HELIOTROPISM 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. It 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 Amphipyre 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 intocrevices depends upon the contact-irritability, which 
was mentioned before. The following experiments always 
succeeded: In the evening, when a lamp was brought into 
Digitized by Microsoft® 
