FAMILY APHIDIDJE 119 



they mature and as many as two or three generations of 

 these agamic apterous forms may be produced. Next, 

 alate (winged) forms are produced, usually asexual, to pro- 

 vide for a migration of the species to another host plant. 

 As a general thing these winged migrants usually produce 

 mainly wingless, agamic forms. In the autumn a winged 

 form appears (usually asexual) which permits the return of 

 the species to its original host. Frequently with aphids 

 having alternate food plants the spring and autumn host 

 plants are perennials — woody or shrubby plants, the sum- 

 mer host plant an annual. A number of cases, however, of 

 alternation between woody plants are known. An example 

 of the former is seen in the case of an apple-plant louse, 

 Siphocoryne avence, which winters over in the egg stage 

 on apple trees, the spring forms occur for two or three 

 generations on apple and then apparently disappear; in 

 reality they migrate to wheat and breed on it during the 

 summer. With the ripening of the wheat the lice leave 

 and return to the apple. In the case of the hop-plant louse, 

 Phorodon humuli, the spring and fall forms occur on wild 

 plum and the midsummer forms by migration on the hop 

 plants, which accounts for their very sudden appearance 

 and in such large numbers in the hop vineyards in summer. 



The return migration in autumn to the original host 

 plant may be followed by two or three generations of 

 asexual, agamic individuals. After that, a short time prior 

 to the end of the breeding season, the sexual forms are 

 produced. The appearance of the sexual forms may possibly 

 be induced (or precipitated) by the ripening and hardening 

 of the plant tissue of the host. The same factor, in case of 

 the summer annual host plant, may determine the time of 

 the return migration to the spring and autumn host plant. 



The sexual forms are often wingless and very small and 

 are less dependent upon the host plant for food than are 

 the other forms. After mating, the female produces one or 

 two eggs or at least a very small number of eggs and these 

 eggs are usually attached to permanent parts of the tree, 

 seldom to leaves or stems. These eggs are usually hard, 



