THE SIGNIFICANCE OF TROPISMS 115 



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 deciduse in non-preg- 

 nant 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 larvae of Chrysorrhcea. When the young larvae are 

 awakened from their winter sleep by the spring sunshine they are ac- 

 tively 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 men- 

 tioned 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 12 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 role 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 direc- 

 tion 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 de- 

 veloped heliotropic sensitiveness. The case must be similar among the 

 bees according to the following experiment described by Kellogg. The 



"The physico-chemical cause of this "restlessness" which is noticeable in 

 many insects and crustaceans is at present unknown. 



