LINKAGE GROUPS 95 



wild fly there is a three-pronged mark on the thorax 

 present in many individuals. Trefoil is a further 

 development and modification of this mark and is 

 due to a special factor. 



The fifth figure (e) is called dachsoid; the body 

 is shortened and the wings are broad and held out. 

 The legs are short. The absence of cross-veins in the 

 wings is characteristic. 



In the sixth figure (/), apterous, the wings are 

 entirely absent, not even the base remaining as in 

 vestigial. The apterous flies are almost completely 

 sterile. 



The seventh figure (g) shows the wings "curved." 

 In addition there is j^resent a minute black speck at 

 the base of each wing, due to another factor called 

 speck. 



In the eighth figure (k) the wings are arched. 

 The factor is called arc. The dark color of the body, 

 and especially of the wings, indicates the factor for 

 black. 



There are also a number of different eye colors in 

 this group — one of which, brown, is darker in old 

 flies than the red of the wild type. 



GROUP III 



The hereditary elements of Group III are carried 

 by the other pair of large bent chromosomes. 



In figure 42 (a) , a mutant type called bithorax is 

 shown. The old metathorax is replaced by another 



