RHEOTAXIS IN ISOPODA 



327 



' TABLE 26 



The effect of raising the temperature upon rheotactic response 



I 



II 



6.15 

 3.93 

 4.55 



18 

 27 

 26 

 26 

 27* 



22 



46 



70 



121 



50 

 50 

 50 

 50 

 50 



was not determined. It has already been stated that the stream 

 isopods are on the average about 3 mm. larger than those found in 

 ponds. One set of twenty pond males averaged 12.75 mm. in 

 length; a set of twenty-eight pond females averaged 9.14 mm. 

 The average of twenty-five stream isopods of each sex gives the 

 males a length of 15.3 mm. and the females 12.2 mm. 



During the course of the experiments, the difference in size of 

 isopods reared in low and high oxygen was quite noticeable and 

 those with the higher amount of oxygen, other things being equal, 

 were of a larger size. Although only a few of these were measured, 

 the measurements were entirely characteristic of the general 

 effect and are shown in table 27, together with the average oxygen 

 content and approximate age of the isopods measured. 



Both sets of isopods were entirely comparable, being taken 

 from the same stocks of animals. Both had plenty of food 

 although perhaps the pond animals were better supplied. The 



