TEMPERATURE 207 



descending scale, the minimum effective temperature is often 

 termed the threshold of develop7nent, and at which the metabohsm 

 of the insect borders on a condition of equihbrium, but with 

 sufficient excess of anabohsm over katabohsm to allow of growth 

 and development not being quite stationary. This point obviously 

 requires prolonged experimentation for its determination, and has 

 been accurately ascertained in very few instances. Below the 

 minimum effective temperature there follows a zone of inactivity, 

 due to rigor or cold-dormancy, but from whose influence recovery 

 takes place at a higher temperature. Still lower on the descending 

 scale is a zone of fatal low temperatures, corresponding with the 

 zone of fatal high temperatures, and, finally, the minimum fatal 

 temperature. 



The observations of Hunter and Pierce (1912), with reference 

 to the cotton-boll weevil, may be taken as an illustration of 

 these temperature ranges. Their experiments, however, were not 

 strictly controlled, but the prevailing humidity only varied between 

 37 and 40 per cent. The zone of effective temperatures (Fig. 66) 

 for this insect lies between 13-5° ^ and 35°. Above 35° up to 50° 

 the insects become inactive, and from 50° to 60° (soil temperatures) 

 is the range of fatal high temperatures in which the weevils died 

 in from fifteen minutes to one second, according to the temperature. 

 The maximum fatal temperature was found, therefore, to be 60°. 

 The lower zone of inactivity, which is due to cold, ranges between 

 13-3° and — 4-4°, while below the latter temperature, and down to 

 — 13-8°, is the zone of fatal low temperatures, with — 13-8° as 

 the minimum. The above observations illustrate the fact that 

 such temperature zones exist, and are to be regarded as accurate 

 for the particular conditions under which they were carried out. 

 Within the zone of effective temperatures there exists an optimum 

 range, at which the greatest number of insects complete their 

 normal development. Its limits naturally vary when applied to 

 the testing of more specific phenomena ; the range of optimum 

 activity, for example, not coinciding with that of longevity. 



^ Temperatures are given in Centigrade degrees throughout ; in many 

 instances Fahrenheit readings are given in the original sources of information, 

 and their conversion accounts for the quotation of decimal degrees. 



