1 88 LIGHT AND THE BEHAVIOR OF ORGANISMS 



age of ten trips across the course made by different indi- 

 viduals. The total average as seen in Table III, with the 

 entire larva exposed in 7 ca. m. light intensity, is 44.24 

 seconds, and that in 7 ca. m. intensity with the posterior 

 third of the body in 3888 ca. m. intensity, is 43.89 seconds, 

 a difference of only 0.35 seconds in traveling 15 cm. In 

 Table IV the total average in 7 ca. m. intensity is 44.02 

 seconds; and in 7 ca. m. with f of the body exposed in 3888 

 ca. m. intensity, it is 43.23 seconds, a difference of 0.79 

 seconds in traveling 15 cm. By comparing these results 

 with those recorded in Table II it will be seen that there 

 is very little difference in rate of locomotion under the 

 different conditions of illumination, i.e. larvae entirely ex- 

 posed in 3888 or 7 ca. m. or the posterior one- third or 

 three-fourths exposed in 3888 ca. m. This seems to indi- 

 cate that the tissue sensitive to light is restricted to the 

 anterior tip of the body. The difference in rate of loco- 

 motion under the different conditions can be accounted 

 for by assuming it to be caused by the light reflected from 

 the highly illuminated posterior end of the body upon the 

 sensitive anterior end. 



The exposure of the side of the body to the very intense 

 light from the three glowers has apparently no effect what- 

 ever on orientation. The larvae continue as directly on 

 their course as though they were exposed only to light of 

 7 ca. m. intensity from the single glower. As a matter of 

 fact all but the very tip of the anterior end can be illumi- 

 nated by this intense lateral light without causing any 

 noticeable deviation in the direction of motion. If how- 

 ever the tip is exposed there is a sudden sharp turning 

 of the anterior end either toward or from the source of 

 light. 



The results recorded in the last two tables indicate either 

 that the tissue sensitive to light in fly larvae is confined to 

 the extreme anterior end, or that light of constant intensity 

 has no effect on the rate of locomotion, the increase in rate 

 due to increase in light intensity when the entire organism 



