210 SOME ASPECTS OF ECOLOGY 



velocity of development at low temperatures, and that the 

 development of a stage commences at a point lower in the scale 

 than that of the intersection of the velocity curve with the 

 temperature axis (Fig. 68). Similarly, at the upper extremity of 

 the velocity curve there is a high temperature retardation effect ; 

 at points ranging from about 35°, or less, development slows down 

 and is rarely completed at 40° (Peairs). Subject to the two 

 deviations just mentioned, the developmental curve of an insect, 

 when plotted as the reciprocal of the time factor, is a straight line 

 which represents the rate of increase in the velocity of development. 



The law of van't Hoff states that the velocity of a chemical 

 reaction increases with the temperature, and that the coefficient, 

 which expresses the rate of increase, is usually between two and 

 three times for each ten degrees of temperature. The relation 

 between temperatures and metabolism in cold-blooded animals 

 has been often stated to follow van't Hoff's law. It has been 

 shown by Krogh with reference to Tenebrio, and by many others, 

 however, that biological processes do not follow the principle of 

 a simple chemical reaction. The rate of increase of biological 

 processes, which accompanies a rise of temperature, cannot be 

 accurately expressed in terms of van't Hoff's formula, because the 

 value of the coefficient (commonly referred to as Q^^q) is not 

 constant enough to represent an approximation. The metabolism 

 of an insect, for example, is so complex and consists of a series of 

 inter-related processes that it is perhaps scarcely surprising that 

 it does not follow van't Hoff's rule. 



Although the study of the effects of constant temperatures 

 yields important data relative to development and metabolism, 

 and most of the critical experiments refer to such conditions, it 

 has to be remembered that it is variable temperatures which 

 prevail in a state of nature. The experimental study of varying 

 temperatures, however, has not progressed very far in relation to 

 insects, and obviously much depends upon the length of exposure 

 to given temperatures and the ranges of the latter. There is 

 evidence that temperature fluctuations exert considerable influence 

 in accelerating development among insects. Thus Bodine (1925) 

 and Parker (1929-30) found that if the eggs of grasshoppers were 



