178 



JOURNAL OF ECONOMIC ENTOMOLOGY 



[Vol. 7 



rig. 14. Eggs of Porthetria dispar, after cold storage conditions. Drawn from 

 data by Sanderson. 



Note.— The co-efficient for a day at 7.6° is .014, at 17.4° it is .05, the value of a 

 day at 17.4° being 3.6 times that of a day at 7.6°. ^ 



.014 X 34 days gives .486 



.05 X 10 days gives .50 

 Total .986 as opposed to the theoretical 1.00. Also: 10 days at 17.4° equal 10 x 3.6 

 or 36 days a*- 7.6°. Adding this to our actual 34 days at this temperatute we have a 

 theoretical 70 days as opposed to 70 days for the lot kept in 7.6° until emergence. 

 Conversely, 34 days at 7.6° equal 34 divided by 3.6 or 9.5 days. Added to the 10 

 days actually spent in the 17.4° temperature we have a theoretical period of 19.5° as 

 opposed to an actual experimental period of 21 days for a temperature of 16.4°. 



Law No. 3 is based to a certain extent on theory. We know that 

 the point in question is, and by definition must be, the zero for the 

 curve. That it is actually, also, the zero for the development of the 

 insect would seem theoretically probable and the evidence all points 

 to the truth of this assumption. For instance, we have insects complet- 

 ing their development at temperatures but one or two degrees above 

 this temperature, while others within the same distance of the point, 

 but beloAv it, never develop. 



Law No. 4 depends to a considerable extent upon the ones preced- 

 ing. In computing the accumulated temperatures it is necessary to 

 use, not the actual temperatures or number of degrees above an 

 arbitrary zero as on the Centigrade or Fahrenheit scales, but the 

 effective temperatures or numbers of degrees above the calculated 

 developmental zero. It will be seen that these accumulations, as 

 stated above, in curves based on experimental data, approach a con- 



