Cercyon Leach, 1817

Ryndevich, S. K., Hoshina, H. & Prokin, A. A., 2019, Review of species of the genus Cercyon of Russia and adjacent regions. VI. Subgenus Cercyon, the C. shinanensis group (Coleoptera: Hydrophilidae), Zoosystematica Rossica (China) 28 (2), pp. 258-266 : 259-262

Subgenus Cercyon Leach, 1817

Group of species Cercyon (s. str.) shinanensis

Diagnosis. Body elongate oval, length 1.6– 2.2 mm, width 1.0– 1.4 mm; dorsal surface convex, without microsculpture. Head black; pronotum dark (dark brown), paler near lateral margins; elytra reddish brown to brown; maxillary palpi pale (yellowish), last segment occasionally darkened on apex. Humeral bulge not distinct; mesoventral plate narrowly elongate, about 4.4– 5.0 times as long as wide; metaventrite without femoral lines; metaventral median pentagonal area about 0.9–1.0 times as long as wide. Median lobe of aedeagus fairly wide, slightly widened apically, with almost parallel sides and widely round- ed apex.

Composition. This group includes two species, C. shinanensis and C. sundukovi sp. nov. The similarity in the structure of the median lobe of aedeagus confirms the affiliation of these species to the same group.

Comparison with other Palaearctic species groups of Cercyon s. str. The C. shinanensis group is closest to the C. lateralis , C. rotundulus , C. olibrus and C. ovillus species groups. All members of these groups share many structural features and are distributed in the Russian Far East and Japan.

Unlike the species of the C. shinanensis group, the members of the C. lateralis group, which includes C. lateralis (Marsham, 1802) , C. inquinatus Wollaston, 1854 , and C. ustus Sharp, 1874 , have pronotum black or dark brown, with broad pale lateral margins (reddish or reddish brown), maxillary palpi pale, elytra from brownish black to reddish brown, and size larger (body length 2.4–2.8 mm). Pale specimens of C. lateralis is somewhat similar in colour to the representatives of the C. shinanensis group, but their size is larger.

The species of the C. rotundulus group ( C. rotundulus Sharp, 1884 and C. saluki Ryndevich, 1998 ) differ from the species of the C. shinanensis group in having body broadly oval, head pal- er (yellowish brown or reddish brown to dark brown), and size larger (2.1–2.9 mm)

A single species of the C. olibrus group is characterised by total body colour yellowish brown, with anterior and central parts of head darker. The species of the C. shinanensis group have black head and darker total colour.

A single species of the C. ovillus group is similar to the members of the C. shinanensis group in small body size (1.6–2.0 mm), dark head and pronotum, with pale maxillary palpi, but differs from them in having elytra yellow or reddish, with black triangular sutural spot basally and with very strongly conspicuous humeral bulge, and body oval, strongly narrowed posteriorly.

Among other Palaearctic species groups, the C. shinanensis group can be recognised as follows. The absence of femoral lines of the metaventrite distinguishes its members from the C. nigriceps group, which includes C. nigriceps (Marsham, 1802) and C. paranigriceps Ryndevich et Hebau- er, 2010; from the C. pygmaeus group, which includes C. pygmaeus (Illiger, 1801) , C. terminatus (Marsham, 1802), and C. bellus Jia, Liang, Ryndevich et Fikáček, 2019 ; from the C. melanocephalus group, which includes C. melanocephalus (Linnaeus, 1758) , C. alpinus Vogt, 1969 , C. haemorrhoidalis (Fabricius, 1775) , C. strandi Roubal, 1938 , and C. tatricus Endrődy-Younga, 1967 ; and from the monotypic C. impressus (Sturm, 1807) group.

Members of the C. tristis group, which comprises C. tristis (Illiger, 1801) , C. abeillei Guillebeau, 1896 , C. bononiensis Chiesa, 1964 , C. convexiusculus Stephens, 1829 , C. granarius Erichson, 1837 , C. kryzhanovskii Shatrovskiy, 1989 , C. korbianus Kniz, 1911 , C. renneri Hebauer, 1997 , C. sternalis (Sharp, 1918) , and C. subsulcatus Rey, 1885 ; and the species of the groups that inhabit marine sediments (the C. algarum , C. depressus , C. dux and C. littoralis groups) have elytral microsculpture that is absent in the members of the C. shinanensis group.

Species of the C. alinae group, which includes C. alinae Ryndevich, 2004 and C. biltoni Jia, Liang, Ryndevich et Fikáček, 2019 , are distinguished from the species of the C. shinanensis group by the broadly oval body, convex elytral intervals, larger size (2.4–2.7 mm), and by the characteristic colour pattern ( Ryndevich, 2004b; Jia et al., 2019).

A single species of the C. borealis Baranowski, 1985 group differs from all other species of the nominotypical subgenus in the very wide metaventral median pentagonal area which is about 0.7 times as long as wide, and in the larger size (2.3–2.8 mm). In the species of the C. shinanensis group, the metaventral median pentagonal area is narrower, about 0.9–1.0 times as long as wide.

The pale maxillary palpi of the members of the C. shinanensis group distinguish it from the C. melanocephalus group mentioned above; from the C. marinus group, which includes C. marinus Thomson, 1853 , C. bifenestratus Küster, 1851 , and C. medvedorum Shatrovskiy, 1999 ; from the C. obsoletus group, which includes C. obsoletus (Gyllenhal, 1808) and C. castaneipennis Vorst, 2009 ; and from the monotypic C. impressus and C. berlovi Shatrovskiy, 1999 groups.

A single species of the C. lencoranus group differs from the members of the C. shinanensis group in the almost total black body colour and in the larger size (2.4–2.8 mm).

The colour of the pronotum and elytra distinguishes the species of the C. shinanensis group from the members of the C. unipunctatus group, which includes C. unipunctatus (Linnaeus, 1758) , C. emarginatus Baranowski, 1985 , C. divisus Hebauer, 2002 , C. kabaki Ryndevich, 2004 , C. quisquilius (Linnaeus, 1761) , C. undulipennis Ryndevich, Jia et Fikáček, 2017 , C. unipustulatus Nakane, 1982 , C. verus Shatrovskiy, 1989 , C. flavimarginatus Ryndevich, Jia et Fikáček, 2017 , and C. kubani Ryndevich, Jia et Fikáček, 2017 . The members of the C. unipunctatus group are characterised by the pronotum either pale with a central dark spot or dark with pale lateral margins; their elytra are pale (yellow, reddish or brownish), in some species with the dark sutural interval or with a dark spot on the elytral disc, rarely with the predominance of dark colouration (because of what the elytra appear dark with pale spots: in C. kabaki and in the dark form of C. unipunctatus ).

extending over pronotal anterior and posterior angles; anterior and posterior margin without bead. Prosternum with strong longitudinal carina medially; antennal groove distinct, moderately large, rounded laterally. Mesoventral plate narrowly elongate, about 4.6–5.0 times as long as wide, widest posteriorly ( Fig. 3). Metaventrite with raised, glabrous, sparsely punctate medi- an pentagonal area; this area about 0.9 times as long as wide; femoral lines absent ( Fig. 3). Elytra with nine punctate striae; intervals flat, with ground punctures on intervals very fine; interval 2 widest, with four or five matted rows of punctation; remaining intervals with no more than three matted puncture rows. Humeral bulge not distinct. Epipleuron flat, horizontal. Femora with sparse and shallow punctures ventrally, each with distinct tibial groove. Front femora with strongly obliterate microsculpture, hind and middle femora with distinct microsculpture (consisting of transverse meshes). Tarsi with densely arranged whitish setae ventrally, metatarsomere 1 about as long as metatarsomeres 2 and 3 combined.

Abdomen. Five exposed ventrites; ventrite 1 longest, about twice as long as ventrite 2, bearing distinct median longitudinal carina; ventrite 5 not emarginate apically.

Male genitalia ( Figs 4–7). Phallobase asymmetrical basally, about 1.2 times as long as paramere. Parameres almost parallel-sided, sharply tapering at very apex, with apical part membranous and with two very short setae subapically. Median lobe almost parallel-sided, only slightly widened subapically, widely rounded at apex, and with short apodemes basally; gonopore large, situated subapically.

Note. All median projections of the sternite 9 of the examined type specimens were lost when mounted by Nakane.

Comparison. Cercyon shinanensis is most similar in colouration to the Japanese species C. ustus , C. olibrus and C. verus . Cercyon ustus differs from C. shinanensis in the larger size (2.6–2.9 mm). Cercyon olibrus differs from C. shinanensis in the pale head and pronotum. Cercyon verus differs from C. shinanensis in the larger size (2.6– 2.9 mm) and in wider (about 3.3–4.0 times as long as wide) mesoventral plate; in addition, its pronotum is black to dark brown, with the lateral margins widely pale (reddish), anterior margin very narrowly pale, and posterior margin very narrowly pale near the posterolateral angles.

Distribution. Japan (Honshu). This species is known only from the type locality (Mount Jonen).

Bionomics. The specimens were collected in a mountain evergreen forest.