316 



W. C. ALLEE 



e. Low temperature 



The general effect of lowering the temperature is to cause a 

 decrease in the positiveness of the animals. This decrease is 

 well shown in the experiments listed in table 19. No attempt 

 was made to find how small a decrease or how short an exposure 

 would cause a reaction, the only care being to find if low tempera- 

 ture would affect the response. 



The results show that a decrease in temperature does affect 

 the rheotactic response in a marked manner, mainly in that 

 it renders the isopods extremely inactive. Although these ex- 

 periments do not show any acclimatization, yet by comparing 

 with results listed in table 2 it will be evident that the isopods do 

 come to give their normal positive response at as low as 4°C. 

 above zero. Again this is a reaction to a change of conditions. 

 General experimental work has shown that a change in 3° of 

 temperature does not usually affect the reaction to current, 

 although in one case a change of 4° did have a marked effect. In 



TABLE 19 



The effect of loiv temperature upon rheotaxis in adult stream Aselli 



TEMPER- 

 ATURE 



c. 



18 



5 



18 



10 



O2 



cc. per liter 



5.69 

 8.41 

 4.99 

 6.08 

 7.57 



NUMBER OF 

 TRIALS 



50 

 50 

 50 

 50 

 80 



TIME EXPOSED TO 



TEMPERATURE 



GIVEN 



7 days 

 field test 



2 days 

 12 hours 

 10 days 



