A review of the omicrine genera Omicrogiton, Mircogioton and Peratogonus of China (Coleoptera, Hydrophilidae, Sphaeridiinae)

Abstract The Chinese species of the genera Omicrogiton Orchymont, 1919, Peratogonus Sharp, 1884 and Mircogioton Orchymont, 1937 are reviewed, diagnosed and keyed. Mircogioton and Omicrogiton are reported for the first time from China, Peratogonus for the first time for mainland China. Five species are recognized: Omicrogiton coomani Balfour-Browne, 1939 (Guangdong, Hongkong), Omicrogiton hainanensis sp. n. (Hainan), Omicrogiton roberti sp. n. (Hainan), Mircogioton coomani Orchymont, 1937 (Yunnan), and Peratogonus reversus Sharp, 1884 (Guangdong, Jiangxi, Taiwan). Lectotype of Omicrogiton coomani is designated. Mircogioton cognitus (Malcolm, 1981), syn. n. is considered a junior subjective synonym of Mircogioton coomani Orchymont, 1939. Species of Mircogioton and Omicrogiton inhabit decaying banana trunks, whereas Peratogonus reversus was always collected from moist forest leaf litter.


Introduction
A total of 15 genera and 104 species of the tribe Omicrini Smetana, 1975 have been described world-wide (Hansen 1999;Fikáček 2011, 2013). Eleven of these genera may be found in the Oriental Region: Oreomicrus Malcolm, 1980, Tylomicrus Schödl, 1995, Nannomicrus Bameul, 1991, Litrosurus Orchymont, 1925, Stanmalcolmia Bameul, 1993, and Mircogioton Orchymont, 1937 are endemic to the Oriental Region, Peratogonus Sharp, 1884 and Noteropagus Orchymont, 1919 are principally Oriental but also reach the Palaearctic or Pacific Regions, respectively. Paromicrus Scott, 1913 andPsalitrus Orchymont, 1919 occur in the Oriental and Afrotropical Regions (Africa), and Aculomicrus Smetana, 1990 that occurs in the Neotropic Region reaches the Oriental Region only in the Malay Archipelago (and this Bornean species may actually belong to a different genus : Fikáček 2010). Only two species of the tribe were so far recorded from China, in both cases from Taiwan: Peratogonus reversus Sharp, 1884 by Knisch (1921) and Psalitrus sauteri Orchymont, 1929by Orchymont (1929. Not a single species of the tribe was so far recorded from the mainland China.
Since 2009, a lot of material of the tribe Omicrini was collected by us or our colleagues in various parts of Southern China, confirming that at least six omicrine genera occur in mainland China and/or Taiwan: Psalitrus, Noteropagus, Paromicrus, Peratogonus, Omicrogiton and Mircogioton. The latter three genera are revised in this contribution, in which we are providing diagnoses, identification keys and biology data of five species, of which two are described as new.

Material and methods
Male genitalia were dissected in a portion of specimens of each species. In specimens deposited in SYSU, dissected genitalia was transferred to a drop of absolute alcohol for removing membranes after 8-10 hours in 10% KOH at room temperature, and subsequently mounted into a drop of glycerine on a piece of transparent plastic slide attached below the respective specimens. In specimens deposited in NMPC and in the holotype of O. hainanensis sp. n., the dissected male genitalia were mounted into a drop of alcohol-soluble Euparal resin on a piece of glass attached below the respective specimens. Specimens from BMNH were dissected by R. B. Angus, the genitalia were placed without any additional treatment into a water-soluble dimethyl hydantoin formaldehyde resin on the same card as the beetle. Male genitalia and morphological characters were examined using a Nikon SMZ800 compound microscope. Genitalia photographs were taken using a Zeiss Axioskop 40 or Olympus BX41 compound microscopes and combined with AutoMontage or Helicon Focus software, respectively. Photographs of habitus and external morphology were taken using a Leica M205C stereomicroscope and combined with AutoMontage software.
Detailed descriptions of the tribe Omicrini and the genera treated in this study were provided by Hansen (1991). Morphological terminology largely follows Hansen (1991) and Komarek (2004), classification follows Short and Fikáček (2013).
Examined specimens are deposited in the following collections:  BAMEUL det. 1985. The specimen is dissected and its genitalia were probably mounted in a drop of dimethyl hydantoin formaldehyde resin, which is still present on a piece of transparent plastic below the specimen. However, we failed to find any genitalia in this drop -either they were never placed there, or they became completely transparent due to the long-term effect of dimethyl hydantoin formaldehyde resin. We were therefore not able to compare the genital morphology of this species with that of O. hainanensis sp. n., as originally planned. New material from Reunion Island is necessary to perform this detailed comparison.

Key to Chinese Omicrini
The following key allows to identify all genera of the tribe Omicrini occurring in China based on our published and unpublished data, and all species of the genus Omicrogiton based on the revision performed in this paper. The generic key is adapted from that of Hansen (1991  Diagnosis. Body length 1.9-2.1 mm, width 1.2-1.3 mm. Head and elytra brown; scapus ca. 3.5× as long as antennomeres 2-5 combined. Interstices of pronotum with fine mesh-like microsculpture; prosternum strongly tectiform. Phallobase ca. half as long as parameres, without distinct manubrium; paramere without strongly sclerotized S-shaped portion, rather wide throughout, weakly sinuate on outer margin, apex semicircular; median lobe slightly narrower than paramere, wide basally, then abruptly narrowing and rather narrow in apical half, apex narrowly rounded, gonopore small, apical.
Differential diagnosis. Omicrogiton coomani differs from all other species of the genus except of O. cheesmanae from New Hebrides by the presence of the fine microsculpture on pronotal interstices. It may be also easily distinguished from all other species of the genus by the morphology of the aedeagus, which lacks the strongly sclerotized S-shaped sclerite of the paramere, and has narrow median lobe with small apical gonopore.
Remark. When Balfour-Browne (1939) checked the material of Omicrogiton insularis Orchymont, 1919 deposited in BMNH, he found that the specimens from "Tonkin (Lac Tho)" (i.e. Lac Tho in the Hoa Binh Province in northern Vietnam) differ from those from Engano Island near Sumatra (type locality of O. insularis) and described them under the name O. coomani. Since the diagnosis of O. coomani was very short and it was only presented in the discussion concerning another species of the genus from New Hebrides, the species remained virtually unknown and unrecognized in the collections. The aedeagus of the dissected type specimen of O. coomani (Fig. 17) really differs from that of O. insularis and we can therefore confirm that O. coomani is a separate species.
Within this study, we are showing that O. coomani may co-occur syntopically with other species of the genus, from which it may be distinguished by male genitalia only. To fix the identity of the species and prevent any future confusion, we are hence designating here the only dissected syntype specimen as the lectotype of O. coomani. Aedeagus of this specimen is shown in Fig. 17.
Biology. All examined specimens were found in the decaying banana trunks, typically in still standing trunk bases which are decaying after the apical part of the plant was cut or broken.
Thorax. Pronotum ca. 2.6× as wide as long. Pronotal punctation similar to that on frons, interstices without microsculpture. Lateral margins with strong bead overlapping to anterior margin, posterior margin of pronotum without bead. Prosternum weakly tectiform; antennal grooves absent. Mesoventrite strongly and abruptly raised medially to form a narrow longitudinal lamina not reaching metaventral process posteriorly; cavities for reception of procoxae absent. Metaventrite weakly convex, without glabrous median portion, with weak posteromedial depression on elevated portion. Elytra widely explanate laterally, with 10 series of large punctures, series 1-5 almost reaching base, series 6-10 abbreviated anteriorly; interval punctures very fine but distinct, similar to those on pronotum, interstices without microsculpture; humeral bulge absent; lateral margin of elytron finely serrate; epipleuron wide throughout. Profemur glabrous, anterior margin angulate near base, with a large basal depression with golden pubescence, tibial groove sharply defined. Mesofemur with sparse and coarse punctures on anterior half, each puncture with a short seta; posterior half glabrous, with fine longitudinal sculpture. Metafemur with fine longitudinal sculpture and scattered fine punctures. Tibiae flat, meso-and metatibiae with long and stout spines along outer face and 1 or 2 pairs of spines on apical half of inner face; metatibial long spur ca. as long as first metatarsomere. First metatarsomere almost as long as metatarsomeres 2-3 combined.
Differential diagnosis. Omicrogiton hainanensis sp. n. belongs to the species with strongly sclerotized S-shaped portion of the paramere, together with the Oriental O. insularis Orchymont, 1919 andO. gomyi Bameul, 1986 from the Reunion Island (Bameul 1986). It differs from O. insularis by the much wider aedeagus (aedeagus is generally very narrow in O. insularis (Fig. 19), much wider parameres with bases of strongly sclerotized parts forming a very obtuse angle (parameres are narrow and bases of sclerotized portions form acute angle in O. insularis) and wide median lobe with subapical gonopore (median lobe is extremely narrow apically and the gonopore is situated at midlength in O. insularis). The aedeagus of O. gomyi is similar to that of O. hainanensis in the proportions (i.e., it is wide and robust in both species), but O. gomyi easily differs by wide median lobe with apical gonopore. Omicrogiton coomani and O. roberti, differ from all above species including O. hainanensis in parameres lacking the strongly sclerotized S-shaped portion, and O. coomani and O. cheesmanae may be distinguished from other species including O. hainanensis by the pronotum with fine mesh-like microsculpture.
Etymology. The species name is patronymic, referring to the Hainan Island where this species is commonly collected.
Biology. All type specimens were collected in decaying banana trunks in primary or secondary rainforests. On the type locality, the specimens of this species were collected in the same banana trunk as two other Omicrogiton species occurring in Hainan (i.e. O. roberti sp. n. and O. coomani), which indicates that multiple species may occur syntopically in this genus. For this reason, we excluded females from the type series of this species.
Thorax. Pronotum ca. 2.6× as wide as long; pronotal punctation similar to that on frons, interstices without microsculpture. Lateral margins with strong bead overlapping to anterior margin, posterior margin of pronotum without bead. Prosternum strongly tectiform, antennal grooves absent. Mesoventrite strongly and abruptly raised medially to form a narrow longitudinal lamina not contacting metaventral process posteriorly, cavities for reception of procoxae absent. Metaventrite weakly convex, with a small glabrous portion, with a posteromedial depression on elevated portion. Elytra widely explanate laterally, with 10 series of large punctures, series 1-5 almost reaching base, series 6-10 abbreviated anteriorly; interval punctures very fine but distinct, similar to on pronotum; interstices without microsculpture. Humeral bulge absent, lateral margin of elytron very finely serrate; epipleuron wide throughout.Profemur glabrous, anterior margin angulate near base, with a large basal depression with golden pubescence, tibial groove sharply defined. Mesofemur with sparse and strong punctures on anterior half, each puncture with a short seta; posterior half glabrous, with fine longitudinal sculpture. Metafemur with fine longitudinal sculpture and scattered fine punctures. Tibiae flat, meso-and metatibiae with long and stout spines along outer face and 1 or 2 pairs of spines on apical half of inner face; metatibial long spur longer than first tarsomere. First metatarsomere almost as long as metatarsomeres 2-3 combined.
Different diagnosis. Omicrogiton roberti is similar to O. coomani Balfour-Browne, 1939 in the aedeagus without the strongly sclerotized S-shaped portion of the paramere. It differs from O. coomani by the morphology of the aedeagus (median lobe very wide with very large subapical gonopore and very short phallobase in O. roberti, rather narrow and with small apical gonopore and rather long phallobase in O. coomani) and by the pronotal interstices without fine mesh-like microsculpture (with fine mesh-like microsculpture in O. coomani).
Etymology. The species is named after Dr. Robert Bagrie Angus, a British specialist on the Helophoridae, who helped us a lot with this study.
Biology. The holotype was collected in a decaying banana trunk together with specimens of O. hainanensis and O. coomani.

Mircogioton coomani Orchymont, 1937
Figs 5-6, 14, 20 2.2× as long as antennomeres 2-5 combined, slightly shorter than antennal club. Head, pronotum and elytra with similar sparse and fine punctation, interstices without fine microsculpture; elytra with 10 series of punctures, series 6-7 abbreviated anteriorly. Prosternum strongly tectiform, sharp anteriorly. Mesoventral elevation much longer than wide, with distinct longitudinal groove medially, posteriorly fused with metaventral process, not projecting posteriad into a process overlapping metaventrite. Metaventrite with a longitudinal glabrous elevated band medially, forming together with mesoventral plate a joint meso-metaventral elevation. Phallobase ca. 0.3× as long as paramere; paramere rather wide throughout, outer margin slightly concave subapically, apex semicircular. Median lobe slightly narrower than paramere, lateral margin almost parallel, apex narrowly rounded, gonopore of moderate size, subapical (Fig. 20). Bameul, 1993, M. seriatus Hebauer, 2006and M. irregularis Hebauer, 2006 in mesoventrite fused with metaventral process (in contrast, mesoventrite is projecting into a long process overlapping metaventrite in the latter three species). It differs from M. grandis Bameul, 1993 andM. julieae (Malcolm, 1981) by the anterior margin of the labrum bisinuate (in contrast, labrum is simply concave on anterior margin in the latter two species). From M. julieae it also differs by apically broad paramere and apex of median lobe not distinctly narrowed. From M. grandis it may be also distinguished by smaller body size (up to 3.5 mm, in comparison with 3.8 mm in M. grandis).

Differential diagnosis. Mircogioton coomani differs from Mircogioton spinosus
Remark. This species was described by d'Orchymont (1937) based on a single female collected by A. de Cooman in "Tonkin, Hoa Binh". The senior author examined one male collected by the same collector and bearing the same label data. Except of clearly being a part of the same material from which M. coomani was collected, the examined specimen agrees in all details with the original description. We therefore consider it represents M. coomani although we have not checked the female holotype. Malcolm (1981) described Ischyromicrus cognitus Malcolm, 1981 based on a female from upper Mekong (later transferred to Mircogioton by Hansen (1991)). The species was redescribed by Bameul (1993). The characters described by Malcolm (1981) and Bameul (1993) are identical with the specimens of M. coomani in our hands. The type locality of M. cognitus is situated in northeastern Laos not far from the border with China rather than in Vietnam as supposed by Malcolm (1981) (Bameul 1993, Hansen 1999. Bameul (1981) moreover noticed that "in the description of M. cognitus, no characters really differ from those given by d' Orchymont (1937)   Diagnosis. Body length 2.1 mm, width 1.5 mm, strongly convex. Head and pronotum with fine microsculpture between punctures. Elytra with 10 striae, striae 1-5 reaching elytral base, striae 6-10 abbreviated anteriorly, not reaching base; elytral intervals with distinct fine punctures, without microsculpture between punctures. Prosternum steeply raised in middle to form a triangular medially carinate tablet. Mesoventrite flat medially, widely fused with metaventrite. Metaventrite laterally with much coarser and stronger punctures than on its median portion. Aedeagus (Figs 21-22) with phallobase ca. as long as paramere, tube-like; paramere broad basally, gradually narrowing towards apex; median lobe slender, parallel-sided, with very long basal struts, gonopore indistrinct, apex slightly emarginate.
Differential diagnosis. This species can be easily distinguished from P. grandis Malcolm, 1981 occurring in India (Sikkim) by punctures on the lateral portion of the metaventrite much deeper and larger than medially. From P. corporaali Orchymont, 1926 occurring in Indonesia (Java), it may be distinguished by pronotum with distinct microsculpture between punctures, elytra with striae 6-10 not reaching elytral base (stria 8 almost reaching base, striae 9-10 reaching base in P. corporaali), elytral intervals with distinct punctures, flat mesoventral plate, and metaventrite with coaser and sparser punctures medially.
Remark. This species was firstly described from Nagasaki, Kyushu in southern Japan by Sharp (1884). It was subsequently reported from Taiwan by Knisch (1921). The comparison of the specimens from Taiwan and southern continental China revealed that they are identical with those from Japan.
When Malcolm (1981) described Peratogonus grandis Malcolm, 1981, he diagnosed it from P. reversus by the different body size (2.21×1.64 mm in P. grandis versus 1.72×1.31 in P. reversus) and by the shallower and smaller punctures on lateral portion of the metaventrite. The material examined by us revealed that the specimen of P. reversus examined by Malcolm (1981) was smaller than its individuals usually are (i.e. body length 1.9-2.2 mm). Therefore, the body size can not be used as a reliable character to distinguish the two species, in contrast to the punctation of the metaventrite, which seems to be a reliable character to distinguish the two species.
Biology. Most specimens examined here were found by sifting forest leaf litter. Distribution. China (Guangdong, Jiangxi, Taiwan), Japan (Honshu, Kyushu). New genus and species for mailand China.