ON THE TERMINOLOGY OF DIPTERA. XXI 



wing, are called the cells of the posterior margin, or posterior cells 

 (cellulde posterior es), and numbered as first, second, etc., beginning 

 with that which belongs to the middle of the wing, and is limited 

 at its base by the small transverse vein. It is evident that in all 

 Diptera there are really only three posterior cells. They exist in 

 their typical simple form in the Muscidse. The first of them be- 

 longs to the middle of the wing, the two others to the first and 

 second section of the posterior part of the wing. The first is 

 usually subject to no partition, but is sometimes closed before 

 reaching the border. The second is frequently divided in two 

 portions by the presence of the anterior intercalary vein, and this 

 happens whenever the fourth longitudinal vein emits a hind branch 

 before its end ; it even forms three portions when this branch 

 exists along with the intercalary vein. In the genera having a 

 posterior intercalary vein, a bipartition of the third posterior cell 

 occurs. Though it would be very convenient to speak in all cases 

 of only three such cells, and to point out in the way indicated the 

 mode of their further partition, yet the ruling usage does not 

 admit of this, but counts all these portions as successive posterior 

 cells, whence their number sometimes amounts to six. When the 

 second posterior cell and the discoidal cell are united in conse- 

 quence of the absence of the posterior transverse vein, the cell 

 formed in this way retains the name of second posterior cell. 



The cells belonging to the third section of the posterior part of 

 the wing are not, usually, completely separated from each other, 

 and then are frequently termed the false cells (cellulse spurise) ; a 

 better term for them might be that of axillary cells {cellulde axil- 

 lares). They are numbered in the direction from the sixth longi- 

 tudinal vein towards the posterior angle of the wing. 



As for the expressions costal border, tip, posterior border, pos- 

 terior or anal angle of the wing„they are understood by everybody. 

 The posterior angle is terminated by the axillary incision (incisura 

 axillaris) towards the base of the wing. The wings of many Dip- 

 tera are provided with a lobiform appendage, the alar appendage 

 (alula), reaching from the axillary incision to the innermost base ; 

 it must not be confounded with the covering scale that lies above 

 the poisers, and which has often been called by the same name. 



In order to understand a very intricate neuration and reduce it 

 to the simple type, we must take care not to assume for parts of 

 the same main vein all those ramifications which run in one direc- 



