356 ENTOMOLOGY 



Exact experimental studies of the effects of temperature, moisture, 

 and other conditions have undoubtedly an important economic bearing. 

 For example: it was found, from experiments made by Professor. T. J. 

 Headlee in Manhattan, Kansas, that the cycle of the codling moth 

 required an average of 39 days with an average of 1,006 degrees of 

 effective temperature (temperatures above the threshold of develop- 

 ment, given at that time as about 50 F.). With as exact a knowledge 

 of the other factors, particularly moisture, one ought to be able, with 

 the aid of weather reports, to foretell when a given brood of the cod- 

 ling moth will appear; which would evidently be of advantage to 

 fruit-growers. 



The subject is, however, not so simple as it was thought to be. 

 Sanderson (1908, 1910), who has given a useful discussion of this sub- 

 ject, showed "that upon purely theoretical grounds there could be 

 no uniform accumulation of temperature or 'thermal constant' for the 

 various stages of insect growth, but that the relation of temperature to 

 growth phenomena was probably different for each species and might 

 be expressed by a curve, the abscissas of which represent degrees of 

 temperature and the ordinates represent the time factor. The impor- 

 tance of considering the so-called law of the velocity of chemical reaction 

 as influenced by temperature was pointed out and it was shown that the 

 velocity of reaction varies at different temperatures. It was shown 

 that both the so-called thermal constant and coefficient of velocity 

 increase as the temperature is lowered from the optimum of the species, 

 and that the curve for each species and phase of growth or activity of 

 that species must be plotted before the influence of temperature can be 

 exactly stated." Sanderson defined the "thermal constant " for insects 

 as "that accumulation of mean daily temperature above the 'critical 

 point' of the species, which will cause it to emerge from hibernation or 

 to transform from any given stage." (i) There is no uniform minimum 

 above which the temperature may be accumulated as effective, but this 

 varies with each species and phase of growth; (2) there is no "thermal 

 constant" as far as a mere accumulation of temperatures is concerned; 

 and (3) the velocity of reaction varies according to the range of tem- 

 peratures (Sanderson) . To illustrate the first of these three statements, 

 Sanderson cites the green-bug, Toxoptera graminum, which, as Hunter 

 and Glenn showed, begins to develop at 1.65 C., while its parasite, 

 Lysiphlebus tritici, shows no activity below about 4 or 5 C. For the 

 bollworm the point of cold rigor is about 10 C. In regard to the second 

 statement, Sanderson adds: "It is evident that any accumulation of 



