346 THE BIOLOGY OF THE FROG CHAP. 



manner by placing their bodies so that they face the region 

 of strongest illumination, and frequently also moving toward 

 it. Animals which orient themselves to light in this manner 

 are said to \>e photo tactic or heliotropic. Those which move 

 toward the light are called positive ; those which move 

 away from it negative. This orientation is generally re- 

 garded as brought about in a reflex manner through the 

 unequal stimulation of the two sides of the body, either 

 through the eyes or other parts of the organism. When the 

 body is oblique to the rays, it receives more stimulus from 

 the light on one side than the other, and if the light has any 

 directive effect upon the animal's movements, it will natu- 

 rally cause the body to turn until equally illuminated on the 

 two sides ; then, as both sides receive the same amount of 

 stimulation, the animal tends to go either toward or away 

 from the light in a straight line. 



Graber, who experimented with Rana esculenta, came to 

 the conclusion that this species is negatively phototactic. 

 The specimens were placed in a box in which were two 

 compartments, one of which was darkened while the other 

 was exposed to diffuse daylight. The animals showed a 

 tendency to collect in the darker of the two compartments. 

 If the frogs were given a choice between red and blue, they 

 collected under the red light. 



The responses of Rana pipiens and R. clamitans to 

 light have been studied by Miss Torelle, who arrived at re- 

 sults quite different from those of Graber. Frogs placed in 

 a box one half of which was exposed to diffuse light, while 

 the other half was shaded, moved into the light end of the 

 box and oriented the body so as to face the incoming rays. 

 When one half of the box was exposed to direct sunlight, 

 the frogs first moved into the illuminated area, and then, 

 after a short time, retreated into the shade, where they sat 



