LIGHT REACTIONS OF TERRESTRIAL AMPHIPODS 337 



negativity of the smallest individuals, and seven minutes rep- 

 resents that of the largest.) 



Specimens are kept in covered glass dishes with some filter 

 paper*moistened with sea water. I do not think that the moist 

 paper has to do with the negative reactions. I have kept two 

 sets of O. pugettensis in the dark for forty-eight hours, one set 

 in dripping wet paper and the other in a very small amount of 

 moisture. When brought to the light, they both were positive 

 at once. 



Effect of temperature. — The most influential factor in the 

 reversal of the sense of phototaxis of O. pugettensis is tempera- 

 ture. I put one set of about a dozen specimens on ice for 

 twenty -four hours, and placed one set in room temperature for 

 the same length of time, keeping both in the dark. When 

 exposed to a light of thirty-two candle power, those kept in 

 the room were positive in one-half minute, and those kept on 

 ice were positive in two minutes, but remained sluggish for 

 five minutes. Those kept in the room had been exposed to 

 darkness for five days, hence the one-half minute's negativity. 

 (The above reaction time denotes that all the specimens were 

 positive in that time.) 



Effect of blackening one eye. — 0. pugettensis always makes 

 circus movements toward the normal eye. This reaction is 

 common in most of the positive forms on which the experiment 

 has been tried. Holmes found it to be true in positive speci- 

 mens of Ranatra, Talorchestia, and 0. agilis. 



When the specimen is picked up repeatedly and started from 

 a certain point in a position directly facing the light, one notices 

 after several trials, a gradual diminution of the curve and a 

 growing tendency to travel to the light in a more or less direct 

 course. Very often it started out in a straight line toward the 

 light, but it can never keep the path straight all the way. If 

 it begins to veer when halfway from the light, it will sometimes 

 stop, turn until it faces the light and then travel on. The same 

 interesting behavior was observed by Holmes in his work on 

 Ranatra. 



Effect of contact. — The inhibition of the light response by 

 thigmotaxis depends upon the character of the contact stim- 

 ulus. When specimens are shaken so that they roll about the 



