REACTIONS TO LIGHT IN VANESSA ANTIOPA 403 



eye was functional. But when placed in non-directive light, 

 essentially the same surface of the functional eye was continu- 

 ously illuminated; while in a horizontal beam, the surface of the 

 eye illuminated changed whenever the animal turned in either 

 direction. The absence of circus movements in the beam of 

 light must, therefore, have been due in some way to the change 

 in the illumination of different regions of the eye, produced by 

 the lateral movements. 



This conclusion is further supported by the fact that all of the 

 animals with one eye blackened used in the experiments de- 

 scribed in the present paper, if placed before a window per- 

 formed circus movements continuously, whenever they moved. 

 In some cases the organisms were kept under these conditions 

 for as long as two weeks, and frequent observations revealed no 

 modification in their behavior. Yet when these same insects 

 were placed in a horizontal beam from a glower they made no 

 circus movements after a certain amount of experience. When a 

 butterfly with one eye blackened, is placed before a w^indow, the 

 light conditions resemble, in many respects, those present in 

 non-directive light. A change in position on the part of the 

 organism, does not involve the illumination of an entirely different 

 area of the functional eye. In every position all large areas of 

 the eye are approximately equally illuminated, and no two posi- 

 tions involve the illumination of entirely different areas of the 

 eye, for the sources of reflected light are countless, and extend 

 on all sides of the animal. 



No definite conclusion can, however, be drawn as to the nature 

 of the stimulus which regulates the movement of the butterflies 

 in non-directive light. Superficially it would appear to be due 

 to the continuous action of light. Yet, it must be remembered, 

 that with every change in the axial position of the butterfly, 

 although there may be essentially no change in the illumination 

 of the surface of the eye, there must be accompanying changes of 

 intensity upon certain of the ormnatidia. There is just one 

 possible type of reaction in which the same ommatidia would be 

 illuminated equally in all of the positions assumed by the insect, 

 and that is movement of the organism in a circle with its center 



