It is not without interest to find that caterpillars exhibit some of the same reaction shown by butterflies. Loeb has made numerous experiments with the caterpillars of _Porthesia chrysorrhœa_. The caterpillars of this moth collect together in the autumn and spin a web or nest in which they pass the winter. If they are taken from the nest and brought into a warm room, they will orientate themselves to the light, and also crawl toward it. If 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 caterpillars 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 degrees C., and above this temperature point they become restless and wander about.