SOME EXPERIMENTS ON THE METHOD OF 

 ORIENTATION TO LIGHT 



S. J. HOLMES AND K. W. McGRAW 



One of the questions raised in recent years concerning the 

 orientation of animals to light is whether light acts as a con- 

 stant stimulus or stimulates mainly through its fluctuations of 

 intensity. We may conceivably explain the orientation of an 

 insect, for instance, by the supposition that when there is a 

 deviation from the line of orientation to the left and a diminu- 

 tion of the light entering the left eye (or an increase of light 

 entering the right one) the change in light intensity produces 

 a reaction that turns the insect to the right. Deviations toward 

 the right of the direction of the rays being responded to by a 

 turn toward the left the insect would automatically keep in a 

 position of orientation. We know that changes in the intensity 

 of light, whether an increase or a decrease, may act as a stim- 

 ulus, especially if the changes are sudden. In many animals it 

 is not so much the intensity of the light that induces a response 

 as the shock of transition from one intensity to another. Is 

 it possible to explain orientation in general as the result of such 

 responses ? 



Some years ago it was found by one of the writers (Holmes, 1 

 '05) that Ranatras with one eye blackened over were sometimes 

 able to go toward the light in a nearly straight line. While 

 there was a tendency to turn toward the normal eye, there 

 were counter movements which held this tendency in check. 

 It was pointed out that "Were the insect so constituted as to 

 respond to an increase of light entering the left eye by a turn 

 to the left and to a decrease of light by a turn to the right, we 

 can understand how, when once pointed towards the light, a 

 straight course might be preserved. If the insect turned towards 

 the right there would be an increase of light entering the left 

 eye which we might suppose stimulates the insect to turn in 

 the opposite direction. Deviations to the left would cause a 

 diminution of light entering the left eye, which we might sup- 



1 Holmes, S. J. The reactions of Ranatra to light. Jour. Comp. Neur. and 

 Psych. 1905, vol. 15, pp. 305-349. 



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