90 EVOLUTION AND GENETICS 



there are five black bands {fig. 6, right) . The wings 

 are gray with a sin*face texture of such a kind that 

 at certain angles thev are iridescent. The eves are a 

 deep, brick-red. The minute hairs that cover the body 

 have a characteristic arrangement that is most ob- 

 vious on the head and thorax. There is a definite 

 number of larger hairs called bristles which have a 

 characteristic position and are used for diagnostic 

 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 secondarv sexual character. 



Some of the characters of the mutant types are 

 shown in figures 40, 41, 42, 43. The drawing of a 

 single fly is often used here to illustrate more than 

 one character. This is done to economize space, but 

 of course there would be no difficulty in actually 

 bringing together in the same individual any two or 

 more characters belonging to the same group (or to 

 different groups) . Without colored figures it is not 

 possible to show many of the most striking differ- 

 ences of these mutant races ; at most, dark and light 

 coloring can be indicated by the shading of the body, 

 wings, or eyes. 



GROUP I 



The hereditary elements of this group are carried 

 bv the A^- chromosomes. The characters are said to 



a. 



be sex-linked. 



