THEORY OF EVOLUTION 103 



THE FOUR GREAT LINKAGE GROUPS OF 

 DROSOPHILA AMPELOPHILA 



The following description of the characters 

 of the wild fly may be useful in connection with 

 the account of the modifications of these char- 

 acters that appear in the mutants. 



The head and thorax of the wild fly are gray- 

 ish-yellow, the abdomen is banded with alter- 

 nate stripes of yellow and black. In the male, 

 (fig. 4 to right), there are three narrow bands 

 and a black tip. In the female there are five 

 black bands ( fig. 4 to left ) . The wings are gray 

 with a surface texture of such a kind that at cer- 

 tain angles they are iridescent. The eyes are a 

 deep, solid, brick-red. The minute hairs that 

 cover the body have a very definite arrange- 

 ment that is most obvious on the head and 

 thorax. There is a definite number of larger 

 hairs called bristles or chaetae which have a 

 characteristic position and are used for diagnos- 

 tic purposes in classifying the species. On the 

 foreleg of the male there is a comb-like organ 

 formed by a row of bristles ; it is absent in the 

 female. The comb is a secondary sexual char- 

 acter, and it is, so far as known, functionless. 



