216 



CHARLES HARLAN ABBOTT 



those of stream life induced a definite positive rheotaxis in pond 

 isopods, while conditions similar to those of pond life decreased 

 the rheotaxis of stream isopods. That these differences are 

 adaptive is shown by the fact that reaction to current is important 

 in the normal life of stream forms, but of less significance for 

 animals that live in quiet water. 



The third result, a reversal of response, has been observed in 

 various animals, due to many different causes. The work on 

 this subject is well summarized by Holmes ('16) in a chapter 

 entitled ''The Reversal of Tropisms." 



The following problems were undertaken, in order to learn 

 whether the behavior of land isopods is constant or is easily 

 changed by external conditions: 



1. Reaction to various intensities of light. 



2. Effect on reaction of repeated stimulation by the same 

 light intensity. 



3. Comparison of reactions following exposure to light and 

 to dark. 



4. Comparison of reactions of individuals from dry and moist 

 habitats. 



5. Reaction in water. 



1. Reaction to different intensities 



a. Average intensities. Studies of the effect of different in- 

 tensities were easily accomplished by placing the isopods at 

 various distances from the source of light. The intensities at the 

 points chosen furnished a range from 3 to 100 CM. The actual 

 intensities, as determined by the illuminometer, are given 

 below. 



