40 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 



somes. In the other types only the female groups are shown. In 

 those species belonging to types A and C that are marked with an 

 asterisk (*), male chromosome-groups have been studied and found 

 to differ from the female groups in such manner as to show that the 

 rod-like pair of chromosomes represented at the bottom of the diagrams 

 is the sex-chromosome pair. Male groups in D. virilis (type F) and 

 D. funehris (type G) suggest that the long pair represented at the 

 bottom in these figures is the sex-chromosome pair, but the differences 

 between the two members are not striking enough to make this con- 

 clusion certain. 



It appears from the studies of Metz and Stevens that the usual 

 muscid chromosome group is quite different from these. It consists 

 of six pairs of chromosomes, of which five are more or less V-shaped, 

 while the sixth and smallest is a short rod or is spherical and is the 

 sex-chromosome pair. This group has been found in numerous 

 calypterate muscids, and in the following acalypterate genera :* 



Piophilinse — Piophila. Lauxaniinse — Physogenua. 



Ortalinse — Camptoneura. Sciomyzinse — Tetanocera, Neuroctena. 



Trypetinse — Euaresta(?) . Cordylurinse — Scatophaga. 



The only muscid not a drosophiline that has been found to have a 

 chromosome group different from this is Chcetopsis fulvifrons Macquart, 

 one of the Ortalinae. Metz finds that this species has a chromosome 

 group like that of type A, figured above, though it is not known which 

 is the sex-chromosome pair. The interpretation of these data will be 

 obscure until Geomyzinae, Milichiinse, or Ephydrinse are studied. 

 Here, if anywhere, one might expect to find intermediate stages in 

 the series. 



* In a few of these it has not been definitely determined that the»smallest pair is the sex- 

 chromosome pair. 



