l82 LIGHT AND THE BEHAVIOR OF ORGANISMS 



thus be forced to move toward a source of light or at any 

 angle with the rays. 



If exposed in a narrow shadow in a field of light con- 

 sisting of rays perpendicular to the substratum they crawl 

 along in the shadow. If the anterior end chances to pro- 

 ject out into the light it is stimulated and turned in the 

 opposite direction. The direction of the rays here is how- 

 ever perpendicular to the substratum; under the preceding 

 conditions it was parallel with the substratum, yet the reac- 

 tion is the same under both conditions. It is of course due 

 to a change of intensity and is not primarily dependent 

 upon the ray direction. In a narrow shadow in the field 

 consisting of vertical rays the larvae can also be made to 

 move toward or perpendicular to light rays in the shadow 

 just as under the conditions described above. Cole (1907) 

 obtained similar results in experiments on Bipaliumkewense. 



These results indicate that in their movements the larvae 

 attempt to keep the sensitive anterior end in the lowest 

 possible light intensity regardless of the direction of the 

 rays or the angle between them and the sensitive surface. 

 Loeb claims that the larvae follow the direction of the rays 

 even if in so doing they go from regions of lower into regions 

 of higher light intensity. He says (1905, p. 58), " I put 

 the almost fully grown larvae into a test-tube and placed it 

 horizontally on the table, with its longitudinal axis perpen- 

 dicular 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 on 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.'' Do these results prove Loeb's 

 conclusions? Can the reactions described in the quota- 

 tion be explained on the assumption that orientation reac- 

 tions are due to difference of intensity? 



