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 larvae.) 



I was able to show that fly larvae 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 larvae 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 



