60 W. C. ALLEE. 



into different relations with the current since when oriented 

 transversely part of the body would be near the surface instead 

 of the whole body being at the bottom, as is the case when the 

 animals are oriented either positively or negatively. Since with 

 the present apparatus it was impossible to obtain comparable 

 results and since stream isopods usually go either positive or 

 negative this method of maintenance will not be discussed further. 



The data so far presented make it evident that the rheotactic 

 reaction per se is not sufficient to account for the maintenance of 

 position of stream isopods but that it has an effect upon the 

 clinging ability of isopods in the current that is very important. 

 In other words the movement accomplished in the rheotactic 

 reaction of isopods is of much less importance in maintaining 

 their position in streams than is the sign of the rheotactic 

 reaction, which becomes of prime importance by means of its 

 influence upon the clinging ability of isopods. Since the sign of 

 the rheotactic reaction is closely correlated with the metabolic 

 rate of isopods it follows that an isopod with a relatively high 

 rate of metabolism would give positive rheotactic responses and 

 hence be better able to maintain its position in a stream than an 

 isopod having a relatively low rate of metabolism which would 

 be accompanied by negative or indefinite rheotactic reactions. 



It has already been noted that there are two periods in the 

 natural life of adult stream isopods when the usual highly positive 

 rheotactic reaction is lowered, namely: the breeding season and 

 the shorter but more frequently recurring molting period. Both 

 of these times are the more critical in that the clinging ability is, 

 or may be, directly affected by them. During and immediately 

 after molting an isopod is easily swept off its feet by a water 

 current and in the breeding season at the time of copulation the 

 pairs have their clinging ability reduced, especially when oriented 

 negatively (cf. No. 171, in Table I., p. 59). After the female 

 liberates the young and before the brood pouch is molted, a 

 bubble of air may get into it and an isopod thus afflicted finds it 

 almost impossible to remain on the bottom even in quiet water. 

 These floaters often occur in the laboratory and have been 

 observed to some extent in nature. Thus both the molting 

 period and the breeding season reduce the maintenance efficiency 



