PHOTOPERIODISM IN INSECTS AND MITES 589 



the light response will show local adaptations to photoperiod. Thus it 

 has been noted that the photoperiod critical for diapause initiation in 

 a Cambridge (England) population of red mites {Tetrcmychus 

 telarius) was about 2 hr less than in a Leningrad population 8° of 

 latitude to the north (Bondarenko, 1950; Lees, 1953a). A system- 

 atic study of these variations has recently been made by Danilyevsky 

 (1957) in the moth Acronycta. Insects from the Leningrad (60"N.) 

 region were found to have a critical photoperiod of nearly 20 hr; 

 whereas in insects from Vitebsk (55°N.), Byelgorod (51°N.), and 

 Sukhumi on the Black Sea coast (43 °N.) the values were 18, 17, and 

 lAVi hr respectively. There is a further difference in that high tempera- 

 tures are more effective in preventing diapause in the more southerly 

 strains. Since the pupae are released from diapause much earlier in 

 the south than in the north (a further adaptation in the diapause 

 mechanism) the net result is that long-day conditions persist for more 

 than four months in the south, and three generations can develop. On 

 the other hand, short-day conditions prevail almost throughout the 

 summer for Leningrad insects which are invariably univoltine despite 

 the latent capacity to respond to photoperiod. As is suggested by 

 Danilyevsky, local variations in the diapause characteristics will un- 

 doubtedly operate as important isolating mechanisms. 



The mode of adaptation to the environment of the Japanese races of 

 Barathra investigated by Masaki (1956) is entirely different. This 

 species is adjusted to a two-generation pattern almost throughout its 

 range. Strains from northern Japan show the usual response to photo- 

 period, a short day inducing an intense diapause and a long day pre- 

 venting diapause completely. But insects taken from populations in the 

 south respond to the long photoperiods of summer by aestivating. This 

 short-term "summer" diapause delays development just sufficiently for 

 the second generation to develop in autumn. Pupae of the latter genera- 

 tion, having been acted upon by short days, then enter an intense 

 "winter" diapause. 



The three examples detailed in this section show that there is con- 

 siderable plasticity in the photoperiodic behavior, even within a single 

 order (Lepidoptera). Clearly, there may well be still other types of 

 response awaiting discovery. 



