Key to Macrostomus species-groups based mainly in male specimens

1 Male and female wings dark with distinct fenestrate hyaline spots (Figs 1B, 2B, 3B, 4B); veins R 2 +3 and R 4 +5 curved, converging to each other, leaving cell r 2+3 narrowed subapically. Female. Legs with mid femur, hind femur, and hind tibia with dorsal and ventral pennate scales (Fig. 4A).................................................... M. megatarsus species-group

- Male and female specimens with different pattern of infuscation; veins R 2+3 and R 4+5 not converging to each other. Female. Legs sometimes as above, or with variable dorsal and ventral pennate scales on hind leg or without pennate scales............. 2

2 Male wings distinctly brown infuscate on costal and distal margins, on crossvein r-m, and slightly on crossvein dm-m (Rafael & Cumming 2010, figs 1A, 2A). Female wings distinctly more infuscated on respective regions of males (Rafael & Cumming 2010, figs 1G, 2E). Female legs with pennate scales on flattened hind femur, hind tibia and often on mid femur (Rafael & Cumming 2010, figs 1E, F, 4D, E), rarely on mid tibia.................................. M. pictipennis species-group

- Male and females wings infuscated on costal and distal margins to almost entirely hyaline (Rafael & Cumming 2009, fig. 2A, F; Rafael & Cumming 2015, figs 54–63); pterostigma slightly darker. Female legs with pennate scales only on hind tibia dorsally, sometimes present ventrally or hind tibia without pennate scales................................................ 3

3 Male tergite 8 with posteromedial projection, with apex acute (Rafael & Cumming 2009, figs 1B, 2B, 3B), or apex blunt (Rafael & Cumming 2009, figs 4B, 6B) or apex bifid (Rafael & Cumming 2009, fig. 5B)................................... 5

- Male tergite 8 without posteromedial projection beyond posterior margin (Rafael & Cumming 2012, figs 5, 29, 36) or with two posterolateral protuberances apically (Rafael & Cumming 2012, fig. 17).......................................... 4

4 No supra-alar postsutural setae; 1–2 pairs of scutellar setae. Cell dm somewhat blunt posteriorly (Rafael & Cumming 2015, figs 54–63). Hypandrium generally narrowed on distal third of posterior margin (Rafael & Cumming 2015, figs 14, 22, 33) or notched, with small sinus on distal third (Rafael & Cumming 2015, fig. 5)................. M. amazonensis species-group

- One supra-alar postsutural seta; 2 pairs of scutellar setae. Cell dm slightly projected posteriorly (Rafael & Cumming 2012, figs 4, 15, 16, 20, 27). Hypandrium somewhat same diameter along entire length (Rafael & Cumming 2012, figs 7, 19) or distinctly widened on distal half (Rafael & Cumming 2012, figs 31, 38)............................ M. limbipennis species-group

5 Hypandrium generally slender, narrowed and nearly same diameter along entire length with pair of distal protuberances (Rafael & Cumming 2009, figs 1E, 2E).................................................... M. ferrugineus species-group

- Hypandrium variable, generally widened, with somewhat translucent membranous area basally (Rafael & Marques 2021, figs 10D, 11J, 13N)..................................................................... unplaced species-group