Oosternum convexum, 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 : 105-106

Oosternum convexum species group

Differential diagnosis. Eyes very small, separated by 10–11 widths of one eye ( Fig. 16 ); pronotum slightly more convex than elytra, without longitudinal sulci and ridges ( Fig. 2 ); median part of prosternum not demarcated from lateral portions by sharp ridges, only bearing different microsculpture ( Fig. 9 ); elytral interval 2 as wide as interval 3, reaching elytral apex ( Fig. 10 ); elytral series 10 completely absent; lateral margins of elytra with minute denticulation, distinct at least basally ( Fig. 11 ); metathoracic wings absent in all specimens in which this character was examined; preepisternal plate of mesothorax 1.7–2.7× as long as wide ( Figs. 19–21 ); median portion of metaventrite narrow, pentagonal; femoral lines of metaventrite absent ( Figs. 19–21 ).

The species of O. convexum group can be easily recognized from other Oosternum by the rather convex body, brown to pale brown coloration ( Figs. 1–2 ), very small eyes and finely denticulate elytral margins (distinct especially at elytral base). In all these characters they are similar to Motonerus apterus Fikáček & Short, 2006 and M. oosternoides Fikáček & Short, 2006 , which are also apterous. These two Motonerus species can, however, be differentiated from the O. convexum group by ventral characters, i.e. the absence of antennal grooves, the median portion of prosternum divided from the lateral parts and bearing an anteromedian excision, very narrow preepisternal elevation of the mesothorax, and absence of anterolateral ridge on the metaventrite ( FIKÁČEK & SHORT 2006).

Description. Body in dorsal view elongate oval, widest near midlength; in lateral view highly convex, gradually narrowing posteriad. Elytra unicoloured, without sharply delimited spots.

Head. Each puncture on clypeus and frons bearing fine semi-erected yellowish seta. Anterior margin of clypeus slightly concave with distinct rim. Interocular area not elevated above remaining head surface; interstices between punctures on frons without microsculpture. Eyes extremely small, separated by 10–11 widths of one eye. Mentum 2.3× as wide as long, narrowing anteriad; anterior margin slightly emarginate; anteromedian part impressed in contrast to lateral and posterior portions; surface of mentum flat, bearing small rounded punctures, interstices without microsculpture ( Figs. 17 and 18 ). Gula wide, gular sutures diverging posteriad, posterior tentorial pits minute. Maxilla of male with sucking disc; maxillary palpus with palpomeres 2 and 4 ca. 1.5× as long as palpomere 3, palpomere 2 strongly widened apically, palpomere 4 spindle-like. Antenna with nine antennomeres, scapus approximately as long as antennomeres 2–6 combined; antennal club compact; antennomere 9 forming distal half of antennal club, indistinctly constricted subapically, blunt at apex.

Prothorax. Pronotum arcuately narrowed anteriad, slightly emarginate on anterior margin, continuously arcuate on posterior margin, evenly convex (lacking longitudinal sulci), slightly more convex than elytra in lateral view ( Fig. 2 ); lateral margins of pronotum not deflexed, smooth, not serrate, with narrow marginal rim; posterolateral corners forming obtuse angle. Pronotal punctation uniform, nearly similar on whole surface, each puncture bearing fine decumbent yellowish seta; transverse row of punctures on posterior margin of pronotum absent; interstices without microsculpture. Prosternum ( Fig. 9 ) with low and narrow median carina; mesal part of prosternum not divided from lateral portions, only bearing different microsculpture; anteromedian excision of prosternum absent; pair of deep pits at the border between mesal and lateral prosternal portions absent. Antennal grooves moderately large, not reaching lateral margins of hypomeron; lateral glabrous part of hypomeron very narrow anteriorly, slightly widened posteriorly.

Mesothorax. Scutellar shield small, in shape of equilateral triangle, interstices without microsculpture. Elytron not deflexed laterally, bearing nine punctural series; series 7 and 8 as widely separated from each other as from adjacent series; series 9 arising subbasally; each serial puncture with minute seta (indistinct under stereomicroscope) and with small but distinct denticle on each side of the puncture at least in basal portion of elytron ( Fig. 14 ). Elytra not costate, intervals weakly convex at suture, becoming more convex laterad and posteriad; interval 2 ( Fig. 10 ) as wide as interval 3 (but see Variability under O. convexum sp. nov.), as high as intervals 1 and 3 and reaching elytral apex. Interval punctation arranged into series, at least on some intervals; series consisting of minute, slightly transverse punctures, each bearing a fine, decumbent yellowish seta; interstices without microsculpture. Lateral margin of elytron finely denticulate, at least basally. Epipleuron very narrow ( Fig. 11 ), much narrower than pseudepipleuron basally, narrowing posteriad, reaching level of metathorax; pseudepipleuron narrowing posteriad, reaching elytral apex. Preepisternal plate ( Figs. 19–21 ) broadly attached to metaventrite, slightly overlapping its anterior margin, 1.7–2.7× as long as wide; interstices without microsculpture. Grooves for reception of procoxae well developed, not reaching anterior margin of mesocoxal cavities ( Figs. 19–21 ).

Metathorax. Metaventrite ( Figs. 19–21 ) with subpentagonal elevate median portion lacking depression in both sexes; interstices shiny, without microsculpture; lateral portions dull, with fine microsculpture. Anterolateral ridges distinctly developed throughout width of metaventrite, not reaching its anterolateral corner, bent posteriad laterally; femoral lines absent. Metathoracic wings absent.

Legs short; tarsi slightly shorter than tibiae, bearing short and sparse pubescence ventrally. Protibiae not emarginate on outer margin distally.

Abdomen with five ventrites covered with hydrofuge pubescence. Ventrite 1 carinate medially, without additional longitudinal ridges laterally. Ventrites 2–5 flat, without longitudinal ridges.

Male genitalia. Tergite 8 with long narrow anteromedian projection ( Fig. 15 ). Sternite 9 without posteromedian projection, angulate ( Fig. 15 ). Aedeagus ( Figs. 3–8 ) with parameres lacking inner projection, bearing two long setae apically. Phallobase narrow, 2.0–2.3× as long as wide. Median lobe slightly longer than parameres, projecting slightly beyond apices of parameres; lateral margin of median lobe bearing fine setae directed posteriad; corona present, situated at apex of median lobe; apex lacking row of setae.

Figures

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. 1–2. Habitus of Oosternum convexum sp. nov.: 1 – dorsal view, 2 – lateral view. Body length: 1.75 mm.

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.

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.