METHOD OF ORIENTATION TO LIGHT 371 



held in the hand in a fixed position often made struggles to 

 turn toward the light. These would frequently manifest them- 

 selves by reaching toward the light with the anterior prehensile 

 legs, and if the insect was in a position to get the feet in contact 

 with a solid object, by pushing and pulling with the other legs 

 also. This fact, together with the persistence for a considerable 

 interval of certain attitudes of the head and body that were 

 assumed in relation to the source of light, seemed to point to 

 a fairly constant stimulating influence of the rays. However, 

 when the insects were fastened to a glass rod and suspended 

 in the air the legs showed no definite response to unilateral 

 stimulation. A considerable number of phototactic insects were 

 held near the light to find if any effect could be noticed in the 

 attitudes of the appendages, but the results, as Radl also found 

 in similar experiments on a number of insects, were entirely 

 negative. 



While no effect of light on the muscular tonus of the legs 

 was manifest so long as the legs were allowed to dangle loosely 

 in the air, it was thought that such an effect might be demon- 

 strated if the legs were given something to act upon which could 

 be moved without altering the position of the insect in relation 

 to the source of light. Profiting by a suggestion from Prof. 

 S. S. Maxwell a device of this kind was constructed consisting 

 of a thin horizontal disk rotating on a pivot like the turntable 

 of the microscopist. The apparatus was made very light and 

 easy running so that even a small insect could set it in motion. 

 By holding an insect over the disk with the head pointing either 

 toward or away from the center, and having a light so that the 

 rays fell upon one side of the body, the movements of the legs 

 which would ordinarily turn the insect toward the light would 

 simply cause the disk to rotate in the opposite direction. With 

 the insect held steadily, the stimulus afforded by the light would 

 naturally remain constant, and if light oriented by its constant 

 stimulating effect we might expect the insect would keep rotat- 

 ing the disk in its attempts at orientation. 



Butterflies proved to be very convenient to experiment with, 

 since by grasping them by the wings held together above the 

 body they could be held quite steadily, especially with the aid 

 of a hand rest, above the disk. A cabbage butterfly, Pieris 

 rapes, was held facing the center of the disk and presenting its 



