Oosternum simplex, Fikáček & Hebauer, 2009

Fikáček, Martin & Hebauer, Franz, 2009, Taxonomic revision of the New World species of the genus Oosternum Sharp (Coleoptera: Hydrophilidae: Sphaeridiinae) II. The Oosternum convexum species group, Acta Entomologica Musei Nationalis Pragae 49 (1), pp. 103-117 : 112-115

publication ID

https://doi.org/ 10.5281/zenodo.5318857



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scientific name

Oosternum simplex

sp. nov.

Oosternum simplex View in CoL sp. nov.

( Figs. 3–4 View Figs , 10, 12 View Figs , 18, 21 View Figs , 24 View Fig )

Type locality: Costa Rica, border of San José and Cartago Provinces, 3 km S of El Empalme, 9°42′30″N 83°57′W, 2350 m a.s.l.

Type material. HOLOTYPE: J ( INBio): ‘COSTA RICA: SanJose/Cart./ km.55, PanAm Hwy, 3 km S. / of El Empalme , R. Anderson / 09°42′30″N, 83°57′W, 2350 m / 8.vi.1997 -008B, oak forest litter’ GoogleMaps . PARATYPES (18 spec.): COSTA RICA: 2 spec. ( KSEM, NMPC): same label data as holotype GoogleMaps ; 1 ♀ ( KSEM): ‘COSTA RICA: S. José / Carta. / Int. Am. Hwy, km 55, 3 km S. / of El Empalme , 2350m, 8.vi.1996 [sic!] / oak forest litter ; R. Anderson / 9°42′30″N, 83°57′W GoogleMaps ; 97-008C’; 7 JJ, 6 spec. ( INBio, KSEM, NHMW, NMPC): ‘COSTA RICA: Cartago / Esperanza El Guarco, 2600m / 09°41′35″N, 83°52′04″W / 26-27-III-2003, R. Anderson / berlese oak forest litter’ GoogleMaps ; 2 JJ ( KSEM, ZMUC):‘COSTA RICA: San Jose / Cart. Prov. / km 45.Pan-Am. Hwy. (09-007D) / 6 km N. E. El Empalme ; 8.vi.1997 / 09°45′N 53°30′W; 1975m / R. Anderson, mixed alder/oak litter’.

Differential diagnosis. Preepisternal plate of mesothorax 2.5–2.7× as long as wide ( Fig. 21 View Figs ); anterolateral ridges arcuately bent posteriad near lateral margin of metaventrite ( Fig. 21 View Figs ); punctation of median portion of metaventrite with moderately sized punctures ( Fig. 21 View Figs ); mentum with dense punctation ( Fig. 18 View Figs ); number of spines of inner ventral mesotibial series similar in both sexes ( Fig. 12 View Figs ); aedeagus small (0.60–0.65 mm); median lobe simple, not constricted or concave subapically; parameres narrowly pointed apically ( Figs. 3 and 4 View Figs ).

Oosternum simplex sp. nov. is externally very similar to O. intermedium sp. nov., differing from the latter only by the narrower preepisternal plate of the mesothorax. Examination of male genitalia, especially the shape of apical portion of the median lobe, is needed for a reliable identification of both species. For differences from O. convexum sp. nov., see under the latter species.

Description. Body elongate oval, body length / body width ratio = 1.7. Body length: 1.25–1.55 mm (length of holotype: 1.50 mm); width: 0.80–0.90 mm (width of holotype: 0.90 mm).

Coloration. Dorsal side brown, clypeus and lateral margins of pronotum and elytra yellowish brown. Ventral side reddish brown, epipleura yellowish brown; femora, tibiae, maxillary palpi and antennal clubs reddish, tarsi and antennomeres 1–6 yellowish.

Head. Clypeus with moderately dense punctation consisting of moderately large punctures; interstices without microsculpture. Frons with dense punctation consisting of large, rounded, uniform punctures. Eyes separated by 10 widths of one eye. Mentum with moderately dense punctation; submentum without poriferous disc-like areas. Antennal club approximately 2.3× as long as wide.

Prothorax. Punctation of pronotum slightly denser than on head, consisting of moderately large to large, slightly transverse to rasp-like punctures. Median carina of prosternum slightly concave in lateral view.

Mesothorax. Scutellar shield bearing few moderately large punctures. Elytral series 1–7 reaching elytral base, series 8 and 9 arising subbasally; serial punctures fine and sparse; denticulation of lateral elytral margins distinct mainly basally. Preepisternal plate ( Fig. 21 View Figs ) narrow, suboval, 2.6× as long as wide; median part flat, bearing a few moderately large, densely arranged setiferous punctures.

Metathorax ( Fig. 21 View Figs ). Punctation of median portion of metaventrite sparse, consisting of small setiferous punctures. Anterolateral ridges arcuately bent posteriad. Anepisternum 4.5× as long as wide.

Legs. Inner ventral series of stout spines on metatibia similar in both sexes ( Fig. 12 View Figs ).

Male genitalia ( Figs. 3 and 4 View Figs ). Aedeagus 0.60–0.65 mm long. Parameres 0.8× as long as phallobase, nearly straight on outer margin, slightly arcuately narrowed towards apex on inner margin. Phallobase 2.2× as long as wide. Median lobe widest in apical 0.4, slightly narrowed both basad and apicad, lateral margins convex subapically; lateral projections of median lobe absent.

Variability. Oosternum simplex sp. nov. is rather variable in the shape of the preepisternal plate of the mesothorax. In most specimens the plate is 2.5–2.7× as long as wide but one male from Esperanza El Gauco (deposited in KSEM) has the plate 3.4× as long as wide. The specimens are, however, identical in other external characters as well as in the morphology of male genitalia.

Etymology. Referring to the simple (i.e. subapically unconstricted) shape of the median lobe of the aedeagus.

Bionomics. The type series was sifted from leaf litter in oak forests at altitudes of 1,975 –2,600 m a.s.l.

Distribution ( Fig. 24 View Fig ). Known from three nearby localities in the northern part of the Cordillera de Talamanca Mts., ca. 7–10 km from the borders of the Tapanti National Park.


National Biodiversity Institute, Costa Rica


National Museum Prague


Naturhistorisches Museum, Wien


Zoological Museum, University of Copenhagen













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