290 W. C. ALLEE 



2. POND ASELLI 



a. Adults 



As has already been mentioned, there are two species of isopods 

 found in the ponds near Chicago. Mancasellus danielsii (Rich- 

 ardson, '05, pp. 417-^19) has been previously reported from La- 

 porte, Indiana, only. It is a much flattened form and is generally 

 found in the grasses in the shallower water of the ponds. Its 

 reactions so far as tested agree with those of A. communis and 

 unless otherwise designated all the discussion of pond behavior 

 will be based on the latter species. 



The pond Aselli are decidedly smaller than those from the 

 stream. In the isopods that have been measured the difference 

 averaged about 3 mm., that is they were about 75 to 80 per 

 cent of the length of the stream forms. However as has already 

 been stated the pond isopods contain all the variations of the pro- 

 podus of the first pair of legs that are to be found in the stream 

 forms. The pond isopods react to light, heat, touch, and gravity 

 in much the same way as the stream animals, although the speed 

 of the reaction and the sensitiveness to the stimuli are probably 

 different. The rheotactic response of the isopods from the two 

 habitats is markedly different. In place of the positive reaction 

 to the current dominating as in the stream mores, these isopods 

 give a high proportion of indefinite responses. Their orientation 

 is less definite and they do not appear to be so capable of ho' ding 

 an orientation once it is attained. Their response is less vigorous 

 than that of the stream isopods and they are much more easily 

 swept off their feet by the current. 



A typical pond isopod response to current is shown in figure 3. 

 This trial and record was made exactly like that for the stream 

 isopod shown in figure 1. The result of twenty trials is given 

 in which the animal went positive 25 per cent, negative 30 per 

 cent, and indifferent to the current 45 per cent of the total number 

 of trials. 



The same response is shown in table 6. It will be noted that 

 the table includes the response made during the breeding season 

 and that this response is not markedly different from that of the 



