REACTIONS OF LAND ISOPODS TO LIGHT 241 



This statement is not at all surprising, because sudden flashes 

 of light are a part of the natural environment of the firefly, and 

 the firefly is adapted to them in a unique way. On the other 

 hand, since most animals are exposed to constant illumination 

 and to gradual changes rather than to sudden changes, the 

 firefly is hardly a typical animal in which to test out this point. 



For this reason an experiment was devised to determine 

 whether Oniscus can be oriented after a delay, in the absence 

 of the original stimulus. Twelve individuals of Oniscus were 

 tested in the position facing the light, according to the usual 

 method. Record was made only when there was a delay in 

 response lasting more than five seconds. In approximately half 

 of these instances of delayed response in each animal, the light 

 was turned off after five seconds; in the other half the animals 

 were exposed to the light until they responded. (If no response 

 occurred during 60 seconds, it was recorded as 'no response.') 

 The course taken by the animal and the interval before response 

 were both recorded in each instance. The individual animals 

 were given repeated stimuli until several delayed responses had 

 been recorded under each of the two conditions. 



In order to observe the animal after the light was turned off, 

 the influence of overhead fights was tested until one was found so 

 dim as not to affect the accuracy of orientation and yet strong 

 enough to make it possible to watch the movements of the ani- 

 mals. In a prefiminary test series with the light finally chosen, 

 five individuals of Oniscus showed an average angle of negative- 

 ness of 126°, which is thoroughly normal, and shows that the 

 overhead light did not disturb the normal reaction to horizontal 

 illumination. This light was, therefore, used during the entire 

 experiment. 



The result is shown in figure 14 and table 12. According to 

 figure 14, the greatest number of responses when the light had 

 been turned off fell into class 1 on the abscissa, with a gradually 

 decreasing number of responses in the other classes. This shows 

 that the animals moved in general in the direction in which they 

 were headed, with no apparent response to the initial light stmiu- 

 lus. On the other hand, the curve of the responses when the 



