84 STUDIES IN GENERAL PHYSIOLOGY 
borer, which live in the dark, are positively heliotropic with- 
out deriving any benefit from this form of irritability. 
VI. There is one form of irritability widely distributed 
throughout the animal kingdom, which has been studied but 
little, and which can easily be confounded with negative 
heliotropism. It consists in many animals being compelled 
to orient their bodies against the surfaces of other solid 
bodies in a certain way, or bringing their bodies in contact 
with other solid bodies on as many sides as possible (stere- 
otropism). Certain animals seek only the concave corners 
and edges of boxes (Forficula auricularia, ants, Amphipyra, 
the larvee of Musca vomitoria, etc.); while others fasten 
themselves only to the convex edges and corners (caterpillars 
of Porthesia chrysorrheea). 
VII. A non-luminous source of heat may influence the 
orientation, but generally it is not able to prescribe the direc- 
tion of the progressive movements of animals. In this way 
it happens that animals which move away from a source of 
heat may be forced by the light to move from diffuse light 
into sunlight, and to remain exposed to the high temperature 
of the sunlight, even though this may cause their death. 
The influence of a non-luminous source of heat can best be 
compared to the influence of a weak source of light, which is 
just sufficient to hinder a negatively heliotropic animal from 
going toward the source of light, but is not sufficient to force 
the animal to move accurately in the direction of the rays. 
We have yet to draw a conclusion from the results of these 
experiments, which could not be formulated until now. We 
have seen that the heliotropic movements of animals possess- 
ing a nervous system are determined in all respects by the 
same external conditions and depend in the same way on the 
external form of the body as do the heliotropic movements 
of plants, which have no nervous system. These heliotropic 
phenomena cannot therefore depend upon specific character- 
istics of the central nervous system. 
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