REACTIONS TO LIGHT IN VANESSA ANTIOPA 413 



to turn toward the functional eye, never in the opposite direc- 

 tion. What causes changes from rest to activity, and what is 

 the significance of the fact that the animals attempt continu- 

 ously to turn toward the functional eye? 



During the periods of rest the changes in illumination in the 

 eyes were exceedingly slight for the light remained practically 

 constant, and there was no change in the position of the eyes. 

 Consequently, if the change from rest to activity is due to light 

 at all, which is questionable, since the butterflies do at times 

 move in total darkness, it must be due to stimuli dependent upon 

 the continuous action of light, not upon the time rate of change 

 in intensity. Regarding the nature of the orienting stimulus 

 the results are unfortunately not so conclusive, for the moment 

 the animals become active, and before they attempt to turn, 

 there is a change in the position of the eye owing to the vertical 

 movements of the head, and this, no doubt, results in changes 

 in the luminous intensity on the various ommatidia. Thus, it 

 is evident that the attempt on the part of the animal to turn 

 toward the illuminated eye may be due to stimuli dependent 

 upon the time-rate of change of intensity. 



Thus, it is clear that these results do not fully settle the ques- 

 tion as to how light acts in producing the orienting stimulus and 

 that the conclusions of Holmes previously stated are not justified. 



The method described above of exposing one eye only to light 

 gives opportunity for testing the effect of illuminating different 

 areas of the eye. The results of this experiment are presented 

 in the following section. 



2. Effect of illuminating different areas of one eye 



We have demonstrated that in Vanessa only one eye is neces- 

 sary in the process of orientation in light. Now the question 

 arises concerning the effect of the illumination of different areas 

 of the eye. If there is any effect then the axial position assumed 

 by the animal ought to bear some specific relation to the area of 

 the eye illuminated. 



This was tested by suspending the insects above a disc, as in 

 the previous experiment, and by allowing the horizontal beam 



