196 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 15 



Fahr. be taken as the threshold of development.^) The sum of the 

 effective day -degrees is not a constant in individual cases, but the aver- 

 age of the sum.s in a large number of observations approaches a constant. 



Variations from the average were comparatively large in individual 

 cases. These were due partly to the use of the day as the unit of time, 

 partly to local conditions by reason of which the temperature of the 

 specimen was not always the same as that of the recording instrument, 

 possibly partly to htimidity and evaporation, and in the case of the 

 larva to differences in the character of food. 



After all allowances were made for variations caused by factors which 

 were measurable, the variation in the siims of effective day-degrees 

 which must be attributed to other causes were for the incubation 

 period from about 156 to 170, for the larval period from about 527 to 

 873 and for the pupal period from about 256 to 274. Adding these 

 together we have for the three periods a minimimi of 939, an average 

 of 1101 and a maximimi of 1317. 



From a computation of the smns of effective day-degrees for the en- 

 tire development period of over 200 individuals kept under obser- 

 vation from the dates when the eggs were laid to the dates when the 

 adults emerged, using 50 deg. as the threshold of development and 86 

 deg. as the degree of maximum rate of developm.ent, the minimiun simi 

 was about 950, the average 1120 and the maximimi about 1350. 



Data on the seasonal history were secured by noting the dates of 

 appearance of the first eggs, larvae and adults of the several genera- 

 tions and computing the total number of effective day-degrees that 

 had accumulated on these dates after January 1. The average accu- 

 mulations are shown in the following table, each of the averages being 

 based upon from 4 to 7 observations: 



^For practical purposes it is necessar>' to use the same threshold of development 

 for the pupa as for the egg and larva. If 50 deg. is used for the pupa instead of 52, 

 the sums of the effective day-degrees will be 265. 



