All animals are not equally sensitive to light; many do not appear to be phototropic, others are very slightly so, and the degree of attraction grows less and less until a good many direct themselves away from the light rays; their phototropism is negative.
This phenomenon was placed in evidence in a masterly way by Georges Pouchet (1872) with the larvae or maggots of the green blow-fly (Lucilia ccesar) and the bluebottle or meat-fly (Calliphora erythrocephala), also with rat-tailed maggots of the drone-fly (Eristalis tenax). Upon a square of black or white paper spread on a horizontal table before a window, the larvae were scattered promiscuously, and all drew away from the daylight. Pouchet also employed artificial lights which he moved about and which always brought about an inverse displacement of the larvae. Loeb gave the experiment a simple and elegant form. "If a shadow," he says, "is thrown upon the table by means of a pencil, the larvae move parallel to the shadow, away from the light." The bedbug and the meal-worms (larvae of Tenebrio molitor) lend themselves equally well to experiments of this kind. I have often made analogous experiments with Peripatus capensis, a curious and very primitive species which is a link between the articulates and the annelids. This wormlike animal is endowed with a very energetic negative phototropism. It directs itself in a straight line away from the light rays, and if one suddenly turns the sheet of paper on which it is walking at an angle of 180 degrees it immediately executes an equal reverse rotation.