Tropisms and Instincts as Adaptations 387 



The following experiments carried out by Loeb on moths 

 show some of the responses of these insects to light. 



Night-flying moths were placed in a box and exposed in a 

 room to ordinary light. As twilight approached the moths 

 became active and began to fly always toward the window- 

 side of the box. They were positively heliotropic to light of 

 this intensity. If let out of the case, they flew toward the 

 window, where they remained even during the whole of the 

 next day, fully exposed to light. If the moth is disturbed in 

 the daytime, so that it flies, it goes always toward the light, 

 and never away from it. These facts show that the moth is 

 always positively heliotropic, and also that the flight toward 

 the lamp is a natural response, misapplied in this case. 

 That the moths do not fly by day is due to another factor, 

 namely, the alternation in the degree of their sensitiveness at 

 different times. But this condition alone does not seem to 

 account fully for all the facts. 



If the moths are given the alternative of flying toward the 

 evening light, or toward the lamp, they always go toward 

 the brighter light. Thus if, when they swarm at dusk, they 

 are set free in the middle of the room, at the back of which 

 a lamp is burning, the moths fly toward the window. If, 

 however, they are set free within a metre of the lamp, they 

 fly toward it. 



The explanation that Loeb offers of the habit of these 

 moths to fly only in the evening is, that, although they are 

 at all times positively heliotropic, they respond to light only 

 in the evening. In other words, it is assumed that there is a 

 periodic change in their sensitiveness to light, which corre- 

 sponds with the change from day to night. Loeb says that, 

 just as certain flowers open only at night, and others only 

 during the day, so do moths become more responsive in 

 the evening, and butterflies during the day. Both moths and 

 butterflies are positively heliotropic, and the sensitiveness of 

 moths to light may be even greater in the evening than is 



