appear in large numbers at the end of the summer, and these fertilize the females; but if kept in a warm green-house, parthenogenetic reproduction may be sustained for as long as four years. Under quite normal circumstances these tiny insects show a singular range of variability, for egg-laying and viviparous individuals are met with; while winged and wingless generations appear sporadically, apparently according to the abundance of food. The winged form is sometimes so abundant as to float about in swarms that darken the air. There are at least three kinds of males-winged males, wingless males with a functional mouth, and small wingless males which have no mouth, and, one need hardly say, are very short-lived. The Aphides are a feeble folk, individually, but collectively a power in the land, causing at times incalculable loss to the farmer and gardener; but on this head and on the subject of their strange habits, and sometimes adventurous lives as slaves in the service of Ants, no more than a hint may be dropped in these pages. But some such aids to faith seem to be necessary when those who are not tolerably familiar with these insects are told of their amazing fertility. Linnæus long since estimated, in regard to one species, that in the course of one year a single Aphis will give rise to a quintillion of descendants—all produced without the aid of a male. Every one of these females begins to reproduce within from ten to twenty days of her birth, but even this statement does not bring home the result of such an astounding fecundity like Huxley’s calculation which was carefully worked out. He estimated that the produce of a single female would, in the course of ten generations, supposing all the individuals to survive—and possess the normal fertility of their race—“contain more ponderable substance than five hundred millions of stout men: that is, more than the whole population of China.”