332 W. C. ALLEE 



gen conditions, the isopods became more tolerant of low oxygen, 

 although their optimum amount of oxygen was still higher than 

 that furnished by the low end of the gradient. 



The results with a carbon dioxide gradient are given in table 

 30. These trials show that stream isopods avoided an amount 

 of carbon dioxide equivalent to the highest quantity found in 

 nature, but that the avoiding is much more pronounced as the 

 amount of carbon dioxide is increased. The result is, that al- 

 though the higher amount of carbon dioxide diffuses against the 

 current to the low end of the gradient, the isopods collect in an 

 amount of carbon dioxide to which they are strongly negative 

 when such a response is possible. 



These gradient experiments show that the stream isopods tend 

 to collect in the amount of oxygen or carbon dioxide to which they 

 are accustomed. In nature this reaction would tend to keep the 

 stream isopods from collecting in conditions that might affect 

 their general state of metabolism, and in this way affect their 

 power of resistance to the stream current. 



5. SUMMARY OF EXPERIMENTAL RESULTS 



The major experiments may best be classified on the basis of 

 the result, of the materials used, upon the rheotactic response. 

 Under this division they fall naturally into two groups; those 

 that decrease and those that increase the positiveness of the rheo- 

 tactic response. These may be summarized as follows : 



A. Conditions that decrease the positive rheotactic response: 



1. Low oxygen 



2. Chloretone 



3. Potassium cyanide 



4. Low temperature 



5. Sudden extreme increase of temperature 



6. Carbon dioxide. 



7. Starvation 



To these may be added the life history effects as shown in the 

 breeding season and the juvenile reactions. 



