ii4 THE POPULAR SCIENCE MONTHLY 



light, they move at once to the side of the watch crystal nearest to the 

 window. They were, therefore, pronouncedly positively heliotropic be- 

 fore 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 inter- 

 preted as a case of " learning." In so far as it is not a case of a lessen- 

 ing of the stickiness of the feet or the removal of some other purely me- 

 chanical factor which retards the rate of movement, it may be brought 

 about by the carbonic or lactic acids produced through the muscular 

 activity. 10 



V 



As far back as twenty years ago I pointed out that the photo-sensi- 

 tiveness 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 dis- 

 tinct 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 larvae of Porthesia chrysorrlicea. The larvae hatch from 

 the eggs in the fall and, as young larvas, 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 tempera- 

 ture. 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 larva? 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. 11 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. 



10 The phenomenon of "steps" ("Treppe") upon stimulation of a muscle 

 is ascribed, probably rightly, also to the formation of acid. The phenomenon 

 of "steps," that is, the increase of the amount of contraction with every new 

 stimulus is, however, comparable to or identical with the increase in the rate of 

 reactions in the experiments described here. 



11 Loeb, I. c, p. 24. (This latter fact has been overlooked by several writers.) 



