126 A. D. LEES 



are most favourable. Nevertheless, the bearing of diapause 

 induction on the phenology can perhaps be more easily appre- 

 ciated. The function here is to forestall the onset of adverse 

 conditions. The following are some examples involving photo- 

 periodic control. 



The life cycle of the red spider mite Panouyclus ulmi is 

 typical of species with several annual generations. The long 

 daylengths of summer are responsible for the appearance of 

 several successive generations of 'summer' females laying non- 

 diapause eggs on the leaves. Under English conditions females 

 laying winter (diapause) eggs appear in the penultimate and 

 last (4th and 5th) generations in response to the reduced day- 

 lengths of September. These individuals normally deposit their 

 eggs on the bark of the tree long before leaf fall. However, 

 heavy damage to the foliage by large mite populations often 

 causes premature leaf abcission. It is interesting to find that 

 under such conditions of semi-starvation diapause is also 

 induced prematurely, even if long day conditions are still 

 prevailing. In this respect the response to nutrition appears as 

 a kind of 'double assurance'. 



Since day length provides a fixed 'point of reference', it might 

 be expected that the composition of the terminal generations, 

 and even the total number of generations in a season, would 

 depend on the environmental temperature — a much more 

 variable component. This has indeed been demonstrated in the 

 moth Polychrosis by Komarova^^. Although this species is 

 always bivoltine in the southern parts of the USSR, the inci- 

 dence of diapause in the second generation varies from year to 

 year and from place to place. In a cool year or at high altitudes 

 the second generation is delayed relative to the critical photo- 

 period with the result that a much higher proportion of indivi- 

 duals enter diapause. 



I have already mentioned that some short day insects have 

 been described. The function of this response may be simply to 

 invert the season of dormancy. In the leafhopper Stejwcranus 

 for example, a long photoperiod induces a summer (aestivating) 



