EYE FACET NUMBER IN DROSOPHILA M ELANOGASTER. 169 



In counting facets, a no. 4 eye-piece and a no. 3 Leitz objective 

 were used. In some cases the facets were counted after the flies 

 had been preserved in alcohol, but mostly the counts were made 

 shortly after the emergence of the flies from the pupa cases. 

 In order to determine the per cent, of error made in counting, 

 a set of ten flies, five males and five females was counted at two 

 different times. The total number of facets in the first count 

 was 1,347, in the second count 1,356, and the per cent, of differ- 

 ence between the two counts was 0.81 per cent. For a larger 

 number of individuals the per cent, of error would probably 

 be less than this; for single individuals it was found to average 

 about 5 per cent, to 7 per cent. 



The area of the eye in arbitrary units was found by making an 

 outline camera lucida drawing of the eye, and measuring the 

 enclosed area with a polar planimeter, which was found to give 

 accurate results to 2/100 of a square inch. 



DATA. 



Experiments i, 2, 3 (Tables I. and II.). These experiments 

 were designed originally to test the effect of different amounts of 

 food on facet number. Three bottles, each containing a different 



TABLE I. 



EXPERIMENTS i, 2, 3, MALES. 



amount of food and thirteen pairs of flies as parents, were placed 

 in a small well-lighted room where the temperature was kept at 

 approximately 27. After seven days the parents were taken 

 out and their facets counted. It was found that the three bottles 

 differed slightly from one another in facet number but that their 



