PHOTIC REACTIONS OF HONEY-BEE 409 



movements toward the fmictional eye. In the absence of signifi- 

 cant intensity changes, these responses must have been produced 

 through the continuous action of Ught. The correctness of this 

 conclusion is further attested by the fact that bees with one eye 

 blackened tend to circle more toward the functional eye in a non- 

 directive light of high intensity than in one of low intensity. The 

 response may thus be made to vary with the intensity of a con- 

 stantly acting stimulus. 



Bohn ('09, a, '09 b) has suggested circus movements as a cri- 

 terion for tropisms. Certainly, if the photic orientation of an 

 animal is the result of a continuous action of the light on both 

 eyes, as the tropism hypothesis postulates, the elimination of one 

 eye should produce circus movements. The form of response 

 will, of course, be subject to the peculiarities of locomotion. 

 However, the elimination of the photoreceptors on one side of 

 the body should result in a more or less asymmetrical response 

 toward or away from that side, depending upon the index of pho- 

 totropism. A failure to obtain such responses means either that 

 the orienting stimulus does not consist in the continuous action 

 of light or that modifying factors are present which interfere 

 with the expected response. As stated in the introductory pages 

 of this paper, the failures to obtain circus movements w^hich have 

 thus far been reported, are, I believe, to be attributed to the lat- 

 ter rather than to the former cause. 



In conclusion, we may say that circus movements, both in direc- 

 tive and non-directive illumination, are produced by the con- 

 tinuous action of light and not by intermittent changes in its 

 intensity. This, together with the general occurrence of cir- 

 cus movements among arthropods and the close relationship of 

 such responses to normal orientation afford strong evidence that 

 in this group of animals photic orientation is normally produced 

 through the continuous action of light. This does not mean that 

 photosensitive arthropods do not respond to sudden changes in 

 illumination. They undoubtedly do. The orientation of the 

 body toward or away from a source of light, however, cannot be 

 fundamentally the result of such responses. 



