If this be so, one conceives that, aside from light, phototropism may be modified by all the factors, either external or internal, which are capable of acting on the photochemical substances of the organism. This is shown, in fact, by observation and experiment.
If a too strong light can suppress phototropism, movements forced upon the animals can make it change in character. The maggots of the green blow-fly are normally, as we have seen, negatively phototropic, but Pouchet has shown that their phototropism becomes positive when one moves them previously in a box. The pelagic copepods, notably Temora longicornis, direct themselves toward the light when they have just been captured and almost all of them move away immediately afterward. But, as Loeb's experiments have shown, an agitation brings back to positive phototropism those which had become negatively phototropic.
Following up his experiments, Loeb has found that these same copepods are very sensitive to changes in temperature. Should this pass a certain point, the phototropism of positive individuals changes rapidly to negative phototropism, which increases with the temperature. If it lowers, on the contrary, below a certain limit the reverse phenomenon occurs.
Hindering the movements leads sometimes to the same result as a mechanical agitation. So it is at least, according to Ostwald (1907), with the little fresh-water Crustacea of the genus Daphnia, known popularly as water-fleas. Indifferent to normal light or having a feeble negative phototropism, these organisms acquire an intense positive phototropism when one increases the viscosity of the water in which they move, by adding a little gelatine.
In the last experiment, as in that where mechanical agitation was used, there is evidently put in play an internal factor, the locomotive activity, which becomes greater from the opposition of the displacement or viscosity of the water or the contractions of the animal. But one of the results of the muscular activity is to render the internal reactions stronger and especially to increase the production of carbonic acid; and we have seen how this gas acts as a sensitizer in phototropic phenomena. In this way can be explained the change of phototropism in Ostwald's experiment and in those produced by mechanical agitation.
But locomotive activity is only one of the forms of work of the body; the organism develops others quite as important from the point of view of the modifications which they provoke, and which, like locomotion, powerfully affect phototropism.
This is especially true of phenomena of nutrition and of reproduction.
The experiments of Loeb on the young caterpillars of the Brown-tail Moth (Euproctis chry-sorrhcea) show very plainly the influence of the nutritive phenomena upon phototropism. Loeb says:
These caterpillars hatch in the autumn and pass the winter in their webs; in the spring, and even in the winter when the temperature is raised, they come out, influenced by the warmth. They then show a very perfect positive phototropism, and I have never found under natural conditions any animal with a more pronounced heliotropism. But, as soon as they have eaten, their positive heliotropism disappears and reappears no more even when they fast again. It is evident that the chemical changes connected with nutrition have acted, directly or indirectly, as an inhibition or a definite suppression of the photochemical reactions which the insect formerly possesed.
It is probably to chemical phenomena dependent on nutrition that we must attribute the change of character of phototropism in the evolution of the individual.
Loeb has shown that larvae of Limulus polyphe-mus and of Balanus are positively phototropic immediately after birth and that soon afterward they are changed in character. We have seen that larval lobsters act in exactly the same way. We know, however, from the experiments of Pouchet that maggots do not react to the light when just hatched and that their negative phototropism develops with their growth.
The influence of the phenomena of reproduction on phototropic sensitiveness is especially shown with the fertile individuals of most ants and of the domesticated honey-bee. With these insects the fecundated females, or queens, pass their whole existence in the obscurity of the nest, where their role is to lay eggs almost continuously. But at the moment when they shed the nymphal skin and acquire sexual maturity they are positively phototropic and leave the nest, to dart toward the light. This flight coincides always with the sexual maturity of the males, which, having the same phototropism, and, guided by the odor, hasten in pursuit of the females. Such is the origin of the nuptial flight of the honey-bee and of the swarming of the winged ants,-that is to say, of the sexual individuals. With these insects, as with all animals, there are found within the organism, at the moment of sexual maturity, special products which modify the physiological condition of individuals; and, according to Loeb, it surely is to the action of these products on the photosensitive substances that we must attribute the development of phototropism.