INSECTS AND OTHER ORGANISMS 391 



into the winged '' sexuaparae " that fly back to the spruce 

 and there lay the eggs, whence the wingless sexual forms are 

 hatched and matured by August. Thus the life-cycle 

 from one sexual generation to the next lasts for two years. 

 A most interesting feature of the Chermes development, 

 however, is a tendency to omit from the normal series of 

 generations, several of the broods. Thus there are races or 

 species of Chermes in which the winged offspring of the 

 foundresses remain on their native tree or fly to other spruces, 

 and from their eggs are developed foundresses like their 

 parents ; in such cases there is a succession of two alternating 

 virgin female generations on the '' principal host " tree, the 

 migratory habit being, at least for a time, abandoned. On 

 the other hand, the offspring of colonists on the larch or 

 pine may develop, not into winged sexuaparae destined to 

 fly back to spruce, but into wingless *' exiles " which remain 

 on the " intermediate host " tree. There may be several 

 generations of exiles on larch or pine through the summer, 

 and in the autumn are hatched young which after wintering 

 without a moult, develop next spring into a new brood of 

 colonists. Here, therefore, the series of generations is 

 restricted to the virgin forms characteristic of the larch or 

 pine. It is of great interest to see such a tendency towards 

 the simplification of a life-cycle normally very complex ; 

 again we are convinced of the plasticity possible in the 

 behaviour of insects so that their relations with the plants 

 whereon they live may become modified in various ways ; 

 and as the successive broods of Chermes are differentiated 

 by special structural features we realise that the dropping 

 out of several generations from the normal cycle must be 

 accompanied by some determining germinal change. This 

 may conceivably be brought about as a response to the 

 changing conditions of the insect's food-plant seasonal or 

 otherwise. A. D. Imms (1925) suggests that the main 

 factor determining the migration of aphids " appears to be 

 those physiological changes in the plant during growth 

 which render it unsuitable as a host," and that '' tempera- 

 ture may possibly exercise an influence on the behaviour 



