Significance of Tropisms for Psychology 47 



V 



As far back as 1889 I pointod out that the photosonsitivo- 

 ness of an animal is different in different physiological conditions 

 and that, therefore, under natural conditions, heliotropisin 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 have left 

 the plant. The influence of the chemical cliangcs which take 

 place in animals upon heliotropism is much more distinct in 

 the larvae of Porthesia chrysorrhoea. The larvae hatch from 

 the eggs in the fall and, as young larvae, hibernate in a nest. 

 The rising temperature in the spring drives them out of the 

 nest, and they can also be driven out of the nest in winter l)y 

 an increase in temperature. When driven out of the nest in 

 this condition they are strongly positively heliotropic and I 

 have never fomid in natural surroundings any animals whose 

 heliotropic sensitiveness was more pronounced than it is in the 

 young larvae of Chrysorrhoea. But as soon as the animals 

 have once eaten, the positive heliotropism disappears and does 

 not return even if they are again allowed to become hungry.^ 

 In this case it is clear that the chemical changes directly or 

 indirectly connected with nutrition lead to a permanent dimi- 

 nution 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 heliotroi)ic 

 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. 



It is a well-knowTi fact that during sexual maturity special 

 substances are formed which influence various organs. For 



1 Loeb, op. ciL, p. 24. (This latter fact has Ijoon overlooked by several 

 writers.) 



