Key to male adults of Rheotanytarsus from South America [emended from Andersen & Kyerematen (2001)]

1. Median volsella with apical plate, reaching apex of superior volsella or shorter; wing length <2 mm ..................... 2

- Median volsella straight and slender with apical subulate setae, extends beyond of superior volsella; wing length> 2 mm. Chile, Argentina (Reiss 1972, fig. 11) .................................................................. Rheotanytarsus globosus Reiss

2. Median volsella with distal setae fused into a rounded plate ....................................................................................... 3

- Median volsella with distal setae fused into a distinct petal-like apical plate. Peru (Andersen & Kyerematen 2001, figs 9–12) ...................................................................................................................... Rheotanytarsus pelus (Roback)

3. Superior volsella oval, apically rounded or pointed, with 5 or 6 dorsal setae .............................................................. 4

- Superior volsella subtriangular, with 4 dorsal setae; wing length c. 1.10 mm. Brazil (Andersen & Kyerematen 2001, figs 1–8) ................................................................................. Rheotanytarsus pantanalensis Andersen & Kyerematen

4. Median volsella reaching apex of superior volsella; superior volsella with 5 dorsal setae, apically rounded; wing length c. 1.40 mm; Puerto Rico, Argentina ................................................... Rheotanytarsus meridionalis (Johannsen)

- Median volsella never reaching apex of superior volsella; superior volsella with 6 dorsal setae, apically pointed; wing length c. 1.31 mm. Chile, Argentina .................................................................. Rheotanytarsus lamellatus Reiss