THE DEADLY PHYLLOXERA 335 



female thus hatched from the winter egg is called a 

 " foundress," or " stock-mother," because she starts a 

 whole colony of young which, by virginal propagation of 

 successive broods, may number many millions in a season. 

 These are known as "virgin-mothers" (Fig. 57), and 

 eventually their later generations always produce males 

 and females, so that we distinguish, in the course of a year, 

 four sets of aphides, starting from the egg, namely (i) the 

 foundresses, (2) the numerous generations of virgin-mothers, 

 (3) the males, and (4) the egg-laying females. 



In different kinds of plant-lice any of these " sets " may 

 be either winged or wingless (Figs. 5 5, 56, 59); many genera- 

 tions 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 



