INSECT DIAPAUSE AND ENVIRONMENT 127 



diapause. Bombyx, however, has a normal hibernation and the 

 reversed response stems from a different cause. This is connected 

 with the early occurrence of the sensitive Qgg stage in spring. 

 A reversed response is therefore necessary to prevent diapause 

 from supervening after one generation. In species such as 

 Panonychus this difficulty does not arise since the sensitive 

 stage occurs much later in the life cycle; and in addition, 

 dormancy is maintained until the short day spring period has 

 been almost completed. 



More complicated relationships are also known. According 

 to Masaki^^ the Japanese magpie moth Abraxas miranda 

 exhibits two separate types of pupal diapause which differ in 

 intensity. The more intense summer diapause is induced by a 

 moderately long photoperiod of 14-16 h, while the weaker 

 diapause is induced in early winter by a somewhat shorter 

 photoperiod of 11-13 h. Like other 'short day' insects, a day 

 length of 7-9 h induces development without diapause. This 

 response has the effect of imposing a bivoltine pattern on the 

 annual cycle. Larvae pupating early in winter have a short 

 dormancy whilst those doing so later on (they feed on ever- 

 green Euonynms) develop without interruption. The apparent 

 function of this adaptation is to synchronize the emergence of 

 the moths in spring. 



GEOGRAPHICAL VARIATIONS 



Since the control of diapause involves rather precise relation- 

 ships with photoperiod and temperature, it is not surprising to 

 find that the character of the diapause in widely distributed 

 species is by no means uniform, and that the differences are 

 determined genetically. 



Diapause races are sometimes surprisingly local in their 

 distribution. For example, there are at least three strains or 

 races of Locusta migratoria in southern France alone-^. How- 

 ever, other species show continuous transitions in the environ- 

 mental response. Thus Danilyevsky'^i has found that the 



References p. 128 



