Cercyon (Cercyon) unipunctatus (Linnaeus, 1758)

Ryndevich, Sergey K., Jia, Fenglong & Fikáček, Martin, 2017, A review of the Asian species of the Cercyon unipunctatus group (Coleoptera: Hydrophilidae: Sphaeridiinae), Acta Entomologica Musei Nationalis Pragae (Acta. Ent. Mus. Natl. Pragae) 57 (2), pp. 535-576 : 537-569

Cercyon (Cercyon) unipunctatus group

Diagnosis. Body elongate oval, length 1.8–3.8 mm, width 0.9–1.9 mm; dorsal surface convex without microsculpture; head black, pronotum pale with central dark spot, or dark with pale lateral margins; elytron pale (yellow, reddish or brownish), in some species with dark sutural interval or dark spot on elytral disc, rarely dark coloration prevailing (resulting in seemingly dark elytra with pale spots in C. kabaki and dark form of C. unipunctatus ); maxillary palps pale (yellowish); mesoventral plate narrowly elongate, 3.3–11.0× as long as wide; metaventrite without femoral lines.

Comparison with other species groups in Cercyon s. str. The pale maxillary palpi and absence of femoral lines of the metaventrite diagnose members of C. unipunctatus group from C. haemorrhoidalis species group (femoral lines present, palpi dark), C. nigriceps group (femoral lines present), C. marinus and C. obsoletus groups (maxillary palps dark) and from C. berlovi Shatrovskiy, 1999 (maxillary palpi dark). The dorsal coloration of the members of C. unipunctatus group is never unicolored, it always has at least the head and parts of the pronotum darker than the elytra; in this they differ from C. olibrus Sharp, 1874 and C. rotundulus Sharp, 1884 (dorsally unicolored), C. borealis Baranowski, 1985 (pronotum and elytra concoloured) and C. cultriformis Wu & Pu, 1995 (head pale dorsally). The dorsal surface of head, pronotum and elytra is always smooth, lacking any microsculpture; this diagnoses the C. unipunctatus group from the C. dux group, C. algarum group, C. tristis group, C. sericatus Hebauer, 2002 and C. emarginatus Baranowski, 1985 . The body is never compressed and the tibiae are not distinctly widened distally and armoured by stout blunt spines; this distinguishes the C. unipunctatus group from C. depressus Stephens, 1829 and related species. See Discussion for a more detailed comparison with similar species ( C. ovillus Motschulsky, 1860 and C. lateralis (Marsham, 1802) etc.).

List of species of the Cercyon unipunctatus group

C. divisus Hebauer, 2002

China (Yunnan, Sichuan), Nepal, India.

C. flavimarginatus sp. nov.

China (Yunnan).

C. kabaki Ryndevich, 2004

China (Sichuan).

C. kubani sp. nov.

China (Yunnan).

C. limbatus Mannerheim, 1842

Not occurring in Asia. Other regions: Western coast of Northern America.

C. quisquilius (Linnaeus, 1760)

China (Gansu, Guangxi, Henan, Inner Mongolia, Jiangxi, Qinghai, Shaanxi, Shanghai, Sichuan, Yunnan), Cyprus, Iran, Israel, Japan, Kazakhstan, Kyrgyzstan, Mongolia, Nepal, Russia (Western and Eastern Siberia, Far East), Saudi Arabia (north), Syria, Tajikistan, Turkey, Turkmenistan, Uzbekistan. Other regions: Europe, North Africa, North America, Caribbean (introduced), Australia (introduced), South America (introduced), Hawaii (introduced).

C. undulipennis sp. nov.

China (Sichuan,? Yunnan)

C. unipunctatus ( Linnaeus, 1758)

Kazakhstan, Russia (Western and Eastern Siberia, Far East), China (Heilongjiang), Japan. Other regions: Europe, North America.

C. unipustulatus Nakane, 1982

Japan, Russia (Far East).

C. verus Shatrovskiy, 1989

Russia (Far East,?Western Siberia).

Key to Asian species of the C. unipunctatus group

1(4) Elytra with very apparent sparse long semi-erect yellowish pubescence at least laterally (may be broken dorsally; Figs 5A,B , 6A in lateral view). Elytral intervals weakly but distinctly convex ( Figs 5A,B , 6A ). Elytra uniformly yellowish or reddish or slightly darker on elytral disc, but without distinct central dark spot or dark sutural interval. Mesoventral plate very narrow (8–11× longer than wide; Figs 5G , 6F ).

2(3) Lateral margins of pronotum narrowly pale ( Fig. 6A ) Sutural interval of elytra not darker than general elytral coloration. Intervals of elytra slightly convex in medial and apical parts. Median lobe of the aedeagus evenly wide throughout, strongly narrowing at extreme apex ( Fig. 6C ). ........................... Cercyon undulipennis sp. nov.

3(2) Lateral margins of pronotum very widely pale ( Figs 5A–B ). Sutural interval of elytra darker than general elytral coloration. Intervals of elytra very slightly convex only in apical part. Median lobe of the aedeagus widest at midlength, very strongly narrowing in apical half ( Fig. 5D ). ................................................ Cercyon kubani sp. nov.

4(1) Elytra without pubescence, or at most with short very inconspicuous pubescence at lateral-most parts. Elytral intervals flat, if weakly convex then elytra with very distinct black central spot. Elytra uniformly yellow or with very distinct black patterns, even darker specimens with a set of pale spots. Mesoventral plate wider (at most 6× longer than wide).

5(6) Elytra largely black, with characteristic pattern of pale spots, or yellow with black and dark brown spots, forming a characteristic pattern ( Figs 4A–B ). Median lobe of the aedeagus narrow with very long apex ( Fig. 4D ). ................................................... ............................................................................... Cercyon kabaki Ryndevich, 2004

6(5) Elytra largely yellowish of reddish, if larger black areas are present they form a central dark spot, leaving apical and lateral portions widely pale ( Figs 1 , 3A , 7A , 8A , 9A , 10A–B , 11–12 ). Median lobe wide throughout, or at least widening at midlength.

7(8) Lateral margins of pronotum very widely pale, dark central pronotal spot may be confined to central area or lobate laterally. Elytra rarely uniformly yellow, usually with dark sutural interval, or with dark spots basally and/or in humeral region, or with combination of these ( Fig. 1 ). Median lobe without series of setae in apical half, basal apodemes short ( Fig. 2B ). ........................................ Cercyon divisus Hebauer, 2002

8(7) Lateral margins of pronotum narrowly pale, dark pronotal spot never confined only to central area and never distinctly bilobate laterally ( Figs 3A , 7A , 8A , 9A , 10A–B ). Elytra uniformly pale, or with dark sutural interval or more or less small central dark spot, but never with basal or humeral dark spots. Median lobe with series of setae in apical half, basal apodemes long.

9(12) Elytra uniformly pale.

10(11) Elytra and lateral parts of pronotum yellowish, pale lateral part of pronotum less extended ( Fig. 7A ). Phallobase longer than parameres, median lobe widest subapically ( Figs 7B–C ). ................................................. Cercyon quisquilius (Linnaeus, 1760)

11(10) Elytra and lateral parts of pronotum reddish, pale lateral part of pronotum wide ( Fig. 10B ). Phallobase ca. as long as parameres, median lobe parallel-sided ( Figs 10C–D ). ..................................................................... Cercyon verus Shatrovskiy, 1989 (part)

12(9) Elytra pale with central dark spot or dark sutural interval ( Figs 3A , 11–12 ).

13(18) Elytra at most with dark sutural interval, without distinct central dark spot.

14(15) Sutural interval nearly completely dark (except at extreme base) ( Fig. 3A ). Median lobe narrower, without abruptly narrowing apex ( Fig. 3C ). ........................................ .............................................................................. Cercyon flavimarginatus sp. nov.

15(14) Sutural interval usually not completely dark (dark coloration confined to narrow stripe at suture or small central area). If sutural interval nearly completely dark, then median lobe wider, with abruptly narrowing apex ( Figs 8C , 10D ).

16(17) Pentagonal area of metaventrite with coarse and dense punctation, distance between the individual punctures in lateral parts ca. 1–3 × their diameters ( Fig. 10F ). Mentum with distinct dense transverse wrinkles ( Fig. 10I ). Pale lateral margins of pronotum wider ( Figs 10A–B ). Coloration of elytra as in Figs 12E–F . ....................................... ..................................................................... Cercyon verus Shatrovskiy, 1989 (part)

17(16) Pentagonal area of metaventrite with shallow and sparse punctation, distance between individual punctures in lateral parts ca. 4–6 × their diameters ( Fig. 8E ). Mentum with very indistinct transverse wrinkles ( Fig. 8G ). Pale lateral margins of pronotum narrower ( Fig. 8A ). Coloration of elytra as in Fig. 11A . ...................................... .................................................................... C. unipunctatus ( Linnaeus, 1758) (part)

18(13) Elytra with distinct central dark spot; spot rounded, triangular or pentagonal in shape.

19(20) Mesoventral plate widely rounded posteriorly, facing a distinct anterior projection of metaventral pentagon ( Fig. 9F ). Median lobe slightly constricted in apical third ( Fig. 9C ). Sutural interval of elytra completely dark including basally, elytral central spot always large, reaching at least to one third of elytral interval ( Figs 12A–D ). ............. ........................................................................ Cercyon unipustulatus Nakane, 1982

20(19) Mesoventral plate sharply pointed posteriorly, anterior part of metaventral pentagon only indistinctly bulged ( Figs 8F , 10G ). Median lobe parallel-sided ( Figs 8C , 10D ). Sutural interval of elytra usually pale basally. Elytral central spot small (on intervals 1–2 only) to very large ( Figs 11 , 12E–J ).

21(22) Pentagonal area of metaventrite with coarse and dense punctation, distance between individual punctures in lateral parts ca. 1–3× their diameters ( Fig. 10F ). Mentum with distinct dense transverse wrinkles ( Fig. 10I ). Pale lateral margins of pronotum wider ( Figs 10A–B ). Coloration of elytra as in Figs 12E–J . ................................................. ..................................................................... Cercyon verus Shatrovskiy, 1989 (part)

22(21) Pentagonal area of metaventrite with shallow and sparse punctation, distance between individual punctures in lateral parts ca. 4–6× their diameters ( Fig. 8E ). Mentum with with very indistinct transverse wrinkles ( Fig. 8G ). Pale lateral margins of pronotum narrower ( Fig. 8A ). Coloration of elytra as in Fig. 11 . ............................... .................................................................... C. unipunctatus ( Linnaeus, 1758) (part)

Figures

Fig. 5. Cercyon kubani sp. nov.A–B dorsal and lateral habitus of paratypes from China: Yunnan: Weibaoshan; C–E – male genitalia of the holotype (C – tegmen; D – median lobe; E – sternite 9). F–I – ventral morphology (F – meso- and metaventrite; G – detail of mesoventral plate; H – prosternum and antennal groove; I – mentum).

Fig. 6. Cercyon undulipennis sp. nov. A – dorsal and lateral habitus of paratype from China: Yunnan: Gaoligong Shan; B–D – male genitalia of the holotype (B – tegmen; C – median lobe; D – sternite 9). E–H – ventral morphology (E – meso- and metaventrite; F – detail of mesoventral plate; G – mentum; H – prosternum and antennal groove).

Fig. 4. Cercyon kabaki Ryndevich, 2004. A–B – dorsal and lateral habitus (A – pale form from China: Sichuan: north of Dehgsheng; B – dark form from China: Sichuan: Taizping Temple); C–E – male genitalia (C – tegmen; D – median lobe; E – sternite 9); F – meso- and metaventrite; G – detail of mesoventral plate.

Fig. 1. Dorsal and lateral habitus of Cercyon divisus Hebauer, 2002. A, C–D – specimens from China: Yunnan, Yulongshan; B – specimen from Nepal: Taplejung Distr., upper Simbua Khola near Tseram.

Fig. 3. Cercyon flavimarginatus sp. nov.A – dorsal and lateral habitus of the paratype from China:Yunnan:Zhondian; B–D – male genitalia of the holotype (B – tegmen; C – median lobe; D – sternite 9). E–H – ventral morphology (E – meso- and metaventrite; F – detail of mesoventral plate; G – mentum; H – prosternum and antennal groove).

Fig. 7. Cercyon quisquilius (Linnaeus, 1761). A – dorsal and lateral habitus of the specimen from China: Yunnan: Lijiang; B–D – male genitalia (B – tegmen; C – median lobe; D – sternite 9). E–H – ventral morphology (E – meso- and metaventrite; F – detail of mesoventral plate; G – mentum; H – prosternum and antennal groove).

Fig. 8. Cercyon unipunctatus (Linnaeus, 1758). A – dorsal and lateral habitus; B–D – male genitalia (B – tegmen; C – median lobe; D – sternite 9). E–H – ventral morphology (E – meso- and metaventrite; F – detail of mesoventral plate; G – mentum; H – prosternum and antennal groove).

Fig. 9. Cercyon unipustulatus Nakane, 1982.A – dorsal and lateral habitus of the paratype from Japan: Chiba Pref., Abiko; B–D – male genitalia (B – tegmen; C – median lobe; D – sternite 9). E–H – ventral morphology of the paratype (E – meso- and metaventrite; F – detail of mesoventral plate; G – mentum; H – prosternum and antennal groove).

Fig. 10. Cercyon verus Shatrovskiy, 1989.A–B – dorsal and lateral habitus of specimens from Kuril Islands; C–E – male genitalia (C – tegmen; D – median lobe; E – sternite 9). F–I – ventral morphology (F – meso- and metaventrite; G – detail of mesoventral plate; H – prosternum and antennal groove; I – mentum).

Fig. 11. Cercyon unipunctatus (Linnaeus, 1758), variability of the elytral coloration.

Fig. 12. Variability of elytral coloration. A–D – Cercyon unipustulatus Nakane, 1982. E–J – Cercyon verus Shatrovskiy, 1989.

Fig. 2. Cercyon divisus Hebauer, 2002. A–C – genitalia of specimen from Nepal: Taplejung Distr. (NMPC) (A – tergmen; B – median lobe; C – sternite 9). D–H – variability in the shape of male sternite 9 (D – China: Yunnan: Habashan; E – Nepal: Karnali Prov.; F – China: Yunnan: Maoniuping; G – China: Yunnan: Yulongshan; H – China: Sichuan: Moxi). I–P – ventral morphology (I–L – specimen from India: West Bengal: Darjeeling; M–P – specimen from China:Yunnan:Maoniuping):I, M – meso- and metaventrite; J, N – detail of mesoventral plate; K, O – mentum; L, P – prosternum and antennal groove.