HELIOTROPISM OF ANIMALS 41 



first two or three days if the animals are kept in the dark. 

 Under these circumstances the moths become restless at the 

 usual time. Reaumur showed that moths begin to fly in the 

 evening when kept in a box. I must leave it undecided for 

 the present whether this periodicity finally disappears if the 

 animals are kept still longer in the dark. I have tried 

 repeatedly to cause Sphinx euphorbia to fly in the daytime 

 by a sudden diminution in the intensity of the light. When 

 I protected the animals from all jarring I never succeeded 

 between 6 and 12 o 'clock in the morning. Yet I was easily 

 successful in the afternoon, long before the beginning of 

 twilight. I will cite here several of my experiments. One 

 morning I placed a Sphinx euphorbias, which had begun to 

 fly at 9 o'clock on the previous evening, on the window cur- 

 tain, where it remained quietly. At 2:45 I returned it to its 

 glass box, which stood in a dark corner and into which light 

 fell only through a narrow slit. An hour went by, but the 

 animal did not leave its place. It then moved to the light 

 side of the box, without flying. I carried the animal back to 

 the window, where it remained quietly. After twenty min- 

 utes I returned it again to the dark box. Half an hour later, 

 at half -past 4, it finally began to fly. 



The next day I allowed it to remain at rest near the win- 

 dow, and it did not begin to fly until 9 P. M. at well-advanced 

 twilight. On the following day I kept it in the dark box, 

 and at half -past 3 in the afternoon it had already begun to 

 fly. At noon on the succeeding day a heavy storm came up 

 and it grew quite dark. The moth, which until then had 

 remained quietly at the window, began to fly. I have had 

 the same experience with other examples of this species. 

 These facts seem to indicate that it is possible to influence 

 the time of waking of Sphinx euphorbise by diminishing the 

 intensity of the light, but only when they would soon wake 

 up without artificial interference. 



