362 



the differences are of the same order of magnitude as the differences in 

 velocity of development of different insects and of different stages of the 

 same insect when the developmental totals are correctly determined. (It 

 must be remembered that these developmental totals are constants only 

 for the same stock and conditions aside from temperature.) 



Other methods. Quite independently, botanical workers and clima- 

 tologists have developed various other methods of estimating stages in 

 life-histories. Koeppen ('86) developed a method of temperature classes. 

 This was modified by Zon ('14) and others. MacDougall ('14) used the 

 area between freezing and the actual temperature tracing as an indicator. 

 Livingston ('13), McLean ('IT), Hildebrant ('17), and Clements ('24) 

 grew standard plants as indicators, using the amount of growth as an 

 index in each case. Animals, especially insects, doubtless could be better 

 used as indicative of the favorability of season to economic pests. 



2. Purpose of the Present Investigation. 



It is the purpose of this paper to show : 



(a.) That various factors besides temperature have important effects 

 on development. 



(b.) That experimental results may be made to have direct bearing 

 on the interpretation of results under actual climatic conditions. 



(c.) That the threshold* of development is a variable point and that 

 the approximations used by various workers in summing temperatures 

 are of little or no physiological significance. 



(d.) That under actual climatic conditions there is no such thing as 

 a "thermal constant" or "sum of temperatures" in the ordinary biological 

 sense, and that temperature should not be summed without various cor- 

 rections and adjustments for the effects of other factors. 



(e.) That interpretations of conditions may be based on equal- 

 velocity charts for combinations of important factors. 



(f.) That conditions of hibernation are of great importance. 



(g.) That rainfall and many other factors are of importance at par- 

 ticular periods of the life history. 



The difficulties of investigating tlie relation of organisms to climate are 

 such that, with a few outstanding exceptions, investigators have tried almost 

 everything in the way of short-cuts. Furthermore, the methods necessarily 

 used in climate-simulation experiments on confined animals are complicated. 

 In view of the necessarily long discussion of these methods, the usual order in 

 scientific papers is here violated; the results and conclusion are presented 

 first and are followed by a discussion of methods. 



In Illinois, hibernating larvae of the codling moth pupate in April and 

 May, emerge in May and June, and deposit eggs within a few days; these 

 hatch quickly, and the larvae enter the apples in May and June. These first- 

 generation larvae pupate chiefly in July, giving rise to a second generation. 

 There is usually also a small third generation, the larvae of which enter the 

 apples in September. 



* The term "physiological zero" should not be used hecau.se metabolism is prob- 

 ably not at a standstill while the animal is alive. The term "threshold" has longr been 

 in use and gives better expression to the facts. 



