136 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 3 



temperature for the pupal stage of the codling moth decrease with the 

 advancing season. Though a fairly constant "total effective tem- 

 perature" for any given phase of an insect's life or activity may be 

 secured for the summer months when there is a fairly constant mean 

 temperature, such an accumulation will have no meaning in regard 

 to the same phenomena in spring and fall when the temperatures are 

 more variable. Thus in the total "effective temperature" in the 

 hatching of the eggs of the cattle tick as given b}^ Hunter and Hooker, 

 eggs laid from September 15 to October 15 require a total of 837.6° 

 to 1,510.8° over 43 °F. to hatch, while those laid in April and May 

 require from 981.6° to 1.139.1° accumulation. Were the moisture 

 factor also considered, it is evident that the range of such an accumu- 

 lation is too large to make it of much practical value, except by always 

 using the minimum possible. 



Thus if we are to relate the phenomena of insect growth and activity 

 to temperature, we may say that they will be about so and so between 

 certain temperatures or at certain seasons when such temperatures 

 normally prevail, as Hunter and Hooker have done in the case of the 

 cattle tick. Or, if we wish to be exact, we must secure the temperature 

 curve for the species, based on the observation of a considerable num- 

 ber of individuals kept at different constant temperatures, or possibly 

 better at temperatures having a diurnal variation with constant maxi- 

 mum and minimum, and with fairly constant moisture conditions. 

 That the moisture factor must not be neglected is shown by the work 

 of Hennings (Ic.) and by that of E. C. Cotton, on the cattle tick, 

 presented before this association, but never published. 



With such a curve plotted it would be possible to give each degree 

 of temperature for whatever time unit used, a definite valuation in 

 relation to the accumulation of temperature necessary for any stage 

 of growth or activity at the optimum temperature. Thus in the case 

 of the cattle tick, see figure 26, if the optimum be considered to be 

 28 °C. at which temperature 21.5 days are required for the eggs to 

 hatch, then each day at 28° C. has a value of 4.65% of the whole, or 

 .0465. As twenty-five days are required at 25°C., each day at 25°C. 

 has a value of .04, and so on the value of a day at 20°C. is .02, at 15°C. 

 is .01, and at liyo°C. is .00666. A table for the value of the degrees 

 between these points may now be made so that the valuation of every 

 degree to be considered may be given. Using these values, when an 

 accumulation of 100% or 1 has been secured the true "thermal con- 

 stant" should have been reached, for all the time relation to the vary- 

 ing temperatures has been reduced to a common unit. Were the 

 effect of moisture similarly studied so that the effect of different 



