Oosternum aequinoctiale

Hebauer, Martin Fiká Č Ek Franz & Hansen, Michael, 2009, 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, Zootaxa 2054, pp. 1-37 : 11-12

Oosternum aequinoctiale species group

Differential diagnosis. Eyes large to small, separated by 5–8.5× of one eye. Pronotum more convex than elytra, without longitudinal sulci and ridges; elytral interval 2 much narrower than interval 3, reaching elytral apex; median part of prosternum sharply demarcated from lateral portions, 1.0–1.5× longer than wide; elytral series 10 completely absent; preepisternal plate of mesothorax 1.3–2.0× longer than wide; median portion of metaventrite narrow, pentagonal, femoral lines of metaventrite absent; median lobe with variably shaped lateral projections.

Description. General habitus and coloration: Body elongate to widely oval, moderately to highly convex in lateral view. Coloration of dorsal side reddish to black, elytra unicolored. Pronotum slightly to apparently more convex than elytra in lateral view.

HEAD: Clypeus with dense punctation; anterior margin of clypeus with narrow rim. Interocular area not elevated above remaining head surface; eyes large to small, separated by 5.0–8.5× of width of one eye. Mentum 2.0–2.8× wider than long, narrowing anteriad, with deep median depression anteriorly; remaining surface flat, bearing sparse punctation. Gula wide, gular sutures divergent posteriad; posterior tentorial pits minute. Maxilla of male with sucking disc (not examined in O. latum ); margins of the disc bare, without fine setae (examined only in O. holosericeum ). Maxillary palpomere 2 strongly widened apically; palpomere 4 spindle-like. Antenna with 9 antennomeres, antennal club compact, 2.0–2.3× longer than wide; ultimate antennomere blunt at apex, slightly constricted subapically, forming distal 0.5 of antennal club.

PROTHORAX: Pronotum arcuately narrowing anteriad, weakly bisinuate on posterior margin, evenly convex (without bulges or longitudinal sulci), not deflexed laterally; posterolateral corners obtusely rounded. Lateral margin of pronotum without denticulation, narrowly rimmed. Punctation of pronotum dense, each puncture bearing fine, decumbent yellowish seta; transverse row of punctures on posterior margin of pronotum absent. Prosternum carinate medially, median carina narrow; median portion of prosternum divided from lateral portions by sharp ridges; deep excavations next to these ridges present. Median portion of prosternum 1.0–1.5× wider than long, without anteromedian excision and transverse ridges. Antennal grooves not reaching lateral margins of hypomeron, well developed, moderately large, sharply angulate laterally. Lateral glabrous part of hypomeron very narrow anteriorly, apparently widening posteriorly.

MESOTHORAX: Scutellar shield small, in form of equilateral triangle. Elytron not deflexed laterally, bearing 9 punctural series; series 10 absent; serial punctures bearing minute setae (not seen under binocular microscope), intervals punctures bearing fine, long decumbent setae. Elytral interval 2 narrower and lower than interval 3, reaching elytral apex. Lateral margin of elytron without denticulation. Epipleuron very narrow, much narrower than pseuepipleuron; pseudepipleuron wide, narrowing posteriad, reaching elytral apex. Preepisternal elevation 1.3–2.0× longer than wide, widely attached to metaventrite posteriorly, slightly overlapping anterior margin of metaventrite. Grooves for reception of procoxae well developed, not reaching anterior margin of mesocoxal cavities.

METATHORAX: Metaventrite with subpentagonal elevate median portion, lateral portions dull, with fine microsculpture. Median part of metaventrite without depressions in both sexes. Anterolateral ridges distinctly developed throughout whole width of metaventrite, bent posteriad laterally and therefore not reaching anterolateral corners of metaventrite; ridges not meeting together mesally. Femoral lines absent. Anepisternum 6.6–7.4× longer than wide. Hind wings present, normally developed (macropterous).

LEGS: Legs short; protibia not emarginate on outer margin apically; tarsi slightly shorter than tibiae.

ABDOMEN: Abdomen with five ventrites; first ventrite carinate medially, with superficial structure as on ventrites 2–5 (not coarser); ventrites 2–5 flat, without longitudinal ridges.

MALE GENITALIA: Tergite 8 with long narrow anteromedian projection. Parameres 1.1–1.5× longer than phallobase, narrowing apicad, bearing two setae apically. Phallobase narrow, 1.4–1.8× longer than wide. Median lobe slightly longer than parameres, projecting slightly further than apices of parameres distally; apical portion of median lobe with lateral projections bearing sparse, small, backward-directed setae; corona absent; apex of median lobe without rows of setae.