359 



degrees of temperatures that were being summed. A day at 60° F.. as 

 shown in curve A of Fig. 8 would give 10 "degree-days" reckoning from 

 50° (which was assumed as the starting point of development in this 

 hypothetical case). As the curve A is drawn, the same number of develop- 

 mental units have accumulated. But the century-old assumption that the 

 velocity curve is always a straight line, is erroneous. 



20 SO « SO i 



\-r so 60 ID eo 90 wo *o so to 



10 iO « SO 

 10 BO SO 



Fig. S. Curves of velocity of the development. (A) Curve ordinarily as- 

 sumed by those who sum temperatures. (B) Curve assumed by Glenn ('23) 

 relative to the codling moth. (C) Type of curve found by many experimental 

 investigators. 



Glenn i'ii) used curve B of Fig. 8 in correcting his sum of tempera- 

 tures. He first corrected temperatures in the usual way by giving all 

 temperatures below his alpJia the value of zero. He found it necessary to 

 assume (see pp. 222 and 233 of his article) that the rate of development 

 increased regularly up to an optimum temperature and then decreased at 

 the same rate. For example, if the maximum rate was at 00°, he con- 

 sidered 100° equivalent to 80° as shown in curve B of Fig. 8. He made 

 no comparable corrections, however, for the lag phase at the lower tem- 

 peratures (see curve C). His work was the first step in the application 

 of correct methods to the summing of temperatures in applied entomology, 

 and his success in the use of temperature data was due to his corrections. 

 He deducted twice the excess above the maximum ; and his data were of 

 such a character that such a correction was the best that could then be 

 made. For the pupa he used 8T° F. as the maximum. It will be seen, 

 however, that, with the lower temperatures uncorrected for ctirvature. and 

 the curve turning down sharply at the upper end (compare curves B and 

 C), errors may be large under certain temperature conditions. Where 

 sums of temperatures are used, even if such corrections as Glenn's are 

 applied, the effects of variations of hiunidity, rainfall, light, and other 

 conditions have ordinarily not been taken into account. The investigation 

 described herein shows that they should be considered. 



Evidences of the nature of the velocity curve. Since summing of 

 temperature would be practicable if the velocity curve were a straight 

 line, it is important to bring in more evidence that it is not. Proof that 

 the curve deviates at either the upper or the lower end is to be found in 



