In certain species of _Daphnia_, fresh-water copepods, and of _Volvox_, a trace of CO₂ suffices to make negatively heliotropic or indifferent specimens violently positively heliotropic.[242] Certain forms of marine copepods and the larvæ of _Polygordius_ can be made positively heliotropic by lowering the temperature[243] and the larvæ of the barnacle can be made negatively heliotropic by strong light.[244] It is quite possible that a change in the sense of heliotropism by temperature and light is to some extent at least responsible for the periodic depth migrations of heliotropic animals. Many if not all positively heliotropic animals can be made negatively heliotropic by exposure to ultraviolet light.[245] [242] Loeb, J., _Arch. f. d. ges. Physiol._, 1906, cxv., 564. [243] _Ibid._, 1893, liv., 81. [244] Groom, Theo. T., and Loeb, J., _Biol. Centralbl._, 1890, x., 160; Ewald, W. F., _Jour. Exper. Zoöl._, 1912, xiii., 591. [245] Loeb, J., _Arch. f. d. ges. Physiol._, 1906, cxv., 564; Moore, A. R., _Jour. Exper. Zoöl._, 1912, xiii., 573. A most interesting example of the rôle of heliotropism in the preservation of a species is shown in the caterpillars of _Porthesia chrysorrhœa_. The butterfly lays its eggs upon a shrub. The larvæ hatch late in the fall and hibernate in a nest on the shrub, as a rule not far from the ground. As soon as the temperature reaches a certain height, they leave the nest; under natural conditions, this happens in the spring when the first leaves have begun to form on the shrub. (The larvæ can, however, be induced to leave the nest at any time in the winter provided the temperature is raised sufficiently.) After leaving the nest, they crawl directly upward on the shrub where they find the leaves on which they feed. Should the caterpillars move down the shrub, they would starve, but this they never do, always crawling upward to where they find their food. What gives the caterpillar this never-failing certainty which saves its life, and for which a human being might envy the little larva? Is it a dim recollection of experiences of former generations? It can be shown that it is the light reflected from the sky which guides the animal upward. When we put these animals into a horizontal test-tube in a room, they all crawl toward the window, or toward a lamp; the animal is positively heliotropic. It is this positive heliotropism which makes them move upward where they find their food, when the mild air of the spring calls them forth from their nest. At the top of the branch, they come in contact with a leaf, and chemical or tactile influences set the mandibles of the young caterpillar into activity. If we put these larvæ into closed test-tubes which lie with their longitudinal axes at right angles to the window, they will all migrate to the window end, where they stay and starve, even if their favourite leaves are close behind them. They are slaves of the light.