194 LIGHT AND THE BEHAVIOR OF ORGANISMS 



acting constantly as a directive stimulus similar to stimula- 

 tion by a constant electric current, or to absolute difference 

 of intensity on symmetrically located points in the sensitive 

 surface, or to changes of intensity? 



According to the idea of Loeb that light acts constantly 

 as a directive stimulation, the organism is continuously 

 stimulated by light on both sides. When one side is more 

 highly illuminated than the other, that side becomes stimu- 

 lated more than the other and causes a more rapid move- 

 ment of the locomotor organs connected with the sense 

 organs of that side. This of course causes the organism to 

 turn until both sides are equally stimulated. We have 

 demonstrated that in a light intensity of 3888 ca. m. the 

 rate of locomotion is only 0.024 mm. per second greater than 

 in an intensity of 7 ca. m. (Table II). If then the rate of 

 motion of the two sides of the organism under discussion is 

 due to the absolute intensity on the two sides in accord 

 with Loeb's theory, and if one side were exposed to an 

 intensity of 3888 ca. m., while the other is exposed to an 

 intensity of 7 ca. m., the former would move only 0.024 nim. 

 per second faster than the latter. It would therefore require 

 several seconds for a fly larva to become oriented even with 

 a difference of intensity on opposite sides amounting to 

 nearly 4000 ca. m. ; whereas it actually requires only a frac- 

 tion of a second for the larvae to orient under conditions in 

 which the greatest difference of intensity could not possibly 

 be more than 5-10 ca. m. The theory that orientation is 

 due to light acting constantly as a directive stimulation is 

 therefore inadequate to account for the orientation of fly 

 larvae. Moreover the fact that the larvae, when exposed 

 to moderate light intensity, respond only when the anterior 

 end comes to be fully exposed to the light in the process of 

 locomotion, shows clearly that the orienting stimulation is 

 not acting constantly. 



The symmetry of the body with reference to the location 

 of the sensitive surface seems to be of no special importance 

 as far as orientation is concerned in this organism. I was 



