VERMES, FLY LARVAE, AND ECHINODERMS 197 



Summary 



(i) Fly larvae are negative in their light reactions in all 

 intensities to which they respond. They become acclima- 

 tized very readily so that after they have been exposed in a 

 given intensity for about half an hour they fail to respond 

 unless the intensity is increased. 



(2) The tip of the anterior end is the only sensitive region 

 on the larvae. On this tip there are two cone-shaped struc- 

 tures which probably are light recipient organs. 



(3) In locomotion the larvae turn the anterior end slightly 

 from side to side with considerable regularity; but if sud- 

 denly exposed to high intensity they throw the anterior end 

 from side to side violently. 



(4) Absolute difference in light intensity has but little 

 effect on the rate of movement. In 7 ca. m. it was found 

 to be 0.321 cm. per second; in 3888 ca. m., 0.345 cm. per 

 second. 



(5) Unilateral illumination of the posterior third or three- 

 fourths of the body has practically no effect on the rate of 

 locomotion. 



(6) The process of orientation in the fly larva is similar 

 in principle to that in Euglena and Stentor. It is brought 

 about by reactions which are similar to the avoiding reac- 

 tions or shock movements of the lower organisms. These 

 reactions are due to changes of light intensity on the sensi- 

 tive anterior end; and the changes of intensity are due 

 largely to the lateral movements of this end. 



(7) Orientation is the result of trial movements, but it is 

 doubtful whether it could be considered as the result of selec- 

 tion of random movements as defined by Holmes (1905). 



(8) In light from two sources they may take a path 

 extending from any point betw^een them. The location of 

 this point depends upon the relation in intensity of light 

 from the two sources. 



(9) There is no evidence indicating differential response 

 to localized stimulation. If the fly larva has the power of 



