THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 213 



The fact that Vanessa antiope ahghts in sunny spots, as shown 

 by Parker ('03, p. 461), is correlated with its reaction to luminous 

 areas, and similar observations made on Lucilia caesar by the 

 writer lead to a similar conclusion. It was frequently noted that 

 when a dead fish was placed under a leafy tree so that a patch of 

 sunlight fell upon it, flies would soon be hovering about the carcass. 

 As the patch of sunlight left the fish, the flies also disappeared. 

 This behavior was noted, and afterwards the fish was always 

 placed in a patch of sunlight or in the open, and not in a position 

 where shade and patches of bright light intermingled. This con- 

 dition does not hold true in large shady areas produced, for exam- 

 ple, by a building or other large object where there is an absence 

 of sunlit patches. 



The experiments with reference to image formation in the sar- 

 cophagid flies are of further interest also because these observa- 

 tions place at least this family of Diptera in line with other groups 

 worked on by Cole ('07), who was, however, unsuccessful with his 

 experiments on the fruit fly, Drosophila ampelophila. 



IV. APPLICATION TO GENERAL THEORY OF ANIMAL BEHAVIOR 



During the progress of the experiments described and discussed 

 in the preceding pages, it was a matter of concern to analyze the 

 movements of the flies in order to ascertain their method of orien- 

 tation and the factors involved. The matter for consideration in 

 the following pages may be briefly stated in the form of three 

 questions, viz., (A) How is the animal oriented by light? (B) 

 How does the animal orient to light? (C) Why is there this be- 

 havior toward light? The first question is concerned with exter- 

 nal, the second and third with internal factors. 



A. How is the animal oriented by light? Sarcophagid fly 

 larvae are stimulated to motion by light and that motion is away 

 from the source of light. 



As early as 1853 the relative importance of intensit}^ and direc- 

 tion of light were made the object of some observation, at which 

 time Cohn ('53) pointed out that Stephanosphaera collected in 

 relatively darker situations and avoided bright fight. Later this 



