QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 277 



deflection was definite and constant, within the hmits of experi- 

 mental error, for a given intensity difference between the Hghts. 



6. Using the average deflections obtained under each one of a 

 graded series of intensity differences, a curve of the response 

 to differential bilateral stimulations may be constructed. The 

 deflection increases regularly with the increase of intensity 

 differences. 



7. The curve of response may be applied as a means of measure- 

 ment to other experimental data obtained under similar con- 

 ditions. 



8. In orientation to a single horizontal light: 



a. The changes of position due to the side to side swinging 

 of the head characteristic of locomotion produce changes in the 

 intensity of the light acting on the sensitive anterior end of the 

 larva, due in a large measure to the shadow cast by the animal's 

 own body. 



b. An abrupt change in the intensity of the light acting on 

 the sensitive surfaces produces a reflex toward a 'physiologically 

 definite side,' the side on which the muscles are passively 

 stretched. 



c. Repetition of this reflex automatically checks motion 

 toward the light. 



9. In orientation to horizontal beams of light from two 

 sources: 



a. The side-to-side swinging of the head does not produce 

 changes in the effective intensity of the light on the anterior 

 end as a whole. 



b. The attainment of orientation depends on the relative 

 amount of stimulation received by symmetrically located sensory 

 areas. 



c. An arrangement of bilateral sensitive areas may be postu- 

 lated whereby bilateral stimulation may be equalized by a 

 change of axial position. This arrangement accords with the 

 facts, so far as they are known. 



10. The phototactic response of the blowfly larva depends, to 

 a large extent, on the stimulating effect of constant light in- 



