INSTINCTS 161 



quently lay their eggs on material on which the larvae 

 cannot thrive would be liable to die out. 



As an illustration of the role of tropisms in the instinc- 

 tive self-preservation the writer wishes to apologize for 

 selecting an example which he has used so often in pre- 

 vious discussions, namely the role of heliotropism in the 

 preservation of the life of the caterpillars of Porthesia 

 chrysorrhcea. 28 ' 1 This butterfly lays its eggs upon a shrub, 

 on which the larvae hatch in the fall and on which they 

 hibernate, as a rule, not far from the ground. As soon 

 as the temperature reaches a certain height, they leave the 

 nest ; under natural conditions this happens in the spring 

 when the first leaves have begun to form on the shrub. 

 (The larvae can, however, be induced to leave the nest at 

 any time in the winter, provided the temperature is raised 

 sufficiently). After leaving the nest, they crawl directly 

 upward on the shrub where they find the leaves on which 

 they feed. If the caterpillars should move down the shrub 

 they would starve, but this they never do, always crawl- 

 ing upward to where they find their food. What gives 

 the caterpillar this never-failing certainty which saves 

 its life and for which the human being might envy the 

 little larva? Is it a dim recollection of experiences of 

 former generations, as Samuel Butler would have us 

 believe ? It can be shown that this instinct is merely posi- 

 tive heliotropism and that the light reflected from the sky 

 guides the animals upward. The caterpillars upon waking 

 from their winter sleep are violently positively heliotropic, 

 and it is this heliotropism which makes the animals move 

 upward. At the top of the branch they come in contact 

 with a growing bud and chemical and tactile influences set 

 the mandibles of the young caterpillar into activity. If we 

 put these caterpillars into closed test tubes which lie 

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