368 S. J. HOLMES AND K. W. McGRAW 



pose acts as a stimulus to turn to the right side. The right 

 eye may be supposed to act, mutatis mutandis, in a similar 

 manner." The general upshot of the discussion was that what- 

 ever role the fluctuations of light intensity might play in the 

 orientation of Ranatra they did not alone afford a satisfactory 

 explanation of orientation, and the tentative conclusion was 

 reached that each of the two factors mentioned "may supple- 

 ment the other in such a way as to cooperate in the maintenance 

 of a direct course towards the light." 



Mast, 2 who is an opponent of the view that phototaxis is the 

 result of light acting as a constant stimulus, is favorably dis- 

 posed toward the alternative supposition which makes orien- 

 tation a function of differential sensibility. In speaking of 

 orientation in many lower forms, he says: "In many of these 

 forms orientation is undoubtedly, and in all it is probably, a 

 response to change of light intensity on some part of the organ- 

 ism. At any rate it has in no instance been demonstrated that 

 it is, as Loeb states, 'a function of constant intensity,' that 

 orientation to light is like orientation to an electric current." 



While there are many facts that indicate that light exercises 

 a stimulating effect on organisms quite apart from the shocks 

 due to variations of intensity, the question as to the relative 

 potency of the two influences mentioned, which Mast has done 

 well to bring into greater prominence, is one that can be answ- 

 ered only by experiment. In the ordinary movements of ani- 

 mals to or from the light both these two factors are free to come 

 into play. The natural method of attacking the problem, there- 

 fore, is to exclude one of the possible agencies, and then to 

 observe the effect of the other alone. 



To this end an apparatus was devised consisting of a jar 

 lined below and at the sides with white paper. This was cov- 

 ered by a cone of the same material in the apex of which was 

 placed an electric light. A small peep hole permitted the obser- 

 vation of insects placed in the jar. In several experiments the 

 insect *was placed in a small circular glass dish in the center of 

 the enclosure. Whether the insect turned to the right or to 

 the left in this apparatus, the amount of light entering the eyes 

 was approximately the same. Insects with one eye blackened 

 over were placed in the jar and stimulated to activity whenever 



2 Mast, S. O. Light and the behavior of organisms. New York. 1911. 



