RHEOTACTIC REACTION OF STREAM ISOPODS. 55 



they become more rare as the streams become larger until they are 

 entirely lacking in many streams of the Thorn Creek stage of 

 development. Also they are not found in streams that do not 

 afford a good chance for lodgment. 



The distribution of isopods shows a decided seasonal variation 

 in the County Line Creek which is the only one of the streams 

 listed above which has been sufficiently studied to give con- 

 clusive evidence on this point. In the dryer periods they are 

 of course limited to the pools. When the stream is flowing they 

 tend to scatter over the bottom among the debris or under 

 overhanging banks where they may congregate in large numbers 

 clinging to protruding roots. During the summer period the 

 isopods are often collected in large groups along the edge of the 

 masses of leaves that have been gathered and forced together 

 by the spring floods. During late summer and early autumn 

 they may be easily collected by finding and securing these 

 groups. Later in the autumn the isopods tend to occupy the 

 whole stream, crawling over the bottom and among the new 

 fallen leaves. In the spring they are still more widely scattered. 



RELATION OF RHEOTAXIS TO MAINTENANCE OF POSITION IN 



STREAMS. 



How much of this seasonal and regional distribution is de- 

 termined by the rheotactic reaction per se? 



In 1908 the entire County Line Creek was dry except the pool 

 nearest the lake. The rainfall of the following spring was normal. 

 Shelford ('n) found that under these conditions the fish had all 

 been driven to the lowest pool and in 1909 only one species, the 

 horned dace (Semotilus atromaculatus) had made any progress 

 upstream and that only about three rods. In July, 1909, the 

 isopod A. communis, was found in the second permanent pool 

 from the source and was abundant a quarter of a mile from the 

 lake. 



The horned dace is much more efficient in its rheotactic re- 

 action than is Asellus, hence this distribution further towards 

 the headwaters of a stream that was dry the preceding year must 

 must be accounted for by some other factor than rheotaxis. 

 Stream isopods if placed on a dirt bottom in a pan of water that 



