30 T. H. MORGAN 



shown in the figure is that of an older fly. When first hatched 

 the brown is more yellowish in shade, and not so easily dis- 

 tinguished from that of the older yellow flies. Most of the 

 brown flies, especially the older ones, show a brown shield on 

 the thorax, in form like that on the thorax of the black fly. The 

 light bands of the abdomen are light brown and differ therefore 

 from the yellow bands of the yellow fly. The dark bands are, 

 brown. The legs also show some brown. The veins of the wings 

 are brown like the thorax. On each side of each vein is a brown 

 band. It is by means of these bands that one can most readily 

 separate under a lens the yellow from the brown fly. The 

 interspaces between the bands are gray-brown, and more trans- 

 parent than the bands. The hairs are also brown. 



In separating the flies into the color groups, (after slightly 

 etherizing) there is never any difficulty in distinguishing the 

 gray from the black and from the yellow, provided they have 

 not just hatched; but the yellow flies and the brown offer greater 

 difficulties, especially when flies of different age are mixed to- 

 gether, and when small flies are also present, due to starving 

 the larvae. I do not feel certain that the separation of these 

 two groups has been always perfect, but the errors are not great 

 enough I think to vitiate seriously the classification. In some 

 cases I have kept the flies alive for several days in order to verify 

 my first separation, and have found occasionally that one or 

 two flies have been put into the wrong group. The errors have 

 been in both directions, and counterbalance to some extent. 

 I have followed the rule of classifiying flies as brown only when 

 they were certainly brown, as shown best by the broad brown 

 bands along the veins of the wings, and this has lead, I fear, 

 to the inclusion of a few brown flies in the yellow group. I 

 have also tested my ability to separate these two groups by 

 breeding doubtful flies, which, in general, I would have placed 

 in the yellow category. If the fly in question is a yellow female 

 and is bred to a black male, all the female offspring should be 

 gray (because only the female producing sperm carries the factor 

 for black) and all the males should be yellow. If the fly in 

 question were a brown female, and were bred to a black male, 



