Page:Popular Science Monthly Volume 79.djvu/118

114 light, they move at once to the side of the watch crystal nearest to the window. They were, therefore, pronouncedly positively heliotropic before they came under the influence of the light.

In experiments with winged aphids I often found that after having gone through the heliotropic reactions a few times they react much more quickly to light than at the beginning. This might be interpreted as a case of "learning." In so far as it is not a case of a lessening of the stickiness of the feet or the removal of some other purely mechanical factor which retards the rate of movement, it may be brought about by the carbonic or lactic acids produced through the muscular activity.

As far back as twenty years ago I pointed out that the photo-sensitiveness of an animal is different in different physiological conditions and that, therefore, under natural conditions, heliotropism is found often only in certain developmental stages, or in certain physiological states of an animal. I have already mentioned that in the aphids distinct heliotropic reactions may only be expected when the animals have developed wings and left the plant. The influence of the chemical changes which take place in animals upon heliotropism is much more distinct in the larvæ of Porthesia chrysorrhæa. The larvæ hatch from the eggs in the fall and, as young larvæ, hibernate in a nest. The rising temperature in the spring drives them out of the nest and they can be driven out of the nest in winter also by an increase in temperature. When they are driven out of the nest in this condition they are strongly positively heliotropic and I have never found in natural surroundings any animals whose heliotropic sensitiveness was more pronounced than it is in the young larvæ of Chrysorrhea under these conditions. But as soon as the animals have once eaten the positive heliotropism disappears and does not return if they are again allowed to become hungry. In this case it is clear that the chemical changes connected with nutrition directly or indirectly lead to a permanent diminution or disappearance of the photochemical reaction. In ants and bees the influence of substances from the sexual organs seems to be the determining factor in the production of positive heliotropism. The ant workers show no heliotropic reactions while in the males and females, at the time of sexual maturity, a distinct positive heliotropism develops, the intensity of which continues to increase.