As in all plant-lice, the advantage for the sake of which sexual reproduction was given up depends upon the fact that a practically unlimited food supply is at the disposal of these parasites of the vine, which can be made full use of during the proper season, and which, since every animal is female and produces eggs, results in an enormous increase in the number of individuals, and thus secures the continuance of the species. These insects emerge in spring from small fertilized eggs, which have lain dormant throughout the winter (Fig. 127, _A_), and they develop rapidly into wingless females (_B_), which, sucking the juice of the vine, multiply by producing large numbers of little white eggs (_C_). These develop without fertilization into similar wingless females. Several generations of females succeed each other, but then, usually from August onwards, differently formed winged females (_D_) make their appearance, and these, flying from plant to plant, effect the distribution of the species. But these, too, lay parthenogenetic eggs (_E^1_ and _E^2_), and from these there emerge, late in autumn, the members of the single bi-sexual generation, males and females (_F^1_ and _F^2_), both very minute and wingless, without a piercing proboscis, and thus incapable of taking food. These pair, and the female lays a single egg (_A_) under the bark of the vine, from which the leaves are now falling; this egg survives the winter, and from it in the following April or May there emerges once more a parthenogenetic female. It could hardly be more plainly shown than it is by this case that the importance of amphimixis is something quite apart from reproduction and multiplication, for here the number of individuals is not only not increased by amphimixis, but is materially diminished, being indeed lessened by a half. By the retention of amphimixis, the species gains in this case no advantage _except_ the mingling of two germ-plasms. Something similar occurs in plants which exhibit alternation of generations, for instance the ferns, in which the sexual generation, the so-called prothallium or prothallus, contributes nothing to the _multiplication_ of the plant, since only a single egg-cell is developed; and the same is true of the mosses. In both cases _multiplication_ depends solely on the asexual generation, which, as the so-called 'moss-fruit' or 'fern-plant proper,' produces an enormous number of spores, in addition to multiplying by runners.