EXTERNAL ANATOMY. 11 



the wing.* In Ptiichopiera, the principal genus of the subfamily 

 Ptyohopterin.e. the 3rd vein issues from the prjefurca at a point 

 where that vein makes such a sharp angle that the general 

 appearance is that of two long veins crossing one another nearly 

 at right angles,t the first being composed of the basal section 

 (ir the 2nd vein and the 3rd vein ; the 2nd vein being com- 

 posed of the anterior cross-vein and the remaining section of the 

 prsefurca. 



The next vein is probably the most important of all in the 

 wing : this is a short cross-vein which almost invariably through- 

 out the Order connects the 3rd and 4th veins when both are 

 present ; and this vein, the anterior cross-vein ,X is. in conjunction 

 with the discal cell, technically, a key to the venation in the 

 whole order of Diptera.§ It divides the wing longitudinally into 

 two nearly equal halves, and no vein found in front of it in one 

 group is ever found behind it in another. When the discal cell is 

 present the anterior cross-vein is placed immediately above it (in 

 TiPULiD.Ti), usually, at or near the base of the cell, rarely beyond 

 it (Conosia). When the discal cell is absent this cross-vein unites 

 the 3rd longitudinal vein to that pai't of the 4th vein which would 

 have formed the anterior side of the cell, had it been present. In 

 the Nematoceka ifc can nearly always be employed as a central 

 vein around which to recognise the adjacent veins ; and a little 

 practice and comparative study of the wings of other families 

 will enable the student to determine it with tolerable accuracy. 



It is usually more or less upright, generally short, sometimes very 

 short, more rarely moderately long ; being wholly absent in only 

 a very limited number of quite abnormal genera of Tipulid^.|| 



Having recognised the anterior cross-vein, joining the 3rd and 

 4th longitudinal veins, it is easy to find the 2nd longitudinal vein 

 and to ascertain, according as that vein be simple or forked, whether 

 there are one or two submarginal cells. The cell on the inner 

 side of the anterior cross-vein is the 1st hasal cell, the cell on its 

 outer side is the 1st posterior cell. This rule is practically 

 inviolate, not only in the TiPULiD.ii, but in all famihes of Diptera 

 with a tolerably complete venation. 



The 4th longitudinal vein begins at the base of the wing, and 

 invariablv encloses, or rather constructs, the discal cell (when 



* This is the only instance I know of showing this abnorniahty, and it is 

 just possible that the vein might be more correctly regarded as the anterior 

 cross-vein. This, however, is doubtful — vide discussion under Mongoma. 



t Note VtyclKypiera in the comparative figures of wings. 



\ Also known as the " small cross- vein, "and tiie "internal transverse vein." 



§ This must not be taken too literally, as exceptions embracing whole families 

 occur, such as the Cecidomyid.e, Simuliid.e, and outside of the Nematoceha, the 

 PiiORiD.E, etc., but all the'se exceptions are instances of incomplete or aberrant 

 venation. 



II The only such Oriental genus is Mongoma. 



