20 STUDIES IN GENERAL PHYSIOLOGY 



tropic at the time of the nuptial flight were practically 

 indifferent toward the light a few days previously. In the 

 same way, later on their heliotropisrn was entirely pushed 

 aside again by another form of irritability, frequently 

 encountered in the animal kingdom, and to which I shall 

 soon return. 



Fly larvae also possess very different forms of heliotropic 

 irritability at different epochs in their existence. Negative 

 heliotropisrn is not very distinct in the newly hatched larvae ; 

 but the animals turn their ventral surfaces toward a suffi- 

 ciently intensive source of light without otherwise being 

 influenced by the direction of the rays of light. Full-grown 

 larvae, however, place their median planes very sharply in the 

 direction of the rays of light, provided the light is suffi- 

 ciently intense. I believe that this periodic appearance of 

 heliotropic irritability plays a great role in the ecology of 

 animals. The periodic migrations of many animals, such as 

 birds of passage, might be explained in this way. 



It is a well-known fact that the irritability of an animal 

 in the larval stage may be entirely opposite in kind to that 

 of the adult stage. This phenomenon is very common. The 

 larva of the fly is negatively heliotropic, while the imago is 

 positively heliotropic; this is also the case with June-bugs 

 and many other animals. I encountered this inversion of 

 the sense of heliotropisrn when the animal changed from 

 the larval stage to the mature state so frequently that 

 for a time I thought it a universal rule. Such, however, is 

 not the case. Caterpillars, for example, behave toward light 

 as does the imago, as I know from my own experience and 

 from what I can find on the subject in the literature. 



The behavior of an animal is determined by the sum 

 of all the forms of its irritability. The heliotropic irrita- 

 bility, therefore, may be obscured by a more powerful irrita- 

 bility of another sort. This is often due to a special kind 



