RHEOTAXIS IN ISOPODA 273 



Much material collected from all places isopods are known to 

 inhabit in this region, was identified by Miss Richardson in 1910 

 as consisting entirely of the two species mentioned. The appear- 

 ance of the two is entirely difTerent, so that there is no chance of 

 any of the results obtained being due to a mixing of species. 

 Regarding A. communis, Miss Richardson says, in a private com- 

 munication : 



Asellus communis is a very variable species. The uropods in some 

 specimens differ considerably from the typical form, in being shorter 

 and of varying lengths, while in other specimens they are narrower. I 

 think these differences ma}^ be due to size, age, and in some cases the 

 parts may be in process of regeneration. Then too the propodus of the 

 first pair of legs in the male differs in the specimens, being larger in some 

 than in others and with spines more pronounced. There are so many 

 intermediate stages that it was not possible to group the specimens into 

 varieties as I had at first supposed could be done. 



A careful study of a large number of specimens from both ponds 

 and streams showed these variations to be equally common in all 

 habitats. Thus the differences found in the reactions cannot be 

 due to taxonomically differing races. 



2. GENERAL REACTIONS 



The experimental work upon which this paper is based was 

 begun in the summer of 1909 and has been in progress continu- 

 ously since that time. The early experiments upon the general 

 responses run parallel with those of Banta ('10). A summary of 

 these general reactions of A. communis will be given, in order that 

 the conditions of the later experiments may be better appreciated. 

 Unless otherwise indicated, the results are my own and in almost 

 every instance they support the results obtained by Banta. 



1. The main breeding period in these isopods extends from 

 the middle of March to the middle of July but this may be ex- 

 tended in scattered cases until the beginning of cold weather. 

 During the copulation the males carry the females for as many as 

 three days. The developmental period is about three weeks and 

 on the average forty offspring are liberated each time. The num- 

 ber of offspring may be much larger, and in one instance two hun- 



