HELIOTROPISM OF 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 larve 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 larve 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 
larvee. 
I placed some fly eggs on smoked glass plates and allowed 
them to hatch. As the larve 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 larve 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 larvee came 
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