216 SOME ASPECTS OF ECOLOGY 



mean temperature exceeds that of the threshold of development, 

 and then summing the number of degrees so obtained for the 

 whole period under consideration. Further investigation has 

 showed that the idea of a constant total of effective temperatures, 

 or the thermal constant as it is often termed, necessary to com- 

 plete a given stage has proved to be approximately correct in 

 certain cases. The thermal constant, however, may exhibit wide 

 fluctuation, especially during exceptionally hot or exceptionally 

 cold seasons. This is owing to the fact that the influence of one 

 degree of temperature during a unit period of time is not the same 

 at different temperatures. As an example, it may be mentioned 

 that the beetle Sitodrepa panicea is stated to require 1,820 day- 

 degrees at 20°, and 3,638 at 17°. As a rule, for practical purposes, 

 temperature summations are based upon mean temperatures 

 over twenty-four-hour periods. In many cases the thresholds of 

 development for the species concerned are unknown, and an 

 arbitrary figure of say 55° (42° F.) has to be adopted. This 

 method is admittedly rough and ready, and its use can only be 

 qualified in cases where no experimental study of a given species 

 in relation to temperature has been carried out. Shelford (1927) 

 has come to the conclusion that temperatures cannot be summed 

 correctly for biological purposes unless readings are taken at 

 intervals of one or two hours, instead of daily, and corrected for 

 the effects of other factors. Such correction he terms temperature- 

 substitution, and is only possible through preliminary observation 

 or experimentation affording temperature and humidity data for 

 the defining of standard conditions. The temperature-substitution 

 method translates the observed conditions into terms of the 

 response of the organism into developmental units, which can be 

 summed for biological purposes. He claims, therefore, that a new 

 method is required which will take into account the effects of all 

 variations of the factors involved in small units of time. Hence 

 he adopts what he terms the developmental unit, which is defined 

 as the difference in amount of development resulting in one hour 

 from a difference of one degree of mean medial ^ temperature 



^ Medial temperatures as implied by Shelford are those whose range lies 

 on the straight line portion of a reciprocal (velocity) curve. 



