Volume 1

Studies in general physiology / by Jacques Loeb.

  • Jacques Loeb
Date:
1905
Catalogue details

Licence: Public Domain Mark

Credit: Studies in general physiology / by Jacques Loeb. Source: Wellcome Collection.

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    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 larvm also possess very different forms of heliotropic irritability at different epochs in their existence. Negative heliotropism 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 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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