NOMENCLATURE OF THE WINGS 79 



Comstock and Needham's system may be taken first, as being the 

 most easily understood, though not necessarily the most easily applied. 

 These workers deduced from an extensive series of ob- 

 servations a primitive condition of the venation, which Om8t !* k a , nd 

 1 Needham's 



they put forward as a 'urotype' (Plate XVII, fig. 1). Nomenclature 



In such a wing all the veins are to be traced to two 

 tracheae, which enter the wing on the anterior and posterior borders 

 respectively. Each of these divides at once on entering the wing to 

 form a number of branches which traverse the long axis and ultimately 

 reach the borders. In the fully formed wing these tracheae remain 

 as the longitudinal veins, of which there are four derived from each 

 trachea. Taking those from the anterior tracheae first, they are named 

 in order from before backwards the costa, subcosta, radius and media. 

 The costa is undivided, and lies near the anterior border of the wing. 

 The subcosta bifurcates near its termination, both branches reaching 

 the border anterior to the apex. The radius divides in the proximal 

 third of the wing into two branches, the anterior one of which is 

 continued undivided to the apex of the wing ; the posterior divides 

 twice, so as to produce four branches, all of which reach the wing 

 margin near the apex. The media divides into two about the 

 middle of the wing, and each branch again divides in the outer 

 third, so that there are four terminal branches. The posterior trachea 

 also gives rise to four veins. The first of these is the cubitus, which 

 divides into two about the middle of the wing, each vein so produced 

 running undivided to the posterior border. The remaining veins are 

 simple and undivided, and are termed the first, second and third anal 

 veins. 



The types of venation seen in modern Diptera are derived from the 

 urotype by coalescence of certain of the original veins with one another, 

 and by the development of certain cross-veins. These unite the longi- 

 tudinal veins with one another, and, with the wing borders, divide 

 up the wing into areas known as cells. The difficulty in the applica- 

 tion of this system is to determine which veins have coalesced when 

 the number is reduced. 



The veins and cells are distinguished by the initial letter of the 

 primary vein from which they spring, followed by the numeral indi- 

 cating the position of the branch, starting from the anterior side. 

 Thus the first radial vein is R1, the second R2, the first division 

 of .the cubital vein Citl , and so on. The cells are distinguished by 

 the initial letter of the vein forming the anterior margin, preceded by the 



