192 LIGHT AND THE BEHAVIOR OF ORGANISMS 



toward the source of light when the exposure is made (Fig. 

 31). If this end is in any other position, there is no reac- 

 tion whatever until the organism, in its normal process of 



• 



locomotion, extends it toward the source of light. Then 

 it is at once turned from the light to such an extent that it 

 frequently makes a right angle w4th the posterior end. 

 Later it is swung back, but only part way. The tip is 

 however exposed and so the animal may be stimulated 

 again, after which it again turns sharply from the source of 

 light. This process is repeated until the organism has 

 turned to such an extent that the anterior end is practically 

 as much exposed when it turns in one direction as it is when 

 it turns in the other. The great preponderance of lateral 

 movements from the source of light and direct orientation in 

 diffuse light therefore do not indicate that fly larvae have 

 the power of differential response to localized stimulation. 



But why does the organism turn toward the light if the 

 lateral illumination is very intense? Whenever the larva 

 is stimulated, it turns the anterior end in a direction oppo- 

 site to that in which this end is when it receives the stimu- 

 lus. The tip of the anterior end is relatively very sensitive; 

 in diffuse light the larvae are stimulated only when this end 

 is extended and fully exposed, but in very intense light, 

 owing to the transluc-ency of the surrounding tissue, it is 

 stimulated no matter in what position the anterior end is; 

 consequently if this end is turned from the source of light 

 when the organism is exposed it is at once turned sharply 

 in the opposite direction, i.e., toward the light. 



j. Discussion. — It has already been demonstrated that 

 neither the direction of the rays through the organism, in 

 accord with Sachs' theory, nor the angle between the rays 

 and the sensitive surface, in accord with Loeb's explana- 

 tion, is of importance in explaining the orienting reactions 

 of the fly larvae. Nor is the direction of the rays in the 

 field of importance except in so far as it may produce 

 difference of intensity on the body. How then are the 

 orienting stimulations produced ? Are they due to light 



