RHEOTAXIS IN ISOPODA 285 



The large brood pouch on the ventral side of the females offers 

 a serious mechanical obstacle to making progress against a strong 

 current, but since the same reaction tendencies occur in the males 

 this mechanical hindrance cannot be the only factor in the re- 

 sponse. The condition of the germinal glands during the breeding 

 season and the exact connection existing between their activity 

 and the rheotactic response will be presented in detail in another 

 paper. 



In comparison with the normal behavior, the large increase in 

 indifference to the current is remarkable, since this is just the con- 

 dition that tends to prevail in pond animals. The action in the 

 current is also decidedly different. The animals are much more 

 easily swept from their footing; they do not reverse so rapidly as 

 the current changes, often failing to reverse at all and the speed 

 of reaction is greatly lowered. In some cases there is no response 

 at all ; the animals are then in the same state as that caused by the 

 strongest depressing agents. 



This breeding behavior brings up some interesting points in 

 the ecology of the isopods. Although they are taken in streams 

 they are rarely found in rapid parts of permanent ones, being 

 limited for the most part to the pools and protected places. Out- 

 side of the breeding season they are fitted by their positive reac- 

 tion to the current and their strong clinging ability to maintain 

 themselves in much stronger currents than those in which they 

 are found. The reason for their absence in these places must be 

 due to the influence of the breeding activities upon their behavior. 

 This is especially significant since their period of least ability to 

 maintain themselves corresponds to the time of the strongest cur- 

 rent in the stream. Hence their breeding behavior limits them to 

 those streams where they can find ample lodging places during 

 this time of weakened responses. 



c. Juvenile rnores 



When the isopods are first liberated they are about 2 mm. iii 

 length and usually give no response to the current but cling 

 passively to the bottom. Consistent rheotactic responses were 

 made by the time the animals were about 3 mm. long; that is when 



