THEIR RELATION TO PLANTS 45 



variable number of individuals develops further and 

 becomes winged. There is not in this case any genera- 

 tion in which all the individuals are winged, although 

 there are species in which such a condition exists. It 

 is simply that out of a dozen individuals born of the 

 same mother on the same day, a number develop 

 wings and fly away to other trees or plants. They are 

 no further advanced, sexually, than the stem-mother 

 and when they reach their new homes they also pro- 

 duce living young, which may become winged or re- 

 main wingless; and this sort of reproduction continues 

 until the end of the season and the gradual decrease 

 of . sap in the trees and plants forces a provision for 

 winter rest. Late in the fall, therefore, a generation is 

 produced in which both sexes are represented and 

 from a union of these the winter egg is produced. 



Now while this in a way epitomizes the usual his- 

 tory of plant lice, there is an infinite amount of varia- 

 tion. A species that is confined to one tree or similar 

 food plant may do very well without much modifica- 

 tion; but a species which feeds during the summer on 

 a plant that dies down completely before winter, needs 

 some provision that enables it to continue its kind 

 elsewhere; hence we have migrations in early summer 

 from and in late fall to the winter host. In a melon 

 field, for instance, there may not be a plant louse until 

 the vines are well developed: then on some warm, 

 almost windless day in June, the air will be found full 

 of drifting, winged aphids and next day a sprinkling 

 of them may be noted all over the melons, giving rise 

 to the summer generations which, in late fall, again 

 produce return migrants that find their way to plants 

 upon which they can pass the winter. Sometimes 

 there seems to be a direct relationship between two 

 plants, as apple and wheat for one form of Aphid that 



