The tonus of the muscles of heliotropic insects is due chiefly to the action of light; it is markedly decreased in the dark. Each eye controls the tonus of a different group of muscles on both sides of the body. Different areas of each eye likewise are related to the tonus of different muscle groups, and the relationship is entirely analogous to that of the otic labyrinth of vertebrates. Asymmetrical conditions of muscle tension are produced by any procedure which establishes an unequal photochemical reaction in the two eyes, by difference in illumination, by partial or complete blackening of one eye, or establishing unequal sensitiveness in the two eyes. The unbalanced condition of muscle tonus expresses itself in unusual postures of the resting insects, and in movements in forced paths—circus motions when one eye has been blackened. These reactions vary directly with the intensity of the illumination, as shown not only by the variation in diameters of the circles, but also by the reactions of the insects on vertical surfaces and on the turntable.
The relation of the results of these experiments to the problem of heliotropic orientation is too obvious to require detailed discussion, which could only lead to a repetition of the description of the mechanism f heliotropism which Loeb has so clearly expounded. The experiments are so completely in accordance with Loeb's muscle tension theory of heliotropism, that they are tantamount to a complete proof of it.