124 A. D. LEES 



Nutrition is of occasional significance. In Panonychus for 

 example, a diet of senescent leaves or foliage damaged by the 

 feeding punctures of other iTiites, exerts a strong diapause- 

 promoting influence and can even overcome the opposite effect 

 of a long day and a high temperature. The larvae of the codling 

 moth Carpocapsa ponwne/Ia are influenced photoperiodically by 

 light which penetrates through the flesh of the apple; but the 

 maturity of the fruit also has some effect^-^' ^^. 



Although, in general, population density has surprisingly 

 little effect, crowding does sometimes induce diapause. This is 

 so, for example, in the grain moth Plodia^^. 



These factors frequently act on the insect long before growth 

 is actually arrested. In the case of species with a pupal diapause, 

 the sensitive period may occur during larval development, the 

 precise instar or instars depending on the species. Egg dor- 

 mancies are controlled by the maternal physiology. However, 

 the direction of the maternal response is often decided far back 

 in ontogeny. In Bombyx, for example, photoperiod acts mainly 

 on the late embryo, while it is still enclosed in the shell. It may 

 well be that the 'directions' provided by the photoperiod are 

 registered in the central nervous system and associated neuro- 

 secretory centres, since these are already differentiated in the 

 embryo. Perhaps even more remarkable are certain hymenop- 

 teran parasites (e.g. Mormoniella) in which the diapause stage 

 is the fully grown larva. The temperature conditions to which 

 the mother is exposed again determine whether dormancy will 

 occur. But in this instance the effect must be transmitted through 

 the cytoplasm of the egg and cannot be dependent on the 

 continuity of any organ system^-^. 



THE TERMINATION OF DIAPAUSE 



In many insects and mites the state of suspended animation 

 is so intense that the diapausing stages finally die unless the 

 appropriate releasing stimuli are forthcoming from the environ- 

 ment. One gains the impression that almost any agency of 



