8. Graber had found that when animals are put into a trough covered half with blue and half with red glass, those that are “fond” of light go under the blue, those that are “fond” of darkness go under the red glass. The writer pointed out that this result should be expected on the basis of his theory of helio­tropism, if the assump­tion be correct that the red light is considerably less efficient than light which goes through blue glass (such glass also allows green rays to go through). The botanists had already shown that red glass is impermeable for the rays which cause helio­tropic reac­tions of plants, and the writer was able to show the same for the helio­tropic reac­tions of animals. Red glass acts, therefore, almost like an opaque body for these animals.

A closer examination of the most efficient rays for the helio­tropic reac­tions of different organisms has revealed the fact that for some organisms a region in the blue λ = 460–490 µµ, for others a region in the yellowish-green, near about λ = 520–530 µµ is the most efficient.[236] For many plants and for some animals, like Eudendrium and the larvæ of the worm Arenicola, a region in the blue is most efficient; for certain, if not most, animals a region in the yellow-green is most efficient. Among unicellular green algæ, Chlamydomonas, has its maximal efficiency in the yellowish-green and Euglena in the blue. According to observa­tions by Mast, some green unicellular organisms like Pandorina, Eudorina, and Spondylomorum seem to behave more like Chlamydomonas, while certain others behave more like Euglena.[237] Wasteneys and the writer suggested that there are two groups of helio­tropic substances, one with a maximum of photo­sensitiveness in the blue, the other in the yellowish-green; and that the latter group may or may not be related or identical with the visual purple which is most rapidly bleached by light of a wave length near λ = 520–530 µµ.

The ophthalmologist Hess[238] has utilized the helio­tropic reac­tions of animals in an attempt to prove that all animals from the lowest invertebrates up to the fishes inclusive suffer from total colour-blindness. This statement was based on the observa­tion that for most positively helio­tropic animals the region in the yellowish-green near λ = 520 µµ seems the most efficient. Since this region of the spectrum appears also as the brightest to a totally colour-blind man, he concluded that all these animals are totally colour-blind. There is no reason why helio­tropic reac­tions should be used as an indicator for colour sensa­tions; if totally colour-blind human beings were possessed of an irresistible impulse to run into a flame Hess’s assump­tion might be considered, but no such phenomenon exists in colour-blind man. Moreover, v. Frisch[239] has shown by experi­ments on the influence of the background on the coloura­tion of fish as well as by experi­ments on bees and on Daphnia that the reac­tions of these animals to light of different wave-lengths indicate different effects besides those of mere intensity. Thus v. Frisch could train bees to go to a blue piece of cardboard distributed among many cardboards of different shades of grey. Bees thus trained would alight on any blue object even if it contained no food. It would be impossible to do this with totally colour-blind organisms.

9. Heliotropic reactions play a great rôle in the preserva­tion of individuals as well as of species. In order to understand this rôle it must be stated that the photo­sensitive substances appear often only under certain condi­tions and that their effect is inhibited under other condi­tions. Thus among ants the winged males and females alone show positive helio­tropism,[240] while the wingless workers are free from this reac­tion. This positive helio­tropism becomes violent at the time of the nuptial flight and this phenomenon itself seems to be a helio­tropic phenomenon since it takes place in the direc­tion of the light. When the queen founds her nest she loses her wings and becomes negatively helio­tropic again. Kellogg[241] has shown that the nuptial flight of the bees is also a purely helio­tropic phenomenon. When a part of the hive remote from the entrance is illuminated the bees rush to the light and can thus be prevented from swarming. These phenomena suggest that the presence of some substance secreted by the sex glands may cause the intensifica­tion of the helio­tropism which leads to the nuptial flight.

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]

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.

The few young leaves on top of a twig are quickly eaten by the caterpillar. The light, which saved its life by making it creep upward where it finds food, would cause it to starve could it not free itself from the bondage of positive helio­tropism. The animal, after having eaten, is no longer a slave of the light, but can and does creep downward. It can be shown that a caterpillar, after having been fed, loses its positive helio­tropism almost completely and permanently. If we submit unfed and fed caterpillars of the same nest contained in two different test-tubes to the same artificial or natural source of light, the unfed will creep to the light and stay there until they die, while those that have eaten will pay little or no atten­tion to the light. Their sensitiveness to light has disappeared; after having eaten they become independent of light and can creep in any direc­tion. The restlessness which accompanies the condi­tion of hunger makes the animal creep downward—which is the only direc­tion open to it—where it finds new young leaves on which it can feed. The wonderful hereditary instinct, upon which the life of the animal depends, is its positive helio­tropism in the unfed condi­tion and its loss of this helio­tropism after having eaten. The latter phenomenon is in harmony with the experi­ments which show that the helio­tropism of certain species of Daphnia disappears when the water becomes neutral.

And finally it may be pointed out that the majority of green plants could not exist if their stems were not positively, their roots negatively, helio­tropic. It is the positive helio­tropism which makes the top grow toward the light, which enables the leaves to get the light necessary for assimila­tion, and the roots to grow into the soil where they find the water and nutritive salts.

10. While we do not wish to deal here with the different tropisms it should be stated that aside from helio­tropism, chemotropism as well as stereotropism play the most essential rôle in the so-called instinctive ac­tions of animals. It is a problem of orienta­tion by the diffusion of molecules from a centre when a male butterfly is deviated from its flight and alights on the wooden box in which is enclosed a female of the same species. We have already alluded to certain phenomena of chemotropism in Chapter IV. Certain organisms have a tendency to bring their bodies as much as possible on all sides in contact with solid bodies; thus the butterfly Amphipyra, which is a fast runner, will come to rest under a glass plate when the plate is put high enough above the ground so that it touches the back of the butterfly. The animals which live under stones or underground or in caves are as a rule both negatively helio­tropic and positively stereotropic. Their tropisms predestine or force them into the life they lead.