Key to Oosternum species groups

1. Pronotum bearing weak (see Fig. 13 in Smetana 1978) to very high longitudinal costae (Figs. 11–12). Frons between eyes elevated above remaining surface (Fig. 27). Prosternal plate with large lateral cavities (Fig. 23). Elevated por- tion of metaventrite nearly reaching lateral margins of metaventrite, not pentagonal (Fig. 29). ..... O. sculptum group

- Pronotum evenly convex, without any trace of longitudinal costae. Interocular portion of frons not elevated (Fig. 28). Prosternal plate without deep lateral cavities (Figs. 24–26). Elevated portion of metaventrite pentagonal, con- fined to median portion of the metaventrite (Figs. 30–31) .......................................................................................... 2

2. Elytral series 10 short but present, situated anteriorly between series 9 and elytral margin (Fig. 17) ........................... ....................................................................................................................................................... O. soricoides group

- Elytral series 10 absent (Fig. 18) .................................................................................................................................. 3

3. Pronotum more convex than elytra (Figs. 15–16) ....................................................................................................... 4

- Pronotum not more convex than elytra (Figs. 13–14). ................................................................................................. 6

4. Median portion of prosternum at most slightly elevated, weakly demarcated from lateral portions (Fig. 24). Elytral interval 2 as wide as interval 3 (Fig. 22)..................................................................................................... O. group A

- Median portion of prosternum demarcated from lateral portions by sharp ridges (Figs. 25–26). Elytral interval 2 much narrower than interval 3 ...................................................................................................................................... 5

5. Prosternal plate narrow, 1.0–1.5× wider than long (Figs. 26, 66–71). Median lobe of aedeagus with lateral projec- tions (Figs. 36–41) .................................................................................................................... O. aequinoctiale group

- Prosternal plate very wide, ca. 2.0× wider than long (as in Fig. 25). Median lobe of aedeagus without lateral projec- tions ............................................................................................................................................................ O. group B

6. Median portion of prosternum demarcated from lateral parts by sharp ridges (Figs. 25–26) ...................................... 7

- Median part of prosternum at most slightly elevated, not demarcated from lateral portions by sharp ridges (as in Fig. 24) ................................................................................................................................................................................. 9

7. Elytral interval 2 slightly narrower than interval 3, quickly narrowing posteriad and not reaching elytral apex (Fig. 21) .................................................................................................................................................... O. sahlbergi group

- Elytral interval much narrower than interval 3, continually narrowing posteriad, reaching elytral apex (Figs. 19–20). ...................................................................................................................................................................................... 8

8. Prosternal plate ca. as long as wide (as in Fig. 26)............................................................................... O. sharpi group

- Prosternal plate very wide, ca. 2.0× wider than long (Fig. 25)....................................................... O. attacomis group

9. Elytral interval 2 much narrower than interval 3 (as in Fig. 20) ................................................... O. pubescens group

- Elytral interval 2 ca. as wide as interval 3 (as in Fig. 22) ............................................................................. O. group C