HELIOTROPISM OF ANIMALS 57 



direction of their progressive movements under the influence 

 of the rays of light is away from the source of light. My 

 description of negative heliotropism need therefore be but 

 brief. I have chosen as an example of negatively heliotropic 

 animals the larvae of Musca vomitoria, which are addi- 

 tionally interesting in that they are completely blind. Helio- 

 tropism in animals is therefore a characteristic of their 

 protoplasm, and not a specific characteristic of their eyes; 

 just as in plants, which have no eyes. 



In order to study the negative heliotropism of Musca 

 larvae it is best to take the almost fully grown larvae fresh 

 from the cadaver on which they were reared. When the 

 light, which may be either diffuse daylight or direct sun- 

 light according to the sensitiveness of the animals, is of the 

 proper intensity, the directing influence of the rays of 

 light can be demonstrated more beautifully in the larvae of 

 the fly than in any other animal. I placed a number of 

 these animals on a horizontal board and exposed them to 

 sunlight. This was at about 4 o'clock in the afternoon, 

 when the rays of light fell obliquely through the window. 

 I shut out that part of the rays which came through the 

 window from above by means of blinds. As soon as the 

 animals came into the sunlight, they were oriented with 

 their oral poles toward the room, and their aboral poles 

 toward the window. They crept with mathematical precision 

 in the direction of the rays. When a shadow was thrown on 

 the board by a penholder, it could be noticed that the 

 animals moved away from the light in a direction exactly 

 parallel to the edge of the shadow. The directing force of the 

 rays was so strong that the animals crept closely along the edge 

 of the shadow without crossing it. They acted as though 

 they were impaled on the ray of light which passed through 

 their median plane. When I turned the board around, the 

 animals immediately turned about also, and again placed 



