100 NORTH AMERICAN DIPTERA. 



15. Femora thickened ; hind tibiae usually much dilated and with tubercles 



on outer side ; basal cells large ; first posterior cell narrowed ; all 



the tibiae with preapical bristle. . . . RHOPALOMBBIDJC. 



Flies without the above characters. . . . . . . 16 



16. Front bristly on the sides and at the vertex 17 



Front bristly at the vertex only ; the auxiliary vein is not steeply in- 

 clined where it joins the costa. 20 



17. Hind tibiae, at least, with a preapical bristle 18 



Hind tibiae without preapical bristle 19 



18. Basal cells of wings small. SAPROMTZII>;E. 



Basal cells of wings of moderate size and distinct. . SCIOMYZID^K. 



19. Posterior basal cells of wings small, the anal cell never produced 



acutely; a single bristle on each side of the front (Lonchceina:). 



SAPROMYZIDJE. 



Posterior basal cells of wings of moderate size, the third (anal) often 

 produced into an acute point. . . . . . TRYPETID^E. 



20. Legs elongate, often very long; abdomen narrow and long, often much 



narrowed at the base. 21 



Legs not elongated; abdomen comparatively short, or, if long, not nar- 

 rowed at the base ORTALID^E. 



21. First, posterior cell narrowed in the margin or closed. MICROPEZID/E. 

 First posterior cell not narrowed in the margin, or but slightly so;* 



palpi rudimentary or wanting. ..... SEPSID^E. 



22. Eyes inserted at the tip of lateral processes. ... DIOPSIIX*:. 

 Head not prolonged into lateral processes 23 



23. Hind metatarsi shorter than the following joint, incrassate. BORBORID/E. 

 Hind metatarsi longer than the following joint, not incrassate. . 24 



24. Discal and second basal cells united, the separating cross-vein rudimen- 



tary or wanting ; posterior cross-vein present.t ... 25 

 Discal cell separated from the second basal cell by a distinct cross- 

 vein, or, if not, the posterior cross-vein also wanting. . . 29 



25. Legs long and slender; abdomen elongate and narrow. MICROPEZID^:- 

 Legs of moderate length, often strong; abdomen ovate or elongate 



elliptical. . ... . . . . . . . . 2(5 



20. Front bare, or, at the most, bristly at the vertex ; third antennal joint 



rounded, or, if elongate, the head triangular in profile. OSCINID/E. 



Front bristly at least as far as the middle ; head never triangular in 



profile. . . . ',-' . - . . . . 27 



* Compare here the Psilidie when there is doubt regarding the auxiliary 

 vein. t The posterior cross-vein is wanting in A stria (Drosophilida), 



Elliponeura (Osc.inidfr), Phytomyza (Agromifzid(i'). 



