HELIOTKOPISM OF ANIMALS 21 



of contact-irritability, which, so far as I know, has not yet 

 been recognized. Many insects are compelled to bring their 

 bodies in contact with the surfaces of solid bodies in a very 

 definite way. My attention was called to this phenomenon 

 in my experiments on animal geotropism, in which I allowed 

 the animals to move about on geometrically simple bodies 

 bounded by plane surfaces. I noticed that the animals 

 rarely remained on the plane surfaces, but collected about 

 the edges, particularly the vertical ones. // is worthy of note 

 that certain animals always seek the concavity of the angle 

 between the sides of hollow cubes, while others just as con- 

 stantly move on the convex side. The caterpillar of Por- 

 thesia chrysorrhcea is an example of the latter type. The 

 other form of this contact-irritability, which leads the ani- 

 mals to the concavity of the angles, is very common. The 

 following observations show how this form of irritability 

 might easily be confused with the irritability toward light, 

 and so lead to a misconception of the behavior of the animal 

 toward light. 



I studied for several weeks a large number of moths of 

 the species Amphipyra. The animals are remarkable in 

 that they are more given to running than to flying. The 

 rapidity of their running movements calls to mind the lively 

 movements of cockroaches and ants. While formerly I had 

 found that all butterflies are positively heliotropic, I observed 

 that Amphipyrse when let loose, did not fly to the window, 

 but to the nearest wall or to the floor, where they ran about 

 nimbly and crept under the first suitable object, like cock- 

 roaches. This looked as though the animals fled from the 

 source of light. Yet it could be shown that the animals 

 move toward a source of light, and that the inclination to 

 creep into crevices depends upon the contact-irritability, which 

 was mentioned before. The following experiments always 

 succeeded : In the evening, when a lamp was brought into 



