Taxonomic revision of New World species of the genus Oosternum Sharp (Coleoptera: Hydrophilidae: Sphaeridiinae) I. Definition of species groups and revision of the Oosternum aequinoctiale group Author Hebauer, Martin Fiká Č Ek Franz Author Hansen, Michael text Zootaxa 2009 2054 1 37 journal article 10.5281/zenodo.186668 7a3d78ed-d4c6-464b-b8ba-a93844a3cf85 1175-5326 186668 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