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 heliotropism was entirely pushed 
aside again by another form of irritability, frequently 
encountered in the animal kingdom, and to which I shall 
soon return. 
Fly larvee also possess very different forms of heliotropic 
irritability at different epochs in their existence. Negative 
heliotropism is not very distinct in the newly hatched larve; 
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 
larve, 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 réle 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 heliotropism 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 
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