Another factor that plays an important rôle in the habits of the earthworm is the response to contact,—the so-called stereotropism. If, in crawling over a flat surface, the worm comes in contact with a crevice, it will crawl along it, and refuse to leave until the end is reached. The contact holds the worm as strongly as though it were actually pulled into the crevice. It can be forced to leave a crevice only by strong sunlight, and then it does not do so at once. If the worm crawls into a small glass tube, it is also held there by its response to contact, and the smaller the tube, the more difficult is it to make the worm leave by throwing strong sunlight upon it. Loeb has found that when winged aphids, the sexual forms, are collected in a tube, and the tube is kept in a room, the aphids crawl toward the light. This happens in ordinary diffuse light, as well as in lamplight. It is stated that the animals orientate themselves towards the light more quickly when it is strong than when it is weak. They turn their bodies toward the light, and then move forward in the direction from which the rays come. It can be shown by a simple experiment that the aphids are turned by the _direction_ of the light, and not by its intensity. If they are placed in a tube, and the tube laid obliquely before a window in such a way that the direct sunlight falls only on the inner end of the tube, the aphids will, if started at the inner end of the tube, first crawl toward the outer surface of the tube, and then wander along this wall, passing out of the region of sunlight into the end of the tube nearest the window, where they come to rest at the end. They have moved constantly towards the direction from which the rays come, passing, as it were, from ray to ray, but each time toward a ray nearer the source of the light. If the tube be turned toward the window, and the window end be covered with blue glass, the aphids crawl into this end of the tube, as they would have done had the tube been uncovered. If, on the other hand, the end of the tube be covered with red glass, they do not crawl into the part of the tube that is covered, unless they are very sensitive to light. Even in the latter case they may remain scattered in the red part, and do not all accumulate at the end, as they do when blue glass is used. In other words, while they respond to blue as they do to ordinary light, they behave toward red as they do towards a very faint light. In diffuse daylight the aphids, as has been said, crawl toward the light, but if they come suddenly into the sunlight they begin to fly. Thus they remain on the food-plant until the sun strikes it, and then they fly away.