TEMPERATURE 209 



which agrees with the previous findings of Krogh (1914) and 

 others. Peairs advanced reasons for concluding that the average 

 time of the first emergence of the insects, at different temperatures, 

 is probably a more reliable index than the mean time of emergence, 

 and both sets of data are plotted for comparison. When experi- 

 ments of this kind are carried out over a sufficiently wide range 

 of constant temperatures, the velocity curve is not a straight line 



Fig. 68. Temperature-velocity curve of development of pupae of 

 Tenebrio. Curve T shows relation between incubation time and 

 temperature. The broken line represents the theoretical 

 velocity curve, with the zero of development at Z. (Adapted 

 from Krogh.) 



throughout its course, but exhibits a definite lag phase at the 

 lower temperatures and inclines downwards at the higher. 

 Although the threshold of development should, theoretically, be 

 the zero of the velocity curve, or the point at which that curve 

 intersects the temperature axis, as a rule this is not actually the 

 case. The investigations of Krogh, Shelford (1927) and others 

 lend reasons for believing that the threshold of development lies 

 below the zero point of the velocity curve. There is a departure 

 from the theoretical curve, indicating a slowing down of the 



