RHEOTAXIS IN ISOPODA 



333 



B. Conditions that increase the positive rheotactic response: 



1. Atmospheric saturation of oxygen 



2. Complete saturation of oxygen 



3. Caffein 



4. Increase of temperature, if not too extreme 



The rheotactic responses given by the isopods under different 

 conditions are summarized in the following series of curves. 

 These graphs are based upon all the experimental data at hand, 

 and plot only the positive responses of the isopods. They neces- 

 sarily show only two factors, namely, time and the per cent of 



Aug. Sept. Oct. Nov Dec. Jan Feb Mar Apr May Jun Jul 



Fig. 5 Normal rheotactic response. A, Adult stream; B, Adult pond isopods 



positive responses. The former is given by the abscissae and the 

 latter by the ordinates. The other accessory data may be found 

 by referring to the different tables. 



Figure 5 shows the curve for the normal rheotactic response 

 for adult stream (A) and pond (B) isopods. The general sim lar- 

 ity of the two curves is noteworthy. The most striking differ- 

 ence, aside from the lower positive response of the isopods, is 

 the different degree in which the breeding season affects the two 

 mores. 



