Key to the species groups of Hesperopenna

1 Pronotum less transverse, 1.30–1.60 times as wide as long. .......................................... 2

– Pronotum more transverse, 1.60–2.20 times as wide as long. ....................................... 4

2 Antennae longer than body, antennomere III about 1.5 – 3.5 times longer than anntenomere II. Pronotum 1.3 – 1.4 times as wide as long. Aedeagus simple, long, thin, with one internal sclerite (Fig. 67). Body length 4.2 – 5.7 mm. .................... Hesperopenna pallida group

– Antennae shorter than body. ........................................................................................... 3

3 Larger species (body length 6.6–10.0 mm). Pronotum with very fine, almost indistinct punctures. Aedeagus long, robust, tubular, with basal orifice large and long (Figs 11–18). ..................................................................................... Hesperopenna medvedevi group

– Smaller species (body length 3.7–7.0 mm). Pronotum with fine dense punctures, often with two or four small slightly elevated areas on disc. Aedeagus with relatively short proximal part, apex often bent down (Fig. 70). ...................... Hesperopenna granulicollis group

4 Aedeagus short, simple (Fig. 68), without long dorsolateral projections. Antennomere III about 1.3–1.5 times longer than anntenomere II. Small species (body length 3.2 – 5.0 mm). ................................................................................... Hesperopenna minor group

– Aedeagus dorsolaterally with two thin processes, ventrally often with apical hook-like process or long projection from basal part directed anteriorly. ...................................... 5

5 Aedeagus ventrally with or without apical hook-like process (Fig. 66), always without long projection from basal part directed anteriorly. Antennomere III about 1.5 – 3.0 times longer than anntenomere II. Body length 5.1 – 7.1 mm. ..................................................... ..................................................................................... Hesperopenna vietnamica group

– Aedeagus ventrally with long projection from basal part directed anteriorly. Antennomere III about 1.3 – 1.5 times longer than anntenomere II. Body length 3.8 – 6.5 mm. ............... ............................................................................................ Hesperopenna bicolor group