HELIOTEOPISM OF ANIMALS 63 



those cases occasionally observed in plants where in light of 

 great intensity the heliotropism of an organ is the opposite 

 of that in light of less intensity. A closer examination, 

 however, showed this not to be the case. When the test- 

 tube lay perpendicular to the plane of the window, positively 

 heliotropic animals contained in it moved, as we have seen, 

 not only to the upper, but also to the window side of the test- 

 tube. This was not the case with the newly hatched fly 

 larvse. They all turned their ventral surfaces toward the 

 source of light, but otherwise moved about irregularly. I 

 placed the animals in a test-tube which was covered with 

 black paper, except for a small slit, and let direct sunlight 

 enter the tube only through the slit. The animals which were 

 on the lower side of the tube left it as soon as the light 

 struck their backs, and crept upward ; but no animal which 

 was sheltered from the light was attracted to the upper, 

 lighted side of the tube, as was the case under similar con- 

 ditions in the positively heliotropic caterpillars of Chrysor- 

 rhoea. When I held the glass vertically, more animals 

 collected on the window side, but they did not all creep up- 

 ward, as did the positively heliotropic animals. When I 

 placed the animals on the outside of a test-tube, they did 

 not move upward, but collected for the most part on the 

 under side of the tube. This experiment was not very 

 decisive, however, as the animals easily fell off the tube. 

 These facts can be interpreted in no other way than that 

 the intense light compels the fly larvse to turn their ventral 

 surfaces toward the source of light, in which condition 

 they are indifferent to the orientation of their median planes 

 toward the rays of light. The ventral position is assumed 

 only when the animals are exposed to light. With this, 

 however, the striking features of the movements of orien- 

 tation in fly larvae are by no means exhausted. While the 

 movements of orientation in all the other animals go on 



