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 larvae of Musca vomitoria, etc.); while others fasten 

 themselves only to the convex edges and corners (caterpillars 

 of Porthesia chrysorrhoea). 



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. 



