INSECT DIAPAUSE AND ENVIRONMENT 123 



inductive dark periods are not greatly affected by short light 

 breaks. The impression gained is of a photochemical reaction — 

 either a synthesis or more probably a breakdown process — 

 which is opposed by a dark reaction. Both the light and dark 

 processes seem to develop slowly. In permanent darkness an 

 intermediate response is often obtained, some individuals 

 entering diapause, others developing without interruption. It 

 seems possible that in these and other unnatural conditions the 

 endocrine mechanism which controls development is no longer 

 fully 'geared' to the photochemical receptor process and there- 

 fore tends to 'drift' erratically. 



The threshold of sensitivity to light is in the region of 0.01 ft.- 

 candles in many insects and is less than 0.0025 ft. -candles in the 

 midge Metriocnemus investigated by Paris and Jenner^o. p^^ 

 important part of the twilight in temperate latitudes therefore 

 comes within the range of the photoreceptors. 



Temperature is nearly always an important factor in the control 

 of diapause. As a rule high temperatures augment long photoperi- 

 ods and low temperatures short ones. This means that in long day 

 insects high temperatures tend to prevent diapause, while in short 

 day insects, such as Bombyx, these conditions favour diapause. 



The modifying role of temperature gives some indication of 

 the possible origins of obligatory and facultative diapause^^. 

 Many insect varieties or sub-species differ in their diapause 

 characteristics. In Bombyx, for example, there are one-genera- 

 tion (univoltine) races with an obligatory diapause, 2-generation 

 (bivoltine) races with a facultative diapause; and multivoltine 

 races which are virtually diapause-free under any environmental 

 conditions. These races may have arisen through selection of the 

 temperature responses. Obviously, if the temperature at which 

 the long- or short-day photoperiodic stimulus can induce a 

 differential response is shifted beyond the normal developmental 

 hmits, diapause would either be prevented entirely or would 

 occur under all environmental conditions. A logical inference 

 is that in insects with obligatory diapause the mechanism for 

 perceiving these factors may still be functional. 



References p. 128 



