588 PHOTOPERIODISM IN INVERTEBRATES 



ture periods in light are rendered equivalent to darkness. That the light 

 reaction has such a marked temperature coefficient had not previously 

 been suspected. Indeed, preliminary results with Metatetranychus sug- 

 gested that temperature sensitivity was confined to the dark period in 

 this species (Lees, 1955a), 



The quantity or quality of the food is also sometimes a modifying 

 influence. In Metatetranychus nutritional deficiency due to leaf senes- 

 cence or to damage by the feeding punctures of other mites causes the 

 female to lay diapause eggs even when exposed to a long photoperiod 

 and high temperature. The codling moth Carpocapsa pomonella pro- 

 vides a further example (Gambaro, 1954). Although it was previously 

 thought that the larvae were influenced only by photoperiod, acting 

 through the semitransparent flesh of the apple, it now seems that some 

 quality connected with the maturity of the fruit is important: thus ripe- 

 ness is conducive to diapause, and immaturity to diapause-free de- 

 velopment. 



PHOTOPERIODIC ADAPTATION IN GEOGRAPHICAL RACES 



It is well known that populations of arthropods with a wide geo- 

 graphical range often show local differences in the character of their 

 diapause. In general, diapause tends to be obhgatory in populations 

 from high latitudes and the life cycle is univoltine; in contrast, popula- 

 tions from lower temperate latitudes often pass through two or more 

 generations a year, this facultative diapause being controlled by photo- 

 period and other environmental factors (Andrewartha, 1952; Lees, 

 1955). These different forms of arrest are also known to be inherited. 

 The recent work of Harvey (1957) on the spruce bud worm Chori- 

 stoneura illustrates the point afresh. In eastern Canada this insect is 

 essentially univoltine although 3-4% of the larvae were found to de- 

 velop without diapause when placed on long days. After selecting this 

 phenotype for six generations a strain was produced which was almost 

 diapause-free in long days but which entered diapause uniformly in 

 short days. 



Even if the species exhibits a facultative diapause consistently 

 throughout the area of distribution, there is every reason to expect that 



