REACTIONS OF LAND ISOPODS TO LIGHT 239 



1 . Direct orientation versus method of trial 



The study of Torrey and Hays ('14) on the orientation of 

 PorcelHo to light resulted in the conclusion that orientation is 

 direct and that the many preliminary movements of the antennae 

 before locomotion, which appear to correspond to the 'random 

 movements' in the forms studied by Holmes ('05), have no rela- 

 tion to orientation. These authors concluded that: ''The con- 

 sistency with which many individuals turned away from the 

 light, whether the latter was on one side or the other, left no 

 room for doubt that the reaction was forced in a definite direction.'^ 



Although the present investigation was not made specifically 

 to deteniiine the mechanics of response, observations in the or- 

 dinary experiments indicated that there is a definite orientation, 

 which is little obscured by random movements. In the usual 

 negative responses there is a time difference which makes pos- 

 sible a division into three classes of responses: 1) those in which 

 orientation occurs without locomotion; 2) those in which orien- 

 tation occurs abruptly at the beginning of locomotion; 3) those 

 in which orientation occurs gradually during locomotion. Of 

 these, the first is not frequent, although a characteristic response, 

 while the second and third are both common. The third method, 

 where orientation is secured by a gradually curved course, is in 

 reality just as direct as the other two. If necessary allowance 

 is made for individual peculiarities in the response, due prob- 

 ably to internal causes, the response seems to involve a direct 

 orientation by the light, in which trial movements do not play 

 any essential part. 



2. Constant intensity versus change of intensity 



The basis of most present discussions of orientation is, how- 

 ever, the second of those quoted from Bancroft, the question 

 whether the response is due to continuous stimulation from light 

 acting at a constant intensity or to the shock resulting from 

 changes of intensity. A possible contribution to the solution of 

 this problem may be found in an analysis of the frequent delays 

 before response. Even when an isopod responds vigorously at 



