RHEOTAXIS IN ISOPODA 317 



a temperature gradient, isopods collect at a temperature near that 

 to which they have been previously exposed so that this optimum 

 shifts with external conditions. 



The effect of a decrease in temperature upon the bunching 

 reaction has already been mentioned. Since the isopods become 

 acclimated in both these cases, it was to be expected that they 

 would show a similar reaction regarding the effect of tempera- 

 ture upon their rheotactic response. 



/. Starvation 



The effect of starvation is shown by the results tabulated in 

 table 20. These experiments were carried on in filtered lake 

 water. The only difference between the experiment and the 

 control was that the latter contained a few leaves for food. The 

 results show that as starvation progressed the positive responses 

 of the isopods were diminished. The decrease shown in the con- 

 trol is due to the approach of the breeding season. As has been 

 explained in other cases this may have helped cause the decrease 

 in the experiment, but by comparison it will be apparent that the 

 breeding season was not the major cause of the change in response. 



2. POND ASELLI 



a. With increased oxygen 



{1} Nor7nal adults. It is obvious that if the amount of oxy- 

 gen present in the habitat is the determining factor in the rheo- 

 tactic response of isopods, increasing the amount of oxygen pres- 

 ent should increase the number of positive reactions. The 

 amount of oxygen present was experimentally increased in three 

 different ways: (1) The animals were placed in running tap water. 

 This method is objectionable because it introduces a current, and 

 the mechanical effect of this might be the stimulating agent; 

 (2) the aerating device already described was used; and (3) the 

 isopods were placed in water containing a large amount of green 

 water moss. Similar results were obtained from all three methods 

 and these results are listed in table 21. 



