162 TROPISMS 



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 mak- 

 ing 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 down- 

 ward. It can be shown that a caterpillar after having 

 been fed loses its positive heliotropism almost completely 

 and permanently. If we submit 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 ani- 

 mal 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 downward — which is the only direction open to it — 

 where it finds new young leaves on which it can feed. The 

 wonderful hereditary instinct upon which the life of the 

 animal depends is its positive heliotropism in the unfed 

 condition and the loss of this heliotropism after having 

 eaten. The chemical changes following the taking up 

 of the food abolish the heliotropism just as C0 2 arouses 

 positive heliotropism in certain Daphnia. 



Mayer and Soule have shown that negative geotropism 

 and positive heliotropism keep the caterpillars of Banais 

 plexippus on its plant (the milk-weed). The chemical 



