24 THE EXTERNAL STRUCTURE OF MCSCA DOMESTICA 



into costal (C.) and first costal (1 C.) cells. The next main 

 nervure — the radial — divides into a number of branches (in the 

 typical insect five); some of these have coalesced in the fly. A 

 nervure joining the costal just past the middle is the first radial 

 (7^. 1) cutting off the sub-costal cell. The next nervure, which 

 joins the costal on the apical curve, represents the fused second 

 and third radial nervures {R. 2 + 3). This cuts off the first radial 

 cell (1 R.). The last nervure, which joins the costal almost at the 

 apex of the wing, represents the fused fourth and fifth i-adial 

 nervures {R. 4 + 5) and so cuts off the third radial cell (3 R.). 

 The fourth main longitudinal nervure is the median, which, in the 

 typical insect, divides into three, but in the fly the nervures have 

 undergone coalescence, as will be shown. The first and second 

 median nervures have coalesced {M. 1 + 2), and do not run direct 

 to the margin of the wing, but bend forwards and almost meet 

 i^. 4 + 5 on the costa. About half way across the wing a trans- 

 verse nervure, the radio-medial {rm.) unites i2. 4 -t- 5 and M. 1-1-2, 

 and cuts off the fifth radial cell (5 R.) from the radial {R.). The 

 next longitudinal nervure represents the coalesced third medial 

 and cubital nervures {M.^-{- Cu.l). It runs to the posterior 

 margin of the wing about half way along the length of the latter. 

 The nervures if . 1 + 2 and M. 3 + (7w. 1 are united by two nervures; 

 a proximal nervure, the medio-cubital (m.cu.) representing part of 

 the original longitudinal vein M. 3, cuts off the small triangular 

 medial cell (M) ; the distal transverse nervure ni. cuts off the first 

 second medial cell (2 MK) from the second second medial cell (2 3P.). 

 The last longitudinal nervure, the anal (^.1), is undivided and does 

 not reach the margin of the wing, thus incompletely separating 

 the first cubital (1 Cu.) and anal {A.) cells. A small nervure, the 

 cubito-anal (cii.a.) representing a portion of the original cubital 

 vein (Cu. 2) slightly more proximal than the medio-cubital cuts oft' 

 the small triangular cubital cell (Cu.) from the first cubital cell 

 (1 Ou.). Running parallel with, and posterior to, the anal longi- 

 tudinal nervure, there is apparently another nervure. This, however, 

 is not a true nervure but is merely a chitinised fun-ow giving 

 additional strength to the posterior angle of the wing. The 

 posterior edge of the base of the wing is divided into a number of 

 lobes. These are the anal lobe, and, as Sharp (1895) proposed. 



