INSECT BEHAVIOR 351 



that Protista ( Strasburger) ,Daphnia (Davenport and Cannon) 

 and the caterpillars of Porthesia (Loeb) move toward a source 

 of light even while, in so doing, they are passing into regions 

 of less intensity of illumination. For this migration as deter- 

 mined by the direction of the light rays, the term pho tot axis 

 is by some authors (as Davenport) reserved. Usually, how- 

 ever, the direction of locomotion does depend on differences 

 of intensity, without regard to the direction whence the light 

 comes. This " migration towards a region of greater or less 

 intensity of light " has been termed photopathy, and organisms 

 are said to be photophil or photophob, according as they move, 

 respectively, toward or away from a more intensely illumi- 

 nated area. 



Verworn and others maintain that differences of intensity 

 are sufficient to account for all phototropic 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 illumination 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 anoma- 

 lies between the response to light of a butterfly and 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 negatively phototropic. " Butter- 

 flies 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, pro- 

 duces 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 



