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 larve 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 
larvee it is best to take the almost fully grown larve 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 larve 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 
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