in] SOME SUCKING INSECTS 19 



shoots, but that when the population becomes 

 crowded, a winged brood capable of emigration is 

 advantageous to the race. 



Many generations of virgin female aphids, some 

 wingless, others winged when adult, succeed each 

 other through the summer months. At the close of 

 the year the latest brood of these bring forth young, 

 which develop into males and egg-laying females; 

 thus the yearly cycle is completed. Variations in 

 points of detail may be noticed in different species of 



Fig. 6. Apple Aphid (Aphis pomi), virgin females, a, wingless ; 

 &, winged. Magnified 20 times. 



aphids. The autumn males and egg-laying females 

 are, for example, frequently winged, and the same 

 species may have constantly recurring generations of 

 different forms adapted for different food-plants, or 

 for different regions of the same food-plant. But 

 taking a general view of the life-story of aphids for 

 comparison with the life-story of other insects, three 



22 



