Page:Popular Science Monthly Volume 79.djvu/119

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THE SIGNIFICANCE OF TROPISMS

According to Kellogg the case of the bees is similar. It is a well known fact that during sexual maturity special substances are formed which influence various organs. For instance, Leo Loeb has found that the substances which are set free by the bursting of an egg follicle cause a special sensitiveness in the non-pregnant uterus, so that every mechanical stimulus causes the latter to form a decidua. In this way he could cause the formation of any number of deciduæ in non-pregnant uteri, while without the follicle substance the uterus did not react in this manner.

It is a common phenomenon that animals in certain larval stages are positively heliotropic, while in others they are not sensitive to light or are even negatively heliotropic. In order to save time I will not now discuss further these facts which are easily comprehensible in the light of what has been said and I refer the readers to my earlier papers.

This change in the heliotropic sensitiveness, produced by certain metabolic products in the animal body is of great biological significance. I have already shown that it even serves to save the lives of the above-mentioned young larvæ of Chrysorrhœa. When the young larvæ are awakened from their winter sleep by the spring sunshine they are actively positively heliotropic. The positive heliotropism leaves them no freedom of movement, but forces them to creep (eindeutig) straight upward to the top of a tree or branch. Here they find the first buds. In this way the heliotropism guides them to their food. Should they now remain positively heliotropic they would be held fast on the ends of the twigs and would starve to death. But we have already mentioned that after they have eaten they lose the positive heliotropism once more. They can now creep downwards, and the restlessness which is characteristic of so many animals[1] forces them to creep downwards until they reach a new leaf, the odor or tactile stimulus of which stops the progressive movement of the machine and sets in motion further eating activity.

The fact that ants and bees become positively heliotropic at the time of sexual maturity plays an important rôle in the vital economy of these creatures. As is well known, the mating of these insects takes place during flights, the so-called nuptial flight. Now I have watched and found that among the male and female ants of a single nest the heliotropic sensitiveness increases steadily up to the time of the nuptial flights and that the direction of their nuptial flights follows the direction of the rays of the sun in the afternoon. I gained the impression that this nuptial flight is merely the consequence of a very highly developed heliotropic sensitiveness. The case must be similar among the bees according to the following experiment described by Kellogg. The

  1. The physico-chemical cause of this "restlessness" which is noticeable in many insects and crustaceans is at present unknown.