Omalium diffine Sharp, 1889

Shavrin, Alexey V., 2025, Review of the genus Omalium Gravenhorst, 1802 (Coleoptera: Staphylinidae: Omaliinae: Omaliini) of Siberia and Far Eastern Russia, with notes on some species from Mongolia and Japan, Zootaxa 5646 (2), pp. 199-235 : 217-219

3.2.4.3. Omalium diffine Sharp, 1889

( Figs 8–9 , 12–13 , 24 , 32 )

Omalium diffine Sharp, 1889: 474 ; Watanabe 1990: 99.

Omalium (Omalium) diffine : Bernhauer & Schubert, 1910: 52.

Type material examined. Holotype of Omalium diffine Sharp, 1889 ♀ ( Fig. 24 ; dissected; underside of the card with handwritten numbers: ‘8.5.81’): ‘Type’ <round label with red margin>, ‘ Japan [underlined by yellow] | G. Lewis. | 1910-320.’ <printed>, ‘Hitoyoshi. | 3.V.–8.V.81.’ <printed>, ‘ Omalium | diffine. | Type | D.S.’ <handwritten in black>,‘NHMUK015009857’ <printed, with barcode on the left side of the label>, ‘ Omalium | japonicum Sharp, 1874 | Shavrin A.V. det. 2025’ ( BMNH).

Material examineD. JAPAN: 1 ♂: ‘Japan Tokyo Tachikawa 6.V.39Y.Yano’, ‘M. Cameron. Bequest. B.M.1955- 147.’, ‘ Omalium japonicum Shp. ’, ‘ Omalium diffine Sharp P.M. Hammond det. 1989’ ( BMNH) .

Redescription. Measurements (n=2): HW: 0.42–0.45; HL: 0.25–0.26; OL: 0.13–0.15; TL: 0.04–0.05; AL (holotype): 0.67; PL: 0.33–0.35; PWmax: 0.52–0.56; PWmin: 0.47–0.50; ESL: 0.65–0.72; EW: 0.67–0.75; MTbL (holotype): 0.32; MTrL (holotype): 0.22 (MTrL 1–4: 0.07; MTrL 5: 0.15); AW: 0.67; AedL: 0.47; BL: 2.20 (holotype)–2.38.

Habitus as in Fig. 24 . Body and antennomeres 6–11 brownish, with distinctly paler elytra; mouthparts, antennomeres 1–5 and legs yellow. Head with sparse and fine punctation, denser in middle (holotype without distinct punctures on infraorbital portions); neck with very sparse and fine punctation; punctation of pronotum denser, larger and deeper than that in middle part of head, sparser in middle and mediobasal portions and finer in basal area; punctation of elytra about as that on pronotum, but slightly larger and deeper, finer and sparser in middle parts of each elytron; abdominal tergites without visible punctation. Head with dense irregular microsculpture: transverse in middle of clypeus and dense and diagonal in each laterobasal parts of it, middle portion with diagonal and isodiametric meshes and infraorbital portions with diagonal and longitudinal microreticulation; neck with fine indistinct longitudinal microsculpture; pronotum, elytra and scutellum without sculpture; abdominal tergites with dense isodiametric microreticulation. Head with several erect and long setae on anterior part of clypeus and with additional separate long erect setae on each laterobasal part of clypeus and posteriolateral parts of infraorbital portions; lateral portions of pronotum with indistinct sparse and short setation; abdominal tergites with fine and relatively dense setation.

Head transverse, 1.6–1.7 times as broad as long, indistinctly elevated in middle, with broad slightly elongate clypeus and distinctly explanated and convex supra-antennal elevations; anteriomedian depressions moderately deep and wide; each posteriolateral margins of clypeus slightly narrowed basad and reaching level of anterior third of eye. Median surface with fine diagonal elevations between punctures, with indistinct narrow elevations along posterior margins of each eye. Anteocellar foveae suboval and moderately deep, reaching level of posterior third of eyes. Temples short, about three times shorter than longitudinal length of eye, relatively strongly narrowed posteriad. Nuchal constriction moderately wide and deep. Ocelli located slightly below level of posterior margins of eyes; distance between ocelli slightly less than distance between ocellus and posterior margin of eye. Antennomeres with slightly transverse antennomeres 6–7 and distinctly transverse 8–10; antennomere 4 about as long as broad, more than twice shorter than 3, 5 slightly broader than 4, 6–7 slightly longer and distinctly broader than 5, 8 broader than 7, 9–10 slightly longer and distinctly broader than 8, apical antennomere 1.4 times as long as preceding segment, from middle gradually narrowed toward rounded apex.

Pronotum 1.5–1.6 times as broad as long, from widest middle slightly more narrowed posteriad than anteriad toward obtuse hind angles. Apical angles not protruded anteriad. Anterior margin slightly concave in middle. Lateral portions narrow, not explanate, with moderately wide and deep elongate impressions behind middle. Surface of disc with two shallow longitudinal depressions; each lateroapical portion with indistinct and short elevation, reaching about middle of pronotum; surface between longitudinal depressions slightly elevated. Middle portion with irregular and fine, transverse elevations between punctures.

Elytra slightly broader than long, twice as long as pronotum, indistinctly broadened posteriad; lateral portions marginate, but not explanate; hind margins slightly rounded. Dorsal surface of each elytron with strong diagonal elevations between punctures.

Metatarsi 1.4 times as long as metatibia.

Abdomen about as broad as or slightly narrower than elytra.

Male. Posterior margin of abdominal tergite VIII straight. Posterior margin of abdominal sternite VIII rounded. Aedeagus with wide basal portion, gradually narrowed toward middle; median lobe narrow and long, from widest middle portion strongly narrowed toward elongate preapical part with subacute apex, from about middle with narrow rounded lateral projections; mediolateral portions with narrow elongate accessory plates, each rounded apically; parameres wide, significantly shorter than median lobe, distinctly narrowed and curved laterad in preapical portions, each with two long apical and two short preapical setae; internal sac wide and long, with two oval and two narrow sclerotized structures in basal portion ( Fig. 8 ). Lateral aspect of the aedeagus as in Fig. 9 ; apical portion of median lobe (lateral view) hook-shaped, with slightly ventrodorsad curved, subacute hind angle and rounded apex.

Female. Posterior margins of abdominal tergite VIII and sternite VIII trunctate. Female accessory sclerite elongate, from widest basal portion gradually narrowed toward small rounded apex ( Fig. 12 ). Spermatheca as in Fig. 13 .

Comparative notes. Based on the general shape and the coloration of the body, proportions of the antennomeres, and general shape of aedeagus and female accessory sclerite, O. diffine is similar to O. subsolanum (see below), from which it can be distinguished by the slightly narrower pronotum, longer median lobe with the presence of rounded lateral projections in about the middle, the hook-shaped apical part of the median lobe (lateral view), and other details of external and internal morphology of the aedeagus. Besides that, based on the general shape of the aedeagus and the presence of the hook in the apical part of the median lobe (if viewed laterally), O. diffine is somewhat similar to O. bambusaphilum Shavrin, 2025 , recently described from Jiangxi, China ( Shavrin 2025). Omalium diffine can be distinguished from the latter species by the smaller body, slightly paler coloration, sparser punctation of the head and the pronotum, longer and narrower median lobe with the presence of rounded lateral projections in about middle, narrower apical portions of the parameres and other details of the morphology of the aedeagus.

Distribution. Omalium diffine is known from two localities in Honshu, Japan ( Fig. 32 ).

Bionomics. Detailed bionomical data are unknown.

Remarks. Omalium diffine was originally described based on the holotype from “Hitoyoshi”. Watanabe (1990) neither re-studied the type specimen and nor redescribed this species, but included it in a key of Omalium of Japan. Thus, based on the study of the holotype and an additional specimen, I decided to redescribe it in the present study.

Figures

FIGURE 32. Distribution of Omalium in the eastern Palaearctic region: O. diffine (squares), O. kurilicum (stars), O. niponense (rounds). Black circles: material studied by author, white circles: previously published material not seen by author.

FIGURES 4–9. Aedeagus of Omalium (Figs 4–5—O. longicorne (Russia: Amur Area, Norskiy Nature Reserve), Figs 6–7—O. golovatchi (paratype), 8–9—O. diffine (Japan: Tokyo)): 4, 6, 8—aedeagus, parameral view, 5, 7, 9—aedeagus, lateral view. Scale bar: 0.1 mm.

FIGURES 10–19. Female accessory sclerite (Figs 10, 12, 14–19) and spermatheca (Figs 11, 13) of Omalium: 10–11—O. curtipenne, 12–13—O. diffine, 14—O. golovatchi, 15—O. japonicum, 16—O. kurilicum, 17—O. longicorne, 18—O. strigicolle, 19—O. subsolanum. Scale bar: 0.1 mm.

FIGURES 22–24. Habitus of Omalium: 22—O. septentrionis (Russia: TyumenArea, Ugut), 23—O. curtipenne (Russia: Irkutsk Area, Vitim Nature Reserve), 24—O. diffine (holotype). Scale bar: 1.0 mm.