308 ENTOMOLOGY 



ing as they move, respectively, toward or away from a more intensely 

 illuminated area. 



Verworn and others maintain, however, that differences of intensity 

 are sufficient to account for all photo tropic phenomena. 



Optimum Intensity. It has been found that there is a certain 

 optimum degree of light, differing according to the organism, toward 

 which the organism will move, from either a region of greater illumina- 

 tion or one of less. The organism appears to be attuned to a "certain 

 range of intensity." This attunement is used by Davenport to explain 

 apparent anomalies between the response to light of a butterfly an< 

 that of a moth. Butterflies are positively phototropic to sunlight and 

 most moths are negatively so. Why, then, do moths fly toward a lamp 

 or an electric light? The answer is given that the moth is positively 

 phototropic up to a certain intensity of light, at which it becomes nega- 

 tively phototropic. " Butterflies are attuned to a high intensity of 

 light, moths to a low intensity; so that bright sunlight, which calls 

 forth the one, causes the other to retreat. On the other hand, a light 

 like that of a candle, so weak as not to stimulate a butterfly, produces a 

 marked response in the moth." (Davenport.) 



The circling of moths and other insects about a light is a matter of 

 common observation, an explanation for which has been given by Loeb. 

 Loeb says, "If a moth be struck by the light on one side, those muscles 

 which turn the head toward the light become more active than those of 

 the opposite side, and correspondingly the head of the animal is turned 

 toward the source of light. As soon as the head of the animal has this 

 orientation and the median-plane (or plane of symmetry) comes into the 

 direction of the rays of light, the symmetrical points of the surface of the 

 body are struck by the rays of light at the same angle. The intensity 

 of light is the same on both sides, and there is no reason why the animal 

 should turn to the right or left, away from the direction of the rays of 

 light. Thus it is led to the source of the light. Animals that move 

 rapidly (like the moth) get into the flame before the heat of the flame has 

 time to check them in their flight. Animals that move slowly are 

 affected by the .increasing heat as they approach the flame; the high 

 temperature checks their progressive movement and they walk or fly 

 slowly about the flame." As Loeb insists, the moth "does not fly into 

 the flame out of 'curiosity/ neither is it 'attracted' by the light; it is 

 only oriented by it and in such a manner that its median-plane is 

 brought into the direction of the rays and its head directed toward the 



