Animal Instincts and Tropisms 281 



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 suffi- 

 ciently.) After leaving the nest, they crawl directly 

 upward on the shrub where they find the leaves on 

 which they feed. Should the caterpillars 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 a human being 

 might envy the little larva ? Is it a dim recollection of 

 experiences of former generations? It can be shown 

 that it is the light reflected from the sky which guides 

 the animal upward. When we put these animals into a 

 horizontal test-tube in a room, they all crawl toward 

 the window, or toward a lamp ; the animal is positively 

 heliotropic. It is this positive heliotropism which 

 makes them move upward where they find their food, 

 when the mild air of the spring calls them forth from 

 their nest. At the top of the branch, they come in 

 contact with a leaf, and chemical or tactile influences 

 set the mandibles of the young caterpillar into activity. 

 If we put these larvae 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 stay and starve, even if their favourite leaves 

 are close behind them. They are slaves of the light. 



