HELIOTROPISM or ANIMALS 61 



In this case, therefore, just as in the case of the positively 

 heliotropic animals, it is chiefly the more refrangible rays 

 which effect the orientation of the animals. The helio- 

 tropic influence of the less refrangible rays, however, is 

 much less in the eyeless fly larvse than in any other ani- 

 mals that I have studied. The animals moved in the direc- 

 tion of the rays, even in diffuse light, at a distance of one 

 meter from the window, but the less the intensity of the 

 light, the more easily did other stimuli (such as contact 

 stimuli) cause a deviation of the animals from the straight 

 line. I have often repeated these experiments in the course 

 of the last two years, and have each time obtained essentially 

 similar results. The irritability of the animal is, however, 

 not always the same; especially does its irritability vary 

 during different periods of its life. I have, however, con- 

 vinced myself that the larvae are negatively heliotropic even 

 immediately after being hatched, although they do not move 

 as precisely in the direction of the rays as the fully grown 

 larvse. 



I placed some fly eggs on smoked glass plates and allowed 

 them to hatch. As the larvse removed the soot in their 

 path, they thus registered graphically the paths they took 

 from the eggs. The glass plates lay on a horizontal table in 

 a room lighted from one side only. The paths followed by 

 the larvee ran, almost without exception, toward the room 

 side of the plates. In the few exceptions the path usually 

 ran first toward the window, then bent, and went toward the 

 room side of the plate. It was neither a mysterious force of 

 nature nor an obscure "inherited instinct" which dictated 

 the direction of the movements of these animals, but only 

 the direction of the rays of light, just as gravity determines 

 the orientation of the Lepidoptera when they emerge from 

 the chrysalis. 



When the diffuse daylight which struck the larvse came 



