The Living Machine 313 



illustrated by the behavior of the caterpillar of the butterfly 

 (Porthesia chrysorrhoaa). "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 crawling 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 recol- 

 lection of experience of former generations, as Samuel Butler 

 would have us believe? It can be shown that this instinct 

 is merely positive 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 ani- 

 mals move upward. At the top of the branch they come in 

 contact with a growing bud and chemical and tactile influ- 

 ences set the mandibles of the young caterpillar into activ- 

 ity. If we put these caterpillars into closed test tubes which 

 lie with their longitudinal axes at right angles to the window 

 they will all migrate to the window end where they will stay 

 and starve, even if we put their favorite leaves into the test 

 tube close behind them. These larvae are in this condition 

 slaves of the light. 



* * The few young leaves on top of a twig are quickly eaten by 

 the caterpillar. The light which saved its life by making it 

 creep upward where it finds its food would cause it to starve 

 could the animal not free itself from the bondage of positive 

 heliotropism. After having eaten it is no longer a slave of 

 light but can and does creep downward. It can be shown 

 that a caterpillar after having been fed loses its positive 

 heliotropism almost completely and permanently. If we sub- 

 mit unfed and fed caterpillars of the same nest to the same 

 artificial or natural source of light in two different test tubes 

 the unfed will creep to the light and stay there until they 

 die, while those that have eaten will pay little or no attention 

 to the light. Their positive heliotropism has disappeared 

 and the animal after having eaten can creep in any direction. 

 The restlessness which accompanies the condition of starva- 

 tion makes the animal leave the top of the branches and creep 



