128 A. D. LEES 



critical photoperiod is smoothly graded in populations of the 

 moth Acronycta rumicis and ranges from nearly 20 h of light 

 daily in the Leningrad populations to about 14i h in populations 

 from the Black Sea coast. In the South moderate or high 

 temperatures are also more effective in preventing diapause- 

 induction by short days. The tendency in nature is therefore for 

 the period of active growth to extend into the winter season and 

 even to continue through it. Very similar relationships have 

 been demonstrated in the red mite Tetranychus urticae'^'^ and in 

 Anopheles maculipennis-^. 



The genetic examination of this interesting material is in its 

 early stages. But it has already been shown in Acronycta that 

 Fi hybrids of local photoperiodic races respond to intermediate 

 critical photoperiods. And no clear-cut segregation emerges 

 when the Fi progeny is backcrossed with the original forms-^. 

 The type of inheritance is therefore polygenic. 



These results immediately indicate the high adaptive value 

 which must attach to the correct adjustment of the diapause- 

 inducing and diapause-terminating mechanisms. They also 

 suggest that there must be considerable selective advantage in 

 retaining the plasticity and variability of the diapause character. 



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5 A. D. Lees, Ann. Appl. Biol., 40 (1953) 449. 



« M. J. NoRRis, Nature, 181 (1958) 58. 



' H. J. MiJLLER, Zool. Anz., 760 (1958) 294. 



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