In different kinds of plant-lice any of these “sets” may be either winged or wingless (Figs. 55, 56, 59); many generations of the virgin-mothers are wingless, but not all, in all species. According to the species or kind of aphis and its requirements in regard to the plants on which it feeds, wings are developed so as to enable the aphis to fly from one tree or locality to another, or are not developed if the aphis has to remain where it was born. The whole series of successive broods of some kinds of aphis remain on one plant and about the same part of it, and then there is little need for wings. Others have their summer broods on the twigs or leaves, but the later broods descend in winter to the roots of the same plant. The woolly aphis of the apple trees and pear trees behaves in this way; other species again produce a late-winged brood, which leaves the plant on which its parents were feeding, and travels some distance to the twigs or to the roots of a quite distinct kind of plant to produce an autumn brood, and from these the final males and females are born, and the winter eggs are then deposited. The hop-louse leaves the hop when the hop-vine dies down in autumn. The abundant wingless form (Fig. 58) of which there have usually been ten generations, produces at last a winged “migrant” brood (Fig. 59) which flies away to plum trees and sloe bushes, perhaps a quarter of a mile distant. There the migrants produce wingless females on the plum tree. They are followed to the plum trees by a final migrant brood from the dying hops which are males—the first yet seen (Fig. 56). The males fertilise the wingless females born on the plum tree—and the latter lay each one fertilised egg in the crevices of the bark of the plum tree near the young buds. Winter now sets in: all are dead except the eggs. In the following late spring a foundress hatches out from each egg so deposited. The “foundress” (Fig. 58) in this species, the hop aphis, is wingless. She produces parthenogenetically and viviparously a brood of wingless females. They similarly produce on the plum tree a third generation of virgin females, but these have wings! (Fig. 55). They fly back to the hop-vines, which are now well risen from the ground and offer abundant juice to the wingless virgin brood which escapes from the winged migrants as soon as they have settled on the hop, and feed and grow and produce new wingless broods (Fig. 57) in rapid succession.