Feb. '08] JOURNAL OP ECONOMIC ENTOMOLOGY 61 



to north temperate climates, have but one generation in the South, 

 while southern species have many generations in the South, but the 

 number decreases as they spread northward. Why should not all 

 have several generations in the South? I have been unable to find 

 any investigations of this problem whatever and would be indebted 

 for any data which will throw any light upon it. 



In some cases, however, the time of emergence from hibernation is 

 controlled by moisture conditions as well as temperature, or independ- 

 ent of temperature. Thus Tower kept the potato beetle in hiberna- 

 tion for 18 months at a high temperature but with a dry atmosphere, 

 and they emerged as soon as normal moisture conditions were pro- 

 duced. Webster and Hopkins have shown a similar effect of lack of 

 rainfall on the emergence of the Hessian Fly in the fall. In relation 

 to hibernation in humid climates the matter of moisture is probably 

 not a controlling factor, but undoubtedly has the most important 

 influence upon the time of emergence of forms in aestivation during 

 the summer or in arid regions. 



A principle which should be considered in any attempt to fix a 

 thermal constant is the law of the velocity of chemical reactions. In 

 general, it may be stated that the velocity of chemical reactions 

 doubles or triples with every increase of 10° C, or in other words, a 

 reaction will take but one half or one third as much time with an 

 increase of 10° C. The application of this law to the velocity of 

 various phases of the physiology of animal and plant life is receiving 

 considerable attention by physiologists at the present time and some 

 of the observations made are shown graphically in figure 2. But all 

 of the observations made go to show that this so-called law is only 

 an approximation of the facts between certain degrees of temperature 

 ranging within 15° or 20° C. of the optimum temperature of the 

 form under study. At low temperatures the velocity rises very 

 rapidly as temperature increases, and above the optimum the velocity 

 decreases. The difference between the observed change in coefficient 

 of increase of velocity as temperature rises, as shown by these curves^ 

 and the theoretical application of this law if the velocity doubled or 

 tripled with every increase of 10° C. is shown by the difference be- 



FiGURE 1. Curves showing the relation of increase of temperature to the 

 time required for stages of growth of animals, more particularly as affecting the 

 emergence from liibernation and transformations of insects. Hypothetical Curve, 

 indicating the empirical nature of the curve of effect of temperature of individ- 

 ual species ; T^ 2 and V 3 Curves based on uniform coefficient of increase of 

 velocity of reaction of 2 and 3 ; Thermal Constant Curve, based on thermal con- 

 stant of 300°C. Numbers on curves are the thermal constants in degrees Centi- 

 grade at points marked. Optimum of 35°0. is empirical and varies for each 

 species. 



