RHEOTACTIC REACTION OF STREAM ISOPODS. 53 



not all stream isopods are positive all the time and some in- 

 dividuals show more daily variations than others ('130). These 

 variations have been analyzed by means of the resistance of 

 isopods to potassium cyanide ('14) and by the rate of carbon 

 dioxide production accompanying different degrees of positive 

 rheotactic reactions (Allee and Tashiro, '14). These analyses 

 indicate clearly that the degree of positiveness of the rheotactic 

 reaction of an isopod is closely correlated with its rate of metabolic 

 activity; that a highly positive rheotactic reaction is correlated 

 with a relatively rapid rate of metabolic activity and that when 

 either internal or external conditions interfere with the metabolic 

 rate of the isopod, the rheotactic reaction becomes less positive. 



There are certain periods in the life cycle of stream isopods 

 when the positiveness of the rheotactic reaction is markedly 

 reduced. These may or may not be due to the metabolic con- 

 ditions obtaining at the time. The young isopod either dis- 

 regards or is very erratic in its response to a water current until 

 it is about a month old ('12). Later at the frequently recurring 

 molting periods the same tendency is shown for about five hours 

 preceding and following the molt ('13, '130). The breeding 

 period is also a period of weakened positive rheotactic response 

 ('12) in which the stream isopods lose about two-thirds of their 

 positiveness. 



The breeding season begins rather later in the stream than 

 in the pond mores ('u) but it is in full swing in April and early 

 May. By late May the adults become rare but the season's 

 young may be taken one third or more grown. These young 

 isopods are at first indefinite in their rheotactic reactions. They 

 become highly positive more rapidly in water which has a high 

 oxygen tension but they will come to be highly positively rheo- 

 tactic even if confined to pools that have a low oxygen content 



('12). 



SURVEY OF STREAM DISTRIBUTION OF Asellus communis IN THE 



CHICAGO AREA. 



I have studied the local distribution of Asellus communis in 

 the Chicago area only (Shelford, 'n, '13, maps). Most of the 

 stream isopods used in the experimental studies summarized 



