42 STUDIES IN GENERAL PHYSIOLOGY 



The day butterflies are positively heliotropic like the night 

 butterflies. The only striking feature is that in certain day 

 butterflies the intensity of the light must be very great to 

 bring about heliotropic movements. Specimens of Papilio 

 machaon (which I had raised) remained at rest during the 

 day at a window where they were exposed to the diffuse day- 

 light and could be carried around on the finger; as soon, 

 however, as they were brought into direct sunlight, they flew 

 toward the window in the direction of the rays of light, 

 and this with such force that they dropped down as if stunned. 

 In direct sunlight they pressed themselves closely against 

 the window pane. In diffuse daylight the animals, if they 

 moved at all, crept toward the source of light ; but in direct 

 sunlight they flew toward it. My attempts to attract Papilio 

 machaon by the weak light of a kerosene lamp were unsuc- 

 cessful. 



I will add at this point my general observations on the 

 caterpillars of butterflies. I have not found these periodic 

 variations in heliotropic irritability in most caterpillars, not 

 even those of Sphinx euphorbiae. The caterpillars which 

 I studied reacted to light at all times of the day and night. 

 The caterpillars agree, however, with the day and night 

 butterflies in so far as they are all, without exception, 

 positively heliotropic. 



This positive heliotropism is most marked in the cater- 

 pillar of the willow-borer, which lives in the stems of the 

 willow where it is not at all exposed to light. Such cases 

 are also known in plants. Koots, for instance, are helio- 

 tropically irritable, and yet, as Sachs points out, under nor- 

 mal conditions their heliotropism is of no use to them. 

 They can certainly not have acquired it through natural 

 selection. According to the Darwinian theory, we would 

 expect that the caterpillars of willow-borers should be nega- 

 tively heliotropic, or at least indifferent to light. But the 



