PAPERS OX ZOOLOGY 265 



and in different generations in the same year also in 

 diff'erent years. The threshold of development for the 

 egg as determined by Mr. Spooner is somewhat lower 

 than that of the pnpa, while the threshold of develop- 

 ment temperature of the larva is perhaps still lower. 

 These threshold values are calculated by the use of the 

 equilateral hyperbola and its reciprocal as already de- 

 scribed and of course the values are only approximate. 

 Calculations of the threshold can usually be made when- 

 ever a change in the factor amounting to about 1/3 of 

 the maximum daily range of that factor halves or doub- 

 les the length of the stage. Thus the ordinary range of 

 temperature to which the codlin moth may be subjected 

 is at most about 30° C, and lowering the temperature 10' 

 usually nearly doubles the length of the stage. This 

 ratio for temperature is known as the quotient for 10° 

 or the Qi:. Likewise the total range of humidity to which 

 the codlin moth may be subjected is almost 90 per cent 

 and lowering the htmaidity 30 per cent under certain con- 

 ditions will double the length of the stage and thus a hu- 

 midity threshold may be calculated under these condi- 

 tions. The total degree-days for the length of codlin 

 moth stages differs greatly with humidity and other con- 

 ditions so that the results of experiments do not coincide 

 exactly with the outdoor variable temperature and hu- 

 midity conditions. 



Having noted the effects of various climatic factors on 

 the codlin moth some comparison with man who has been 

 particularly studied will be valuable. Turning to Figure 

 3 we note that the line representing a death-rate 10 per 

 cent below normal passes through a considerable nmnber 

 of diff'erent temperatures and humidities just as does the 

 minimum death-rate of the codlin moth pupae and that 

 both are oblique iu the same direction but that man being 

 a warm blooded animal does not show as great a tempera- 

 ture range as does the cold blooded codlin moth. On the 

 same figure we have drawn the 10 per cent below normal 

 death-rate of negroes and the center of this area falls 

 on temperature about 6S'F. and humiditv about S3, while 



