THE PHOTIC REACTIONS OF SARCOPHAGID FLIES 209 



9. The adult sarcophagid 



In an earlier paper on the sarcophagids, I ('07, p. 49) assumed 

 that the eyes of these animals are of much importance in orienta- 

 tion, because of their relatively large size. But attention was also 

 called to the fact that the chemical sense is probably of more im- 

 portance in detecting the presence of food, since it could hardly 

 be assumed that the vision of these animals is so acute that so 

 small an object as a fish could be seen at any great distance. On 

 several occasions a dead fish wrapped tightly in folds of paper was 

 carefully enclosed in a tight box so that odors from the fish could 

 not be detected. The package was then carried to a location far 

 removed from the beach where no flies were to be seen and there 

 opened up and the fish exposed. In ten minutes many sarcophagid 

 flies were hovering about, and some eggs had already been de- 

 posited. It can hardly be assumed that these flies found the fish 

 through sharpness of vision. 



It is the object of the following series of experiments to test 

 the eyes of these flies for their image forming powers. Other 

 insects have been worked on to ascertain their powers in this 

 respect, among them the mourning cloak butterfly (Vanessa 

 antiopa) by Parker ('03) and Cole ('07), which species will form a 

 basis for comparison. 



A second question to be considered is one of distribution, alluded 

 to in the Introduction, viz : Is there any relation between the pho- 

 totropism of the two species and the fact that L. caesar is primarily 

 a fly of the fields and C. vomitoria more or less a household form. 



An answer to both of these questions may be sought by the use 

 of an apparatus described by Cole ('07, pp. 340-346), but some- 

 what modified to meet the present need. 



The method and apparatus may be briefly described as follows. 

 Two lights of different areas were used, one, a single Nernst fila- 

 ment on a 110-volt circuit placed back of a metal sheet in which a 

 narrow slit was cut ; the size of this slit could be regulated by means 

 of a sliding shutter. The second light was produced by reflection 

 upon a vertical plate of ground glass. The source of this reflected 

 light was a two-filament Nernst lamp whose rays were thrown upon 



