58 STUDIES IN GENERAL PHYSIOLOGY 
their median planes in the direction of the rays. (That this 
was due to the effect of the light, and not a compensatory 
movement that might have been produced by the rapid 
turning of the board is shown by the fact that compensatory 
movements do not exist in Musca larve.) 
I was able to show that fly larve are compelled to move 
from less intense light into more intense light under the 
influence of the rays of light, just as it could be shown that 
positively heliotropic animals do not go from dark places to 
light ones, but follow the direction of the rays, even when by 
so doing they move from a region of greater intensity of light 
to one of less. I put the almost fully grown larvee into a test- 
tube and placed it horizontally on the table, with its longitu- 
dinal axis perpendicular to the plane of the window. The 
sun’s rays made a small angle with the window. By means 
of a screen I arranged the test-tube so that only diffuse 
light fell through the window upon the half turned toward 
the window, while direct sunlight fell upon the half turned 
toward the room. At the beginning of the experiment the 
animals were all on the window side of the test-tube. They 
immediately moved from the shaded part into the direct 
sunlight on the room side, and remained there. 
Incidentally I was able to observe that the light stimuli 
which strike the oral pole of these completely blind animals 
are most important in the orientation of the animals toward 
light. When the animals crossed the boundary from diffuse 
light into direct sunlight, the reaction caused by the increase 
in the intensity of the light did not take place until a half 
or a third of the body of the animal was in the sunlight 
(because in all phenomena of stimulation some time elapses 
between the application of the stimulus and the reaction to 
it). The animal checked its movement and turned its head 
through an angle of 90-130° from side to side. If in so 
doing the head again came into the shade, the animal 
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