HELIOTROPISM OF ANIMALS 37 



perature for demonstrating their heliotropic activity lies 

 between 20 and 30. 



The experiments on the caterpillars of Porthesia chry- 

 sorrhcea are typical. I have repeated them on some hundred 

 species of insects, but I have never found a positively 

 heliotropic insect whose dependence upon light was of a 

 different kind from that found in Chrysorrhoea. This 

 fact has given me the impression that all animal proto- 

 plasm, as perhaps all plant protoplasm, is heliotropically 

 irritable, and that where this is apparently not the case 

 the heliotropic reaction is inhibited, either temporarily or 

 permanently, by other causes. For this reason it would 

 be useless to publish here every single experiment I have 

 made. This would result in repeating each time the same 

 phenomena, only under the name of a different insect. 

 Since there are only negatively and positively heliotropic 

 animals, it would be of secondary interest to know to which 

 of the two classes the individual animals belong. But I 

 believe it necessary to show by concrete examples what part 

 heliotropism plays in the habits and ecology of animals. 



VI. THE POSITIVE HELIOTROPISM AND THE SLEEP OF 

 BUTTERFLIES 



Our knowledge of the behavior of butterflies toward 

 light has, on the whole, remained at that point which is 

 marked by the statement of Keaumur that "it- is a singular 

 fact that those butterflies which shun the daylight are pre- 

 cisely those which fly into lighted chambers." The paradox 

 has not yet been explained why those butterflies which are 

 not to be seen by day fly into the flame at night, while the 

 day butterflies apparently do not possess the tragic "instinct" 

 of the night Lepidoptera. There is no lack of conjecture 

 on this point. Eomanes believes that the lamp is a "strange 

 object" to the moths, and that "the desire to examine this 



