PHOTIC REACTIONS OF HONEY-BEE 407 



3. General evidence 



In experimenting with locomotor organisms, it is not an easy 

 matter to regulate absolutely the conditions of photic stimulation. 

 Thus, a photopositive animal as it moves toward the light is 

 acted upon continuously by the light. But it is also subjected 

 not only to a gradual increase of intensity with every forward 

 movement, but also to sudden decreases and increases with every 

 lateral deviation, however slight. Whether the orientation of 

 the organism, therefore, is effected through a continuous action 

 of the stimulus or only through sudden changes in its intensity 

 from time to time, is frequently difficult to determine with cer- 

 tainty. This difficulty in separating the two conditions experi- 

 mentally has led to much confusion in solving the problem of 

 orientation. Among arthropods, however, there is a growing 

 body of evidence tending to show that orientation is produced by 

 continuous photic stimulation and not by interixiittent changes of 

 intensity. 



No stronger evidence is to be found in this connection than 

 that afforded by the general phenomenon of circus movements. 

 The importance of this evidence has been repeatedly emphasized 

 by Parker ('03, '07), Loeb ('06, '13), Bohn ('09 a, b). Holmes 

 and McGraw ('13), Garrey ('17), and others. Parker ('07, p. 

 548), in reviewing one of the earlier expositions of the 'change of 

 intensity theory' (Jennings, '06), states the case clearly when he 

 says, ''If the modern tropism theory were as weak as Jennings 

 would have us believe, the experimental evidence upon which it 

 rests ought easily to be explained away. Yet it has always 

 seemed to the reviewer that the characteristic circus movements 

 performed by animals immersed in a homogeneous stimulant, 

 but with sense organs unilaterally obstructed, are explainable 

 only on the basis of this theory." 



This statement is certainly justified by the facts. Circus 

 movements seem quite incapable of explanation in terms of the 

 'change of intensity theory' of orientation. Let us examine the 

 case of a photopositive arthropod, with the right eye blackened, 

 as it creeps toward a source of light. From experiment we know 



