HELIOTROPISM 51 



animals only. In the case of unequal illumination of the 

 two eyes or of the two sides of the body of a negatively 

 heliotropic animal the tension in the muscles turning the 

 animal to the source of light is diminished. The impulses 

 for locomotion which are equal for the muscles of both 

 sides of the body will, therefore, result in turning the 

 head of the animal away from the source of light. As 

 soon as the plane of symmetry of the animal goes again 

 through the source of light, the symmetrical photosen- 

 sitive elements of the head receive again equal illumina- 

 tion, and the animal will now continue to move in a 

 straight line away from the source of light. The fully 

 grown larvae of the housefly when they are ready to pupate 

 show this negative heliotropism. 



Negatively .heliotropic animals, e.g., the fully grown 

 larva? of the blowfly, can be made to move from weak light 

 to strong light, e.g., from the shade into direct sunlight, 

 if in so doing the illumination on the two sides of the 

 body remains equal. 287 This was shown by the writer 

 by an arrangement similar in principle to the one de- 

 scribed above. Thus the idea that the intensity gradient 

 of light determines the direction of motion was disproved 

 also for negatively heliotropic animals. 



Thus far we have shown only that a heliotropic animal 

 is oriented in such a way to a source of light that its plane 

 of symmetry goes through the source of light. This does 

 not yet explain why a positively heliotropic animal cannot 

 go away from the source of light, since in going to or 

 going away from the source of light both sides of the 

 animal receive equal illumination. The fact that a posi- 

 tively heliotropic animal cannot go away from the light 

 finds its explanation by observations of Holmes 228 and 

 Garrey, 177 showing that when light falls from behind 



