INSECTS AND OTHER ORGANISMS 389 



insects and the plants on which they feed is to be noticed 

 where a species of insect passes from one kind of plant to 

 another and back again in the course of the yearly life-cycle. 

 Many aphids or greenfly afford well-known examples of 

 such complex adaptation. In previous chapters (pp. 138-9) 

 we have noted the general yearly course of the genera- 

 tions of these insects ; the autumn sexual forms, the hard- 

 shelled winter eggs, the early spring stem-mothers, and the 

 successive generations of virgin females giving birth to 

 active young through the spring and summer months — 

 the latest of these being the *' sexuaparae " or parents of the 

 autumnal sexual brood. It will be remembered that the 

 early virgin families in the spring consist largely of wingless 

 insects, but that soon a number of winged aphids are 

 developed which fly away to other plants, and in this way 

 extend the feeding range of the species. While the winged 

 members of some aphid races fly only to plants of the same 

 kind as that on which they were born, others seek plants 

 of quite a different kind whereon the successive virgin 

 broods of the summer feed and grow and multiply. These 

 latter are known as migratory aphids, and it may reasonably 

 be believed that the migratory habit, by putting two distinct 

 kinds of plant under contribution at different seasons of the 

 year, is definitely beneficial to the insects. The common 

 green aphid of the apple {Aphis pomi) has been generally 

 regarded as a non-migratory species ; all the virgin genera- 

 tions, as well as the sexual forms, may be found living 

 and feeding on orchard trees. E. M. Patch, however, has 

 lately demonstrated (1923) that in the northern United 

 States, large numbers of this species occur on a wide 

 variety of plants — Polygonum, Crucifers, Clovers, Mallow 

 and Carrot, for example, as well as on rosaceous trees. 

 Another apple-feeding aphid {Siphocoryne avenae) is 

 migratory. Its autumn sexual brood, winter eggs, stem- 

 mothers, and one or two early virgin generations live on 

 the apple-tree, but the winged spring migrants fly away 

 to cornfields and spend most of the warm season of the 

 year feeding on oats, often penetrating into their flowers. 



