39° 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 



