In the presence of two sources of light, says Georges Bohn, it orients itself toward neither the one nor the other, but in an intermediate direction, so that the two sides of its body receive the same light. It is incapable of resisting the luminous reaction and one can point out in advance how it will behave during the reaction. This is pure automatism. As Loeb has shown by many experiments, the phototropic phenomena of animals differ not at all from those of plants, and they carry no more psychism in a polyp or in a pyralis than in a geranium or in a volvox.
It is not, then, from curiosity, as is often said, that certain animals come to the light, and Pouchet is surely self-deceived when he attributes the retreat of the maggots before the light to a dazzled condition. Phototropic creatures are neither drawn toward the light nor repulsed by it; they go or they come by a simple physical mechanism. The fly larvae, says Loeb, are no more frightened by the light than the negatively phototropic end of a tree branch. He adds:
One can show this by putting the larvae on a table, near the window, in such a position that the half of the table nearest the window is lighted by the diffused daylight and the other half by the direct rays of the sun. If at the beginning of the experiment the animals are in the shade, on the window side, they will mechanically bend their heads to leave the influence of the diffuse light and will begin to move in the direction of the light rays. They will also depart from the window, the source of light, which will lead them "from the shade into the sunlight."
Occurring among the plants and among the lower animals, as well as among those in which eyes are well developed, phototropism is not bound up with the existence of eyes, which serve only as propeller centers when they exist. At times, as in fly larvae, eyes are totally lacking, but then the phototropic sensitiveness is localized in the anterior region of the body in the cerebral centers, which is truly an inheritance, since the insects with blind larvae have developed from more primitive forms whose larvae possessed sight. In all cases the sensitive articulates usually orient themselves according to the luminous source, so that the front end of the body (that which carries or should carry the eyes) faces the source or is opposite to it according as the phototropism is positive or negative. This rule is very general, and probably constant. Georges Bohn (1905) showed that the larvae of the European lobster are endowed with positive phototropism on hatching and become negatively phototropic; but the larvae always hold themselves with their eyes away from the light, so that they direct themselves toward the source or depart from it swimming backward. According to the researches of Hadley, the larvae of the American lobster act in exactly the same way.