RHEOTACTIC REACTION OF STREAM ISOPODS. 57 



tribution thigmotaxis seems to be of primary importance rather 

 than rheotaxis. 



INTERRELATIONS OF THIGMOTAXIS AND RHEOTAXIS. 



In testing for interrelations between the rheotactic and thig- 

 motactic reactions of isopods that might prove of ecological 

 importance the following methods were employed : Isopods were 

 tested individually in an intermittent circular current for their 

 rheotactic reaction; immediately following this the individuals 

 were tested in a straight trough to find how strong a water 

 current they could resist. 



These tests were made in a wooden trough painted with water 

 proof paint over which fine quartz sand had been sprinkled while 

 the paint was soft in order to give a good surface for the isopods. 

 The sides met at the bottom forming a V-shaped trough with an 

 enclosed angle of about thirty degrees. The trough was 75 cm. 

 long and was used with a pitch of 8 cm. The current was pro- 

 duced by tap water. 



In making the test the isopod was oriented as desired in a very 

 small stream of water. A rubber tube 5 mm. in diameter con- 

 nected with the tap faucet was introduced 5 cm. above the isopod. 

 The faucet valve was gradually opened until the current swept 

 the isopod off its feet. This occurred in less than a minute so 

 that fatigue did not affect the result. Then the flow in cubic 

 centimeters was measured. In the early work five successive 

 tests were averaged to give the resistance of the isopcd for the 

 day but this was found to be too severe and in the later work 

 only one test was made per day. 



In the weak current the isopods would usually start to move in 

 the direction in which they were headed ('12) although those 

 oriented negatively attempted to turn more often than the others. 

 They usually continued moving until the current became fairly 

 strong, then stopped and clung to the boards in the angle at the 

 bottom of the trough. 



When headed upstream the head was placed close to the bottom 

 as movement ceased the head and front of the thorax made an 

 arch which kept the water running over the relatively smooth 

 convex back of the isopod. The anterior legs are used more in 



