Key to subfamilies and genera of Mesembrinellidae

1 A large, dark brown fly (13–15 mm in length); abdomen robust, shiny black or dark brown with ST2–4 wider than long; each segment with two horizontal rows of stout black setae, one row along the posterior margin, the other in the posterior third of the segment (Fig. 281); presutural intra-alar seta absent; setae on facial ridge ascending about half way to antennal base (Fig. 193); male frons width 0.02/5 of head width at narrowest. Male terminalia with surstylus slender and straight, cercus long and slender with uniform curve forward (Figs 101–102). Phallus in lateral view with epiphallus short and angled forward (Fig. 191); in dorsal view with pair of lateral processes just behind hypophallic lobes (Fig. 192). Female: spermathecae bulbous (Fig. 410); T6 of terminalia of FU shape, T7 large and rectangular, recessed midway anteriorly (Fig. 325); [known from Brazil, Costa Rica, Ecuador, Guatemala, Panama, Trinidad, Venezuela].............................................................................................. Souzalopesiellinae [genus Souzalopesiella; one species, Souzalopesiella facialis]

- A usually smaller fly (most species 12 mm or less in length); abdomen usually shiny, with metallic highlights; setae on sternites usually much weaker and not limited to two rows; sternites usually rounded, not quadrate. Male epandrium, cerci and surstyli as in Figs 15–100; phallus without lateral processes (Figs 105–190). Females usually with spermathecae filiform or tuberform, not bulbous (except in M. flavicrura); T6 of terminalia usually not of FU shape (except Laneella)..................... 2

2 Male with surstylus and cercus both curving forward (Figs 15, 17, 19, 21, 23); phallus in lateral view with epiphallus long and slender, curving backward (Figs 105, 107, 109, 111, 113); phallus in dorsal view with hypophallic lobes short and circular or semicircular (Figs 106, 108, 110, 112, 114); frons narrow, 0.01–0.02 of head width at narrowest. Females with tuberform spermathecae (Figs 370–374), shorter but still tuberform (Fig. 371) in L. fuscosquamata; T6 of FU shape (Figs 284–286). Five of six species in this genus with a more or less shiny brown thorax (one species shiny blue), all medium-sized (10–13 mm); two species with basal half of tarsal claws pale white to pale yellowish (Fig. 10). [Known from Brazil, Colombia, Costa Rica, Ecuador, French Guiana, Guatemala, Mexico, Paraguay (Marinho et al. 2017) and Venezuela]...................................................................................... Laneellinae [genus Laneella; six known species]

- Male normally with surstylus curved backward or straight and cercus curved forward; phallus in lateral view usually with epiphallus short to moderate in length; hypophallic lobes rarely circular but usually narrow or, if wider, more elongate; frons width variable. Females usually with filiform spermathecae (except bulbous in M. flavicrura; Fig. 397), most species with some metallic blue to black colors on abdomen in both sexes; female with T6 usually not so flattened, usually of distinct inverted U or V shape (Figs 287–313); widespread from southern Mexico to Argentina ............................................................................... Mesembrinellinae [genus Mesembrinella; 6 species-groups and 48 species]