INSECT BEHAVIOR 358 
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 Jess intensity of illumination. Tor this migration as deter- 
mined by the direction of the light rays, the term phototaxis 
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 hght 
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 
