REACTIONS OF LAND ISOPODS TO LIGHT 



219 



TABLE 5 



Reactions to low intensities of light. Numbers refer to the average reactions of 



individual isopods 



almost invisible, although scarcely 30 cm. from the eye of the 

 observer. A faint reflection on the chitin covering made it 

 possible, but not easy, to determine the position of the animal, 

 particularly during locomotion. Of course the actual hori- 

 zontal illumination which reached the eyes of the animal was 

 considerably greater than was reflected to the observer above. 

 In spite of the fact that the light was so dim, Oniscus turned 

 away from it as definitely as from the stronger intensities be- 

 tween 1 and 100 CM. Nearly all of the individual animals, on 

 starting locomotion, turned directly away, without describing as 

 large an arc as is common when higher intensities are used. 



As a control for this experiment, ten individuals of Oniscus 

 were tested immediately afterwards in total darkness. The 

 result showed no orientation to any external stimulus, and in 

 most instances locomotion was in the direction in which the 

 animals were already headed. The actual course taken was 

 determined by means of a flashlight after the isopod had been 

 in the dark long enough to start in a definite direction. As all 

 conditions except exposure to light were the same in the control 

 as in the experiment, the results with low intensities must have 

 been due solely to the light. 



According to Patten ('17, p. 260), ''It is the abruptness with 

 which orientation is attained, rather than the final accuracy of 



