404 DWIGHT E. MINNICH 



such action is equal on the symmetrical photoreceptors of oppo- 

 site sides of the body. In its present form this theory is perhaps 

 best summed up by Loeb ('16, p. 259). ''If a positively helio- 

 tropic animal is struck by light from one side, the effect on tension 

 or energy production of muscles connected with this eye will be 

 such that an automatic turning of the head and the whole animal 

 towards the source of light takes place ; as soon as both eyes are 

 illuminated equally the photochemical reaction velocity will be 

 the same in both eyes, the symmetrical muscles of the body will 

 work equally, and the animal will continue to move in this direc- 

 tion. In the case of the negatively heliotropic animal the picture 

 is the same except that if only one eye is illuminated the muscles 

 connected with this eye will work less energetically." 



The 'change of intensity theory/ however, accounts for ori- 

 entation in an entirely different manner. According to it, the proc- 

 ess depends not upon the continuous action of light, but upon 

 the intermittent action of rapid changes in its intensity (Jennings 

 '04, '06, '09; Mast, '11). In positive organisms the effective 

 stimulus is assumed to be a sudden decrease of intensity on the 

 photoreceptor; in negative organisms, a sudden increase. A 

 photopositive animal, such as the honey-bee, for example, orients 

 and maintains its orientation through sudden swervings away from 

 the side experiencing a decrease of illumination, and orientation 

 is attained when neither eye is undergoing such a decrease. A 

 similar explanation is applied to photonegative organisms except 

 that the effective stimulus for them is assumed to be an increase 

 of intensity. 



There is thus a wide diversity in the explanations of orientation 

 offered by these two theories. In the concluding pages of this 

 paper, therefore, I propose to discuss the evidence afforded by 

 my own experiments, as well as that afforded by the observations 

 on circus movements in general, with a view to ascertaining 

 which of the two theories more correctly applies to the orienta- 

 tion of arthropods. 



