THE ECOLOGICAL IMPORTANCE OF THE RHEO- 

 TACTIC REACTION OF STREAM ISOPODS. 



W. C. ALLEE, 

 THOMPSON BIOLOGICAL LABORATORY, W.LLIAMS COLLEGE. 



CONTENTS. 



1. Earlier work on the rheotaxis in isopods 52 



2. Survey of streams distribution of Asellus communis in the Chicago area. . 53 



3. Relation of rheotaxis to maintainance of position in streams 55 



4. Interrelations of thigmotaxis and rheotaxis 57 



5. Discussion and summary 63 



EARLIER WORK ON RHEOTAXIS IN ISOPODS. 



Some time ago it was shown that the stream mores of the 

 isopod Asellus communis (Say) is usually positive in its rheotactic 

 reaction ('12). 1 When measured in an intermittent circular 

 current 2 this was shown by the relatively high percentage of 

 positive responses (79 per cent, of 1,570 trials). In a continuous 

 straight current the same tendency was shown by the relatively 

 long time (7 hrs., 12 min.) which stream isopods would remain 

 positive before their ultimate reversal. Pond isopods of the same 

 species, on the other hand, gave weak positive reaction in the 

 intermittent current (25 per cent, of 775 trials) and there was an 

 average of only 51 minutes of positive reaction before reversal in 

 a continuous straight current. 



The difference in the reaction of the isopods from the two 

 habitats is closely correlated with certain natural conditions, the 

 most important of which is the oxygen content of the water. 

 The oxygen tension of the streams which isopods inhabit is much 

 higher than that of the ponds. Laboratory tests show that 

 increasing the amount of oxygen present in water makes pond 

 isopods more positive and decreasing the oxygen tension tends 

 to make stream isopods less positively rheotactic. 



But even under uniform and favorable external conditions 



1 Numerals standing alone have reference to earlier papers by the author. 



2 For details of the method see Alice, '12, p. 276; '13, p. 261. 



5 2 



