THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 205 



pp. 49-50) describes what he calls a typical motor reaction for 

 Euglena when, swimming toward the source of light, the illumina- 

 tion is suddenly decreased. Mast ('06, p. 370) found that ''Sten- 

 tors which are oriented to a given light respond with the motor 

 reaction to an increase in intensity of the light, they are for the time 

 being thrown out of orientation." It was also found by Walter 

 ('07, p. 63) that Planaria gonocephala "showed a decided response 

 — either some change in course or a wigwag motion of the anterior 

 end — more frequently when suddenly subjected to dark than to 

 light." 



To test the larvae of the sarcophagids to sudden changes of 

 intensity, the light grader was employed so as to throw light from 

 above upon the animals as they crawled on the glass stage at the 

 focus of the lens. By means of a rectangular opening, instead of 

 one triangular in outline, an ungraded field of 70 C\M. throughout 

 was produced. The larvae were caused to crawl in the direction 

 of a given area which could be suddenly illuminated by with- 

 drawal of a diaphragm, when the individual had reached the proper 

 position. 



The results were quite uniform; whether the larvae were crawl- 

 ing in the dark, in 0.5 CM., or in 50 CM. directive light, all pro- 

 duced random movements, and were more or less thrown out of 

 orientation. The after-effects of the directive light on the larvae 

 traveling in the dark, were sufficient to cause such individuals to 

 retain the original general direction even after having been sud- 

 denly illuminated. The accompanying figure (fig. 12) shows 

 clearly the nature of the reaction. On the other hand by a com- 

 parison of fig. 16 to 19 it is quite evident that sudden decrease in 

 intensity, e.g., decrease from an intensity of 960 CM. to total 

 darkness, does not throw the larvae out of orientation, nor does 

 it cause any apparent disturbance at the moment. 



8. Localization of the function 



It has already been pointed out by Loeb ('90, pp. 71-72) that 

 only the rays of light striking the oral pole of the larva are effective 

 in orientation. This conclusion, however, was reached by means 



