Feb. '08] JOURNAL OF ECONOMIC ENTOMOLOGY 57 



logical work, if not indeed for meteorological studies. The weekly 

 sheets are readily filed and the temperature which occurred at any 

 time during breeding experiments can thus be readily referred to or 

 computed in the future. 



We have been endeavoring to determine whether there be a "ther- 

 mal constant" which governs the emergence of insects from hiberna- 

 tion. The "thermal constant" for insects may be defined as that 

 accumulation of mean daily temperature above the ''critical point" 

 of the species, which ivill cause it to emerge from hibernation or to 

 transform from any given stage. The "critical point" above which 

 the temperature is accumulated is a matter of vital importance and 

 seems to have been largely neglected in entomological work. Euro- 

 pean botanists have established the critical points of a long list of 

 plants and shown that a large variation exists between species.^ The 

 "critical point" may be defined as that point of temperature above 

 which active metabolism occurs and above which the accumulation of 

 temperature affects the time of definite transformations in the or- 

 ganism, such as the leafing and flowering of plants, and the emergence 

 from hibernation, hatching and transformations of insects. In recent 

 studies of the relation of temperature to insect life it has been assumed 

 that all temperatures over 43° F. are "effective temperatures," or that 

 43° F. is the critical point. But the records based upon this assump- 

 tion go to prove that the critical point varies and that it must be 

 determined for each species before accurate conclusions concerning 

 the relation of temperature to that species can be secured. Thus, as 

 was pointed out by Simpson in his excellent studies of the Codling 

 Moth, it was impossible for him to draw definite conclusions from a 

 considerable mass of data published by him concerning the influence 

 of temperature upon the duration of its various stages.'' The same 

 fact has been brought out by Hunter and Hooker in their recent 

 paper on the North American Fever Tick in their study of the relation 

 of temperature to the period of incubation. '^ In the case of the brown- 

 tail moth, our present observations go to show that 34° F. is the 

 critical point above which the temperature accumulates in determining 

 the time of emergence in the spring. The critical point is doubtless 

 much lower for northern species and much higher for southern species, 

 and it seems quite probable that it may vary for the same species 

 which has become acclimated to diverse climatic conditions. Thus the 



aSee Cleveland Abbe, First Report on the Relations Between Climates and 

 Crops, Bulletin 36, U. S. Weather Bureau, 1905. 



bSee Simpson, Bulletin 41, Division of Entomology, Tables IV and V, 

 pages 37-39. 



cHuuter and Hooker, Bulletin 72, Bureau of Entomology, page 20. 



