Gravity acts on the protoplasm by its tendency to separate heavy substances from less dense elements. But, contrary to the ideas of Verworn, Davenport does not believe that this action is purely mechanical. It acts by stimulation in reducing the friction with the milieu in the species which follow its direction, increasing with those which act in an opposite manner. However it may be, it provokes in organisms a directive orientation which is currently called geotropism.
Geotropism manifests itself powerfully with plants, where it determines the form and manner of growth. It is positive for the action of roots, which it buries in the soil, negative for the main shoot, which it directs toward the sky. Well developed, also, in fixed animals, it is also seen with insects where one can easily demonstrate it by a very simple experiment devised by Loeb. Certain Coleoptera, coccinellids (or ladybirds) are placed in a closed box which has a dark compartment, in order to avoid all influence of fight. The insects cling to the walls of their prison, and when the box is opened they are fonnd grouped at the highest points; their geotropism is negative. A geotropism of the same kind is common with many flying insects, notably with certain flies. Wheeler points out in this connection (1899) two Dolichopodids of the United States (Neurigona superbiens and Medeterus veles). Resting, or walking on a tree, he says, "these insects always hold the head directed upward, the main axis perpendicular to the ground and parallel to the long axis of the trunk. When they are disturbed, they fly away, but soon return, nearer the ground, and begin again to direct themselves upward whatever may be the condition of the light." It is easy to make similar observations in our country. When the physiological condition is modified by maturing, the geotropism of the insect may appear to become modified. Thus it is, according to Fab re, with the processionary caterpillars of the pine. In the month of March, when they are ready to transform, all leave the silken nest where they have lived together and proceed, more or less directly, to the ground, which they enter to transform to pupae. They are, then, positively geotropic.