222 The Mechanistic Conception of Life 



sense and intensity of the heliotropism of these animals. It 

 is of course immaterial for the result, whether the carbon 

 dioxide or any other acid diffuse into the animal from the out- 

 side or whether they are produced inside in the tissue-cells of 

 the animals. Davenport and Cannon found that Daphniae, 

 which at the beginning of the experiment react sluggishly to 

 light, react much more quickly after they have been made to 

 go to the light a few times. The writer is inclined to attribute 

 this result to the effect of acids, e.g., carbon dioxide, produced 

 in the animals themselves in consequence of their motion. 

 A similar effect of the acids was shown by A. D. Waller in the 

 case of the response of a nerve to stimuli. 



The writer observed many years ago that winged male and 

 female ants are positively heliotropic and that their heliotropic 

 sensitiveness increases and reaches its maximum toward the 

 period of nuptial flight. Since the workers show no heliotropism 

 it looks as if an internal secretion from the sexual glands were 

 the cause of their hehotropic sensitiveness. V. Kellogg has 

 observed that bees also become intensely positively heliotropic 

 at the period of their wedding flight, in fact so much so that by 

 letting light fall into the observation hive from above, the bees 

 are prevented from leaving the hive through the exit at the 

 lower end. 



We notice also the reverse phenomenon, namely, that 

 chemical changes produced in the animal destroy its heli- 

 otropism. The caterpillars of Porthesia chrysorrhoea are very 

 strongly positively heliotropic when they are first aroused from 

 their winter sleep. This heliotropic sensitiveness lasts only as 

 long as they are not fed. If they are kept permanently without 

 food they remain permanently positively heliotropic until 

 they die from starvation. It is to be inferred that as soon as 

 these animals take up food, the formation of a substance or 

 substances in their bodies takes place, diminishing or annihilat- 

 ing their heliotropic sensitiveness. 



