THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 223 



larvae could seldom be induced to enter a field of high intensity 

 (non-directive). 



13. By means of a low intensity of directive light (0.5 CM.) 

 the larvae could be forced farther into the darker end of a field 

 grading at the rate of 7 CM. per cm. than into one grading at the 

 rate of 35 CM. per cm. Though the relation was not constant, 

 the usual distance was about five times as far in the former field. 



14. In a field of light which is at the same time directive and 

 graded to higher intensities in the direction of the rays, the larvae 

 creep with the rays, thus passing into more and more intense 

 regions, not, however, without a disturbance in orientation, in- 

 dicated by many random movements and a strong tendency to 

 take a diagonal course with relation to the rays. 



15. In comparison with the records of other investigators, it 

 appears that more perfect orientation is possible on the part of 

 the fly-larvae because of the localizating of the photoreceptive 

 organs. 



16. The rate of locomotion increases rather uniformly with 

 the intensity of the light, excepting for the lower and higher 

 extremes. 



17. The increase in rate in higher intensities is not alone due 

 to precise orientation, but also to photodynamic effects. 



18. By sudden increase in illumination, whether the individ- 

 uals were previously creeping in the dark or in 0.5 CM. or in 

 50 CM., the larvae invariably produced random movements 

 at the moment and were more or less thrown out of orientation. 

 The reverse condition (sudden decrease of illumination) did not 

 result in any preceptible change of state at the moment. 



19. The functional photoreceptive organs are highly concen- 

 trated and equally distributed bilaterally in the immediate region 

 of the oral pole. 



20. The more frequent presence of the adults of C vomitoria in 

 houses and like situations may be accounted for by the fact that 

 this species is less responsive to light and also less strongly photo- 

 tactic than is L. caesar. Therefore the former is of more impor- 

 tance as a household scavenger or pest, as the case may be, while 



