HELIOTROPISM 1 29 



Copepods and other animals, under certain conditions, as we shall 

 see later. For the negatively heliotropic animals the rule holds also, 

 that the blue rays are more effective for hehotropic reactions than the 

 red rays. It is easy to show that these animals move away from the 

 source of light in the direction of the rays of hght. The fact can be 

 demonstrated nicely in the case of the fully grown larvae of the fly, by 

 compelling them to move on a table on which strong light, e.g. direct 

 sunlight, falls. If a shadow is thrown on the table by means of a pencil, 

 it will be found that the larvae move parallel with the shadow, away 

 from the source of light. It can also be readily demonstrated that 

 these animals are not, as the anthropomorphists would probably state, 

 afraid of the Hght, or fond of darkness, but they are purely negatively 

 heliotropic, which means that the head is bent or turned away auto- 

 matically by the light from the source of light, exactly like the tip of a 

 negatively heliotropic root. This can be shown by putting the larvae 

 on a table which has been placed near the window in such a position 

 that the half of the table which is nearer the window is struck by diffused 

 daylight, the other half by the direct sunlight. If the animals, at the 

 beginning of the experiment, are at the window side of the table in the 

 shade, their heads, under the influence of the light, will be machanically 

 bent away from the window, and all the animals will begin to move 

 in the direction of the rays of light. They go from the shade into the 

 sunlight. I have modified this experiment by putting the larvae in 

 long glass tubes, one end of which lay in the shade near the window, 

 the other in direct sunUght. The animals went from the shade on the 

 room side of the tube into the end which was in the direct sunhght; 

 here they remained permanently, notwithstanding the fact that they 

 soon died from the effect of the sunhght (or the heat?). When the 

 animal is struck by hght on one side only, those muscles which turn 

 the head away from the source of hght contract more strongly than 

 their antagonists ; the consequence is a bending of the head away from 

 the light. As soon as the symmetrical points of the photosensitive 

 surface of the animals are struck by the hght at the same angle, the 

 tension of the photosensitive surface becomes equal ; hence the animals 

 remain in this orientation. If they move, their locomotions will there- 

 fore occur in the direction of the rays of hght, and away from the source 

 of hght. 



We mentioned before that the laws of hehotropic curvature in plants 

 can be successfully demonstrated in animals. We may add that the 

 hehotropic motions of animals to and from the hght can be demonstrated 

 in free-moving plants. Under certain conditions, which are not yet 

 completely analyzed, the cells of algae are transformed into smaller 



