Key to male Hercostomus sensu lato from mangroves in Southeast Asia (after Zhang et al. 2008) 1 Wing with grey to brown pterostigma at tip R 1; R 4+5 thickened from basal quarter onwards (Fig. 13).................... 2 - Wing lacking stigma at tip R 1; R 4+5 not thickened............................................................ 3 2 Pterostigma elongate, extending beyond start of thickening of vein R 4+5 (Fig. 13); knee of hind femur darkened above (Figs 13, 14); aedeagus with small denticle, in more basal position........................... H. lanceolatus Zhang et al., 2008 - Pterostigma shorter, reaching level of thickening of vein R 4+5; knee of hind femur not darkened above; aedeagus with larger denticle in more anteriad position................................................. H. limosus Zhang et al., 2008 3 First flagellomere distinctly elongated, at least 2.0 times as long as wide (Figs 1, 10)............................... 4 - First flagellomere not elongated, at most 1.6 times as long as wide.............................................. 7 4 First flagellomere with blunt tip.......................................... H. obtusus Samoh & Grootaert sp. nov. - First flagellomere with pointed tip (Fig. 10)................................................................ 5 5 First flagellomere 2.3 times as long as wide (Fig. 4); all coxae blackish; femora blackish except tip of fore and mid femora; cercus with at least 3 strong bristles on outer margin......................................................... 6 - First flagellomere 3.0 times as long as wide; fore coxa yellow, mid and hind coxae brown, femora yellow; cercus without strong bristles on outer margin.......................................................... H. meieri Zhang et al., 2008 6 Hind femur with two long ventral bristles at base of hind femur, nearly as long as femur is deep; cercus with only three strong, black squamiform bristles on outer margin..................................... H. brevidigitalis Zhang et al., 2008 - Hind femur without long ventral bristles at base; cercus with seven strong, black squamiform bristles along outer margin.................................................................... H. squamatus Samoh & Grootaert sp. nov. 7 First flagellomere as long as wide (Fig. 8)................................................................. 8 - First flagellomere longer than wide: 1.3–1.6 x as long as wide................................................. 9 8 Fore coxae yellow, mid and hind coxae black; cercus triangular; aedeagus with strong subapical tooth............................................................................................ H. tiomanensis Zhang et al., 2007 - All coxae yellow, mid coxae at most pale brownish with black anterior stripe; cercus strongly curved, almost geniculate; aedeagus with long slender tip, with 2 inner denticles.............................. H. brevicornis Zhang et al., 2008 9 Fore tarsomere 1 shortened, concave ventrally with 2 long curved ventral bristles; mid tarsomere 1 concave ventrally, with 1 strong ventral bristle near middle and 5 short ventral bristles basally; cercus large, with short apical tail........................................................................................ H. singaporensis Zhang et al., 2008 - Fore tarsus unmodified (Fig. 15); cercus triangular, without apical tail................... H. plumatus Zhang et al., 2008

Published as part of Samoh, Abdulloh & Grootaert, Patrick, 2024, New species and records of the genus Hercostomus Loew (Diptera: Dolichopodidae) from Thailand mangroves, with notes on the Hercostomus fauna of Singapore mangroves, pp. 179-204 in Zootaxa 5446 (2) on pages 195-200, DOI: 10.11646/zootaxa.5446.2.2, http://zenodo.org/record/11101578