1. Eye bare or very sparsely haired.................................................................................. 2

– Eye densely haired (Figs. 1–3)...................................................................................... 4

2. Wing vein R 1 bare dorsally ...................................................... Ceromasia auricaudata

– Wing vein R 1 haired dorsally along most of its length (Figs. 12–13)........................... 3

3. Wing vein CuA 1 usually bare dorsally (very rarely with 1–3 hairs) and distal portion of CuA 1 0.27–0.38 length of proximal portion (Fig. 12); male with flagellomere 1 not enlarged (Fig. 37) .................................................................................... Actia diffidens

– Wing vein CuA 1 usually haired dorsally most of distance to crossvein dm­cu (very rarely with as few as 4 hairs) and distal portion of CuA 1 0.37–0.51 length of proximal portion (Fig. 13); male with flagellomere 1 enlarged (Fig. 38) ........... Actia interrupta

4. Prosternum bare .................................................................................. Lypha fumipennis

– Prosternum haired (Fig. 14)......................................................................................... 5

5. Katepisternum with four setae (Fig. 4) ........................................................ Nilea erecta

– Katepisternum with two or three setae (Figs. 5–6, 11)................................................. 6

6. Facial ridge haired on 1/2 or more of distance between vibrissa and lower margin of scape (Figs. 1–2).......................................................................................................... 7

– Facial ridge haired on 1/3 or less of distance between vibrissa and lower margin of scape (Fig. 3)............................................................................................................... 11

7. Eye very large, distance between eye and lower margin of head in lateral view not wider than basal width of maxillary palpus (Fig. 43); two katepisternal setae; katepimeron haired along most of length ......................................... Hemisturmia parva

– Eye smaller, distance between eye and lower margin of head in lateral view several times wider than basal width of maxillary palpus (Figs. 1–2); three katepisternal setae (Figs. 5–6); katepimeron bare or with one to several hairs anteriorly (Fig. 11)........... 8

8. Maxillary palpus (Fig. 2) dark coloured; katepisternum with lowermost seta nearly equidistant between outer two setae (Fig. 6)................................................................ 9

– Maxillary palpus yellow; katepisternum with lowermost katepisternal seta closer to anterior than posterior seta (as in Fig. 5)..................................................................... 10

Abbreviations: ad s, anterodorsal setae; a kepst s, anterior katepisternal seta; ar 3, aristomere 3; a scut s, apical scutellar seta; d scut s, discal scutellar seta; fc rd s, facial ridge setae; l fr s, lowermost frontal seta; l kepst s, lowermost katepisternal seta; max plp, maxillary palpus; oc s, ocellar seta; pafc h, parafacial hairs; p kepst s, posterior katepisternal seta.

Abbreviations (not including veins and crossveins): f cx, fore coxa; h cx, hind coxa; kepm, katepimeron; kepst, katepisternum; trn sut, transverse suture.

9. Facial ridge with hairs decreasing to tiny size on upper portion of haired area (Fig. 1); aristomere 3 evenly tapered to tip (Fig. 1) ........................... Phryxe pecosensis, in part

– Facial ridge with strong setae only (Fig. 2); aristomere 3 thickened along most of length and tapered near tip (Fig. 2) ............................................ Madremyia saundersii

10. Scutellum (Fig. 11) with posterior half orange; discal scutellar setae narrowly separated, distance between them not greater than distance between apical scutellar setae (Fig. 7); female with tiny non­piercing ovipositor; ocellar setae well developed (as in Fig. 3) ............................................................................................... Cyzenis incrassata

– Scutellum mostly gray; discal scutellar setae more widely spaced, distance between them twice the distance between apical scutellar setae (Fig. 8); female with long piercing ovipositor (Fig. 40); ocellar setae usually absent or very weak............................... ................................................................................................... Compsilura concinnata

11. Parafacial with fine hairs on middle portion (Fig. 3) ..................... Smidtia fumiferanae

– Parafacial bare (Figs. 1–2) or with a few hairs directly below lowermost frontal seta........................................................................................................................................ 12

12. Mid tibia with one anterodorsal seta (Fig. 9)............................................................. 13

– Mid tibia with two or more anterodorsal setae (Fig. 10)............................................ 15