94 THE LIFE-STORY OF INSECTS [CH. 



ages, and these, except in very severe frosts, can 

 continue their occupation of feeding on roots. But 

 in the case of the t turnip-flies ' the food-supply is 

 cut off in winter, and all those beetles of the latest 

 summer brood that survive hibernate in some shel- 

 tered spot, waiting for the return of spring, that they 

 may lay their eggs, and start the life-cycle once 

 again. Among the Diptera, most species pass the 

 winter as pupae, the sheltering puparium being a 

 good protection against most adverse conditions, or 

 as flies. But where there is a prolonged parasitic 

 larval life, as with the bot- and warble-flies, the 

 maggot, warm and well-fed within the body of its 

 mammalian host, affords an appropriate wintering 

 stage. 



Among the Hymenoptera an especially interesting 

 seasonal life-cycle is afforded by the alternation of 

 summer and winter generations in many Gall-flies 

 (Cynipidae) as H. Adler (1881, 1896) demonstrated 

 for most of our common species. The well-known 

 ' oak-apples' are tenanted in summer by grubs, which 

 after pupation develop into winged males and wingless 

 females. The latter, after pairing, burrow under- 

 ground and lay their eggs in the roots, the larvae 

 causing the presence there of globular swellings or 

 root-galls within which they live, pass through their 

 transformations and develop into wingless virgin 

 females. These shelter until February or March in 



