RHEOTAXIS IN ISOPODA 275 



meters. If exposed to the colors of the spectrum they collect in 

 the red end and animals exposed to red light act almost as if 

 in complete darkness. Aselli are nocturnal in their habits and 

 in ponds or aquaria there is a diurnal movement, depending, in 

 part at least, on light conditions. 



7. Banta (1. c, pp. 440-467) found that they were sensitive 

 to mechanical stimulation with bristles; with localized currents 

 of water; with concussion; and with vibrations at the rate of 100 

 per second. The results with localized currents of water showed 

 the most sensitive parts of Asellus to be on the head and at the 

 base of the antennae, and these responses were only gained with 

 the strongest currents used. 



3. METHODS 



At the beginning of the work on rheotaxis it was necessary to 

 devise some method that would permit rapid testing of a large 

 number of ^animals and in which the personal equation should be 

 reduced to a minimum. Also the method used must be applic- 

 able to both field a,nd laboratory work. For this reason it was 

 thought best to use a circular current, although Allen ('10) in an 

 unpublished master's thesis showed that the current set up in a 

 circular pan is not straight but forms a diverging spiral. 



The method used is as follows : The isopods were placed in an 

 enamel-ware pan 25 cm. in diameter and 6 cm. deep. In order 

 that they might have a firm foot hold for crawling the pan 

 bottom was covered with a layer of bees-wax. The animals were 

 placed in 2 cm. of the same water in which they had been kept. 

 In the laboratory the pan was then set in a dark box under an 

 illumination of one candle meter. One side of this box was cur- 

 tained so that later the tests could be made without introducing 

 outside light. Usually five isopods were used although this num- 

 ber was varied with the size and condition of the animals. The 

 animals were undisturbed for fifteen minutes in order to allow 

 them to become accustomed to the new conditions and to per- 

 mit a recovery from the shock of handling. In case they had been 

 kept at a temperature differing from that of the room the pan was 



