LIGHT REACTIONS OF TERRESTRIAL AMPHIPODS 341 



twenty minutes, while smaller specimens became positive in 

 eleven minutes. 



Cold, moisture and inactivity tend to prolong negative photo- 

 taxis in 0. traskiana, while heat, dryness and a state o f activity 

 decreases the duration of the negative response. 



On March 29th, three sets of specimens were put away in 

 the dark under conditions of temperature and moisture as 

 follows : 



Set I — 10 traskianas — very little moisture. 

 Set II — 6 — very moist. 



Set III— 18 " —on ice. 



On April 3rd, they were exposed to the light o^ an electric 

 lamp in the dark room, with the following results: 



Set I — positive immediately. 

 Set II — positive in eight minutes. 



Set III — one-half positive in fifteen minutes, rest positive in 

 twenty -two minutes. 



Twelve specimens kept in the dark in as dry a condition as 

 possible for twenty-four hours, became positive, when exposed 

 to the light, in seven minutes, while five specimens kept in a 

 condition of great moisture for the same length o f time became 

 positive in twelve minutes. 



Effect of heat. — The effect of heat upon the phototactic re- 

 sponse is shown by the following experiment: A set of about 

 two dozen specimens, which had been kept on ice for twenty- 

 four hours, was divided into two lots, one of which was put. 

 back on ice, and the other put on top of the radiator. The 

 dish was wrapped up in a dark cloth so as to exclude the light 

 After about six or seven minutes, the set on the radiator was 

 brought into the dark room and exposed to strong light. The 

 specimens were found to be positive in five and one-half minutes. 

 Those kept on ice became positive in fifteen to twenty minutes. 



It seems to be the rule in regard to amphipods that high 

 temperature causes a positive phototaxis, while low tempera- 

 ture brings about a negative reaction of longer duration. Holmes 

 found that O. agilts became positive very quickly when the 



