146 



INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



temperature threshold by application of any algebraic formula to time- 

 temperature curves. 



Another question frequently treated in past literature is whether or not 

 the summation aspect of temperature is the same for constant and varying 

 temperature conditions. This is the familiar 'day-degrees accumulation,' 

 widely used to forecast probable dates of hatching, of pest outbreaks, of 

 flowering and harvesting, and even of the amount of fuel needed to heat a 

 building during winters of different degrees of coldness. Genesis of this 

 idea dates back to Reaumur in the first half of the 18th century. A large 

 body of data attests the adequacy of these arithmetric calculations when 

 the different temperatures are above developmental threshold. As an ex- 



o 

 o 



25 



20 



*-»15 



LU 



I- 

 < 



cc 



10 



30 

 25 

 20 



15 UJ 



10 I- 



< 



15 



20 



25 



30 



35 



TEMPERATURE, °C. 



Fig. 1. Development of Oncopeltus fasciatus (Dallas) eggs tluough hatching for the 

 \iable range of temperatures. A, time-temperature curve ; B, rate-temperature curve. 

 After Lin et ciL, 8. 



ample, this is well shown by Oncopeltus eggs when they are incubated for 

 various periods at 15°C and then transferred to 30°C (fig. 2, curve B). 

 There is an essentially linear decrease in time required at the higher tem- 

 perature when the eggs are held for longer periods at the lower tempera- 

 ture. Little literature deals with summation effects when one of the tem- 

 peratures is below the constant temperature threshold, but here again we 

 find linear decreases indicating that summation effects can be calculated 

 provided the subthreshold temperature is only a little below the threshold 

 (fig. 3, curve B). 



The whole concept of temperature thresholds for development, at least 

 in insects, has lacked definiteness, perhaps because of the complexity of 

 phenomena involved (.8). Perhaps the most useful definition is "...the 

 lowest temperature at which a given physiological process, or development 

 through a given stage in the life history, can be carried through to com- 



