2 DIPTEEA OP NORTH AMERICA. [PART II. 



according .to the type of the Diptera acalyptera (compare Mono- 

 graphs, etc., I, page xxiv, fig. 1), are the following in this family : 

 The costal vein extends as far as the tip of the fourth longitudinal 

 vein ; the auxiliary vein does not run towards the costa, but to- 

 wards the first longitudinal vein, stopping quite far from its tip, 

 and either coalescing entirely with it or disappearing without 

 having joined it. The third longitudinal vein emerges from the 

 second not far from its root, and both veins form at this place a 

 more or less knot-shaped swelling; the small cross-vein (often 

 quite obsolete) is at, or close by, this swelling, so that the first 

 basal cell is unusually short ; the discoidal cell, much narrowed 

 towards its base, is coalescent with the anterior of the two small 

 basal cells in consequence of the absence of the vein separating 

 them. The posterior small basal cell (the so-called anal cell) is 

 small and rounded at the end. The sixth longitudinal vein does 

 not generally reach the border of the wing ; sometimes it is en- 

 tirely wanting, and in this case the anal cell becomes indistinct. 

 The alar appendage (alula) is so narrow that it may be considered 

 as wanting. 



Although all Dolichopodidse agree in the above-described pe- 

 culiarities of neuration, still -this same neuration affords very good 

 and abundant characters for the distinction of the genera and the 

 species ; the- length and direction of the first longitudinal vein 

 vary considerably, the position of the posterior transverse vein 

 also ; the most striking characters, however, are taken from the 

 structure of the last portion of the fourth longitudinal vein, which 

 is subject to considerable variation, being sometimes parallel to 

 the third longitudinal vein, sometimes more or less convergent 

 with it, sometimes straight, often curved or bent abruptly in the 

 middle, or even broken. In the latter case, the angle formed by 

 the strong flexure or fracture sometimes bears a rudiment of a 

 branch-vein, which in some cases (as in most Psilopus) is large 

 enough to make the vein appear forked. 



For the purpose of distinguishing the Dolichopodidse from the 

 other families of diptera the characters taken from the neuration 

 are sufficient, and it is superfluous to have recourse to others 

 taken from the rest of the body. It is different, however, if we 

 intend to establish the natural character of this family. I will 

 confine myself here to characters taken from the external organi- 

 zation, and as to the internal one, I will merely notice in passing 



