34 TYPES OF MENDELIAN HEREDITY 
rear margin of the scutellum, point constantly in an 
abnormal direction in club stock. The head of club 
flies is often flattened, the eyes are smaller, and the 
thorax and abdomen are somewhat distorted. Here 
we have an example of a single germinal difference, 
the factor for club, producing several distinct effects, 
Fig. 18.—Rudimentary wing (to left), and truncate wing (to right). 
some of which are constant features of the stock, while 
others are occasional or variable. 
Another and similar example is found in the rudi- 
mentary winged flies (Fig. 18, a). The wing is usually 
shorter than the abdomen, but may be longer and even 
approach the normal wing in length and shape. The 
