INSECT ECOLOGY 



355 



Development in General. The effects of temperature on the 

 levelopment of insects are known in a qualitative way, and considerable 

 >rogress has been made in the quantitative study of the subject. At 

 certain degree of low temperature during development an insect 

 >ecomes physiologically inactive, or dormant, without being killed, 

 id may resume activity when the temperature rises. This point 

 is termed the threshold of development (critical point, developmental 

 zero). Temperatures above this point that are conducive to develop- 

 ment are termed effective temperatures, and in ascertaining the number 

 of temperature units requisite to development, all temperatures below 

 the threshold of development are disregarded. The effects of high 

 temperatures in accelerating development, and of low temperatures in 

 retarding development, are known to all who have raised butterflies 

 or moths from pupae. 



The theory used to be that the entire development of an insect, 

 from the time the egg is laid until the adult emerges, requires a fixed 

 number of effective degrees of temperature; the same being true also 

 for any stage of the insect, as egg, larva or pupa; that the entire develop- 

 ment, or any phase of the development, will not be completed until a 

 definite number of temperature units have been experienced, whether 

 the time required be long or short. The number expressing the total 

 temperature, or temperature constant, is obtained by multiplying the 

 mean daily temperature by the number of days required for the develop- 

 ment. Needless to say, the effects of temperature are obscured by 

 those of humidity, light, and several other influences in nature, and 

 become evident only under the exact conditions of experimentation. 



For the development of the boll weevil, Hunter and Hinds (1905) 

 give the following summary: 



1 For the average mean temperatures, add 43 degrees to the 

 temperatures. 



average effective 



