Oosternum intermedium, 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 : 111-112

Oosternum intermedium sp. nov.

( Figs. 5, 6 , 13, 16 , 20 , 24 )

Type locality. Costa Rica, San José Province, Estación Cuerici, 4.6 km E of Villa Mills, 09°34′N 83°40′W, 2600 m a.s.l.

Type material. HOLOTYPE: J ( INBio): ‘COSTA RICA: San Jose Prov. / Estacion Cuerici, 4.6 km E. of / Villa Mills, 19.vi.1997 -019E / 09°34′N, 83°40′W, 2600 m / R.S.Andreson, oak for. litter’ . PARATYPES (29 spec.): COSTA RICA: 1 J, 1 ♀ ( KSEM): ‘COSTA RICA: San Jose Prov. / Cuerici Station, 4.6 km E. of / Villa Mills , 26.vi.1997 (97-036D) / 09°34′N 83°40′W ; 2600m / R. Anderson; oak forest litter’; 6 JJ, 1 ♀, 8 spec. ( INBio, KSEM, NMPC, NHMW): ‘COSTA RICA: Cartago / 2.0 km E. Villa Mills / 2750 m, 9°34′0″N, 83°41′50″W / 15 FEB 1998 ; R. Anderson / CR2A98 002 ex: oak forest litter’; 1 J, 1 ♀ ( KSEM): ‘COSTA RICA: Cartago / 2.0km E. Villa Mills, 2750m / 9°34′N, 83°41′50″W, / 15.ii.1998, R. S. Anderson / oak for. litter CR2A98 002 ’; 1 J, 1 spec. ( KSEM): ‘COSTA RICA: San Jose Prov. / km 117 Pan-Am. Hwy / 19km N. San Isidro ; 15.vii.1998 / 09°28′N 83°42′20″W; 1800m. / R. Andersen [sic!], cloud forest litter’; 3 JJ, 1 ♀, 4 spec. ( KSEM, ZMUC): ‘COSTA RICA: San Jose Prov. / km 117 Pan-Am. Hwy. 97-021C [sic!] / 19km N. San Isidro , 20.vi.1998 / 09°28′N 83°42′20″W ; 1800m / R. Anderson; cloud forest litter’.

Differential diagnosis. Preepisternal plate of mesothorax 1.9–2.2× as long as wide ( Fig. 20 ); anterolateral ridges arcuately bent posteriad near lateral margin of metaventrite ( Fig. 20 ); punctation of median portion of metaventrite with moderately sized punctures ( Fig. 20 ); mentum with dense punctation (as in Fig. 18 ); number of spines of inner ventral mesotibial series sexually dimorphic (as in Figs. 12 and 13 ); aedeagus small (0.55–0.65 mm); median lobe with slightly concave lateral margins subapically; parameres narrowly pointed apically ( Figs. 5 and 6 ).

Oosternum intermedium sp. nov. is externally very similar to O. simplex sp. nov., differing from the latter only by the slightly wider 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 = 1.65. Body length: 1.20–1.55 mm (length of holotype: 1.55 mm); width: 0.80–0.90 mm (width of holotype: 0.90 mm).

Coloration. Dorsal side uniformly pale brown. Ventral side castaneous, epipleura pale brown; femora, tibiae and maxillary palpi brownish, tarsi and antennae yellowish.

Head. Clypeus with moderately dense punctation consisting of moderately large punctures, interstices without microsculpture. Frons with dense punctation consisting of large, rounded, uniformly shaped punctures. Eyes separated by 10 widths of one eye. Mentum with moderately dense punctation (as in Fig. 18 ); submentum without poriferous disc-like areas. Antennal club approximately 2.5× as long as wide.

Prothorax. Punctation of pronotum as dense as on head, consisting of rather large, slightly transverse to rasp-like punctures. Median carina of prosternum straight in lateral view.

Mesothorax. Scutellar shield bearing three minute punctures. Elytral series 1–7 reaching elytral base, series 8 and 9 arising subbasally; serial punctures fine and sparse; denticulation of lateral elytral margin distinct mainly basally. Preepisternal plate ( Fig. 20 ) narrow, suboval, 2.2× as long as wide; median part flat, bearing a few moderately large, sparsely distributed setiferous punctures.

Metathorax ( Fig. 20 ). Punctation of median portion of metaventrite sparse, consisting of small setiferous punctures. Anterolateral ridge arcuately bent posteriad. Anepisternum 4.4× as long as wide.

Legs. Inner ventral series of stout spines on metatibiae sexually dimorphic, with more apical spines in males (as in Figs. 12 and 13 ).

Male genitalia ( Figs. 5 and 6 ). Aedeagus 0.55–0.65 mm long. Parameres 0.6× as long as phallobase, nearly straight on outer margin, arcuately narrowed apicad on inner margin. Phallobase 2.3× as long as wide. Median lobe slightly widened from base towards apical 0.75, then narrowing towards apex, lateral margins slightly concave subapically; lateral projections of median lobe present, wide, but developed only subapically.

Variability. Specimens examined vary slightly in the shape of the preepisternal plate of mesothorax, which is 1.9–2.2× as long as wide. Male genitalia vary slightly in the extent of the constriction of the apical portion of the median lobe (i.e., lateral margins are more or less concave subapically); most males examined except of the holotype are, however, teneral and the observed variability can therefore be artificial.

Etymology. Referring to the shape of the apical portion of the median lobe of the aedeagus, which is slightly constricted and therefore intermediate between O. simplex sp. nov. and O. convexum sp. nov.

Bionomics. All specimens were sifted from leaf litter in oak forests at altitudes of 1,800 –2,750 m a.s.l.

Distribution ( Fig. 24 ). Known only from three closely situated localities in the northern part of the Cordillera de Talamanca Mts., close to the border of the Chirripó National Park.

Figures

Figs. 3–8.Aedeagophores.3, 5, 7 – whole aedeagus; 4, 6, 8 – detail of apical portions of parameres and median lobe. 3–4 – Oosternum simplex sp. nov.; 5–6 – O. intermedium sp. nov.; 7–8 – O. convexum sp. nov.

Figs. 9–16. Diagnostic characters of the Oosternum convexum species group. 9 – prosternum; 10 – elytral apex (arrow: elytral interval 2); 11 – detail of basal portion of epipleuron (note marginal denticulation); 12 – ventral surface of mesotibia (female); 13 – ditto male (arrow: multiplied spines in inner row); 14 – detail of elytral surface (arrows: small denticles at each serial puncture); 15 – terminal abdominal sclerites of male (st8: sternite 8; st9: sternite 9; ter8: tergite 8); 16 – head in lateral view. 9, 11, 14 – O. convexum sp. nov.; 10, 12 – O. simplex sp. nov.; 13, 15, 16 – O. intermedium sp. nov.

Figs. 17–23. Differential characters of species of Oosternum convexum group. 17–18 – mentum; 19–21 – meso- and metaventrite (arrow: different shapes of lateral part of anterolateral ridge of metaventrite); 22–23 – right elytron (arrow: elytral interval 3). 18, 20, 22–23 – O. convexum sp. nov.; 19, 22 – O. simplex sp. nov.; 20 – O. intermedium sp. nov.

Fig. 24. Distribution of the Oosternum convexum species group.