390 Evolution and Adaptation 
placed in a tube, they crawl to the upper side of the glass 
and then along this side toward the light. If a covering is 
placed over the end of the tube that is turned toward the 
window, the caterpillars will crawl only as far as the edge 
of the cloth. They also react negatively to gravity. If 
kept in a dark room, they will crawl upward to the top of 
the receptacle in which they are enclosed. If subjected 
to the influences of both light and gravity, they respond 
more strongly to the light. The caterpillars also show a 
contact reaction. They tend to collect on convex sides or 
on corners and angles of solid bodies. They may even pile 
up one on top of the other in response to this reaction; the 
convex side of a quiescent animal acting on another animal 
crawling over it as any convex surface would do and holding 
the animal fast. 
These three kinds of reactions determine the instincts of 
these caterpillars. In the spring, when they become warm, 
they leave the nest. Positive heliotropism and negative 
geotropism compel them to crawl upward to the tops of the 
branches of the trees, and there the contact reaction with the 
small buds holds them fast in this place. That they are not 
attracted to the end of the branches by the food that they 
find there is shown by placing buds in the bottom of the 
tubes in which the caterpillars are contained. The caterpillars 
remain at the top of the tube, although food is within easy 
reach. If, however, they are placed directly on the buds, the 
contact reaction will hold them there, and they will not crawl 
farther upward. Curiously enough, as soon as the cater- 
pillars have fed and the time for shedding approaches, the 
responsiveness to light and to gravity decreases, and at the 
time of shedding they do not respond at all to these agents. 
These same caterpillars react also to warmth above a certain 
point. In a dark tube placed near a stove, the caterpillars 
collect at the end farthest away from the source of the heat. 
They react to light best at a temperature between 20 and 30 
