388 Evolution and Adaptation 



that of butterflies, for the light of the evening to which the 

 moth reacts is less than the minimal to which the butterfly 

 responds. 



Moths appear to pass into a sort of sleep during the day, 

 while butterflies are quiescent only at night. The periodicity 

 of the sleeping time continues, at least for several days, 

 when the insects are kept in the dark. For instance, moths 

 kept in the dark become restless as the evening approaches, 

 as Reaumur observed long ago. It has been found in plants 

 that this sort of periodicity may continue for several days, 

 but gradually disappears if the plants are kept in the dark. 

 By using artificial light, and exposing the plants to it during 

 the night, and putting them in the dark during the day, a new 

 periodicity, alternating with the former one, may be induced '; 

 and this will continue for some days if the plants are then 

 kept continually in the dark. 



Loeb tried the experiment of exposing the quiescent moths 

 suddenly to a lower intensity of light, in order to see if they 

 would respond equally well at any time of day. It was found 

 that if the change was made in the forenoon, between six 

 o'clock and noon, it was not possible to awaken the moths by 

 a sudden decrease in the intensity of the light. But it was 

 possible to do so in the afternoon, long before the appearance 

 of dusk. It appears, therefore, that in this species, Sphinx 

 euphorbia, it is possible to influence the period of awaken- 

 ing by decreasing the intensity of light, but this can be done 

 only near the natural period of awakening. It seems to me 

 that this awaking of a positively heliotropic animal by decreas- 

 ing the light needs to be further investigated. 



The day butterflies are also positively heliotropic. Butter- 

 flies of the species Papilio machaon, that have been raised 

 from the pupa, remain quietly on the window in the diffuse 

 daylight of a bright day. They can be carried around on 

 the finger without leaving it, but the moment they come into 

 the direct rays of the sun they fly away. 



