A review of Himalcercyon stat. nov., with description of a new species from the Chinese Himalaya and an updated key to Asian genera of Megasternini (Coleoptera, Hydrophilidae)

Himalcercyon Hebauer, 2002 stat. nov. is elevated to genus rank based on the unique form of its mesoventral elevation. The genus is reviewed, redescribed, and illustrated in detail. Two species are recognized: Himalcercyon mirus (Hebauer, 2002) comb. nov. from Nepal and H. franzi sp. nov. from Chinese Himalaya (Xizang Autonomous Region). Both species are illustrated and diagnosed. An updated key to the Asian genera of the tribe Megasternini (Coleoptera, Hydrophilidae, Sphaeridiinae) is provided, along with the SEM micrographs of ventral morphology of these genera. New replacement name Oreosternum nom. nov. is proposed for Oreocyon Hebauer, 2002 which is preoccupied by Oreocyon Marsh, 1872 (Mammalia, Oxyenidae) and Oreocyon Krumbiegel, 1949 (Mammalia, Canidae).


Introduction
Megasternini is the largest clade of terrestrial water scavenger beetles, containing approximately 580 described species currently classified in 52 genera (Jia et al. 2011(Jia et al. , 2019Ryndevich 2011;Short and Fikáček 2011;Fikáček et al. 2012aFikáček et al. , 2015bFikáček and Rocchi 2013;Makhan 2013;Deler-Hernández et al. 2014;Arriaga-Varela et al. 2017, 2018aRyndevich and Prokin 2017;Shatrovskiy 2017;Szczepański et al. 2018). Since the 1980s, 20 new genera of Megasternini have been described from the Afrotropical, Australian, Oriental, and Neotropical regions by Hansen (1989Hansen ( , 1990Hansen ( , 1999a, Hebauer (2002aHebauer ( , 2003, , and Arriaga-Varela et al. (2018a). Nearly half of the described megasternine species are classified in the genus Cercyon. This led d' Orchymont (1942), Smetana (1978), and Hebauer (2002aHebauer ( , 2003 to divide Cercyon into numerous subgenera, 11 of which are currently considered valid (Hansen 1999b;Short and Hebauer 2006). However, most of these only contain one to a few species, and the majority of Cercyon species are still members of the nominotypical subgenus Cercyon s. str. A phylogeny of the Hydrophilidae based on molecular data from six genes , which included only four Cercyon species, indicated that Cercyon is very likely a polyphyletic genus. Moreover, preliminary studies have revealed that even some of the small subgenera are not monophyletic (e.g., Arriaga-Varela et al. 2018a). Additional studies are therefore necessary to establish a natural classification of the group and allow for reliable identification of genera and species.
The mountains on the southern margin of the Qinghai-Xizang (Tibetan) Plateau are known for their highly diverse and endemic faunas (e.g., Huang et al. 2007;Deng et al. 2020), of which terrestrial Hydrophilidae are a component. More than 80 species of terrestrial hydrophilid beetles have been reported from Nepal and Bhutan (Hansen 1999b;Hebauer 2002a, b), most of which are until now only known from the Himalayas. Recently, some of the species originally described from the Himalayas have also been recorded from the mountains in the Chinese provinces of Yunnan and Sichuan (e.g., Cercyon divisius Hebauer, 2002, indicating that the mountain systems on southern and south-eastern margin of Qinghai-Xizang are interconnected, thus forming the so-called Sino-Himalayan subregion (for details see Procheş and Ramdhani 2012). Other species originally known from the Himalayas are widespread at high elevations on the Qinghai-Xizang Plateau (C. berlovi Shatrovskiy, 1999: Jia et al. 2011) and seem to be plateau endemics that reach lower altitudes at the margins of their range, which seems uncommon for endemics of the plateau (see, e.g., Angus et al. 2016).
Recently, we received a small sample of terrestrial hydrophilids from Motuo County, Xizang Autonomous Region, China, a region in the Himalayas at the southern margin of the Qinghai-Xizang Plateau. In contrast to the more northern regions of the Xizang Autonomous Region, Motuo County includes middle to low elevations and is affected by monsoon rains; it is, therefore, warmer and more humid than the main plateau areas. The material contained a species of the Megasternini which is unique in the morphology of its mesoventral plate. We originally considered it to be an undescribed genus, but a detailed survey of megasternine taxa described from the Himalaya region revealed that Cercyon mirus Hebauer, 2002 from Nepal, which was assigned to the monotypic subgenus Himalcercyon Hebauer, 2002 in the original description (Hebauer 2002a), shares the unusual mesoventral morphology with our specimens. Hence, we here redescribe Himalcercyon and elevate this subgenus to the rank of genus based on its unique ventral morphology; we (re)describe and illustrate both species. We also provide an updated key to the Asian genera of the Megasternini.

Material and methods
We examined the type series of Cercyon mirus and the small series (10 specimens) of the new species from Motuo County. Male genitalia of the holotypes of both species were examined and photographed in the original position (i.e. with the median lobe inserted in the tegmen). Due to the very limited material available, separation of the median lobe is not always easy and sometimes results in partial damage of some parts of the aedeagus. Genitalia were photographed in glycerol. The aedeagus of the holotype of C. mirus was subsequently embedded in a drop of alcohol-soluble Euparal resin on a piece of glass glued to a small piece of cardboard attached below the respective specimen. Habitus photographs were taken using a Canon D-550 digital camera with attached Canon MP-E65mm f/2.8 1-5 macro lens. Genitalia were photographed using a Canon D1100 digital camera attached to an Olympus BX41 compound microscope (C. mirus) or using an Olympus SZX7 stereomicroscope (new species); combined, focus-stacked images were made with Helicon Focus (Helicon Soft Ltd, Ukraine) software. Scanning electron micrographs of C. mirus and of the Asian genera of the Megasternini were taken using a Hitachi S-3700N environmental electron microscope at the Department of Paleontology, National Museum in Prague; SEMs of the new species were taken using a Phenom Prox scanning electron microscope in the Biological Museum of the Sun Yat-sen University. Images were combined into figures using Adobe Photoshop CS6. All original images, including additional views not presented in this paper, are included in the dataset submitted to the Zenodo archive (https://zenodo.org/ under https:// doi.org/10.5281/zenodo.3693743. SEMs of the megasternine genera for the identification key are mostly based on specimens deposited in NMPC, except for rare genera (Kahanga, Gillisius) for which holotypes were examined.
Redescription. Body broadly oval and moderately convex; body outline not interrupted between pronotum and elytra. Head. Excised in front of eyes laterally, antennal base exposed. Labrum concealed under clypeus, not exposed dorsally. Clypeus not deflexed, truncate anteriorly, without anterolateral extensions; anterior margin narrowly beaded. Frontoclypeal suture obsolete, only visible as impunctate bar. Frons with even surface. Eyes rather small, rounded, projected laterally; interocular distance ca 5-6× the width of one eye in dorsal view. Dorsal punctation of head consisting of punctures each bearing a long seta. Maxillary palpus slightly longer than half of width of head, with ventral sucking disc in male; palpomere 2 strongly swollen, longer than palpomere 3; palpomere 4 symmetrical, slightly shorter than palpomere 2, but longer than palpomere 3. Men-tum ca 2.1-2.4× as wide as long, trapezoidal, anterior margin not emarginate medially (Figs 2B, 3A). Labial palpomere 3 slightly longer and as broad as palpomere 2, symmetrical. Gula well developed throughout, wide posteriorly, moderately narrowed anteriorly. Antennae with nine antennomeres, ca 0.7× width of head; scape a little longer than antennomeres 2-6 combined; club compact, pubescent, ca 2× as long as wide (Fig. 3D), slightly longer than scape.
Discussion. Hebauer (2002a) proposed Himalcercyon as a subgenus of Cercyon, mentioning that it corresponds to Cercyon in all characters except for the shape of the mesoventral plate. The form of the mesoventral elevation is one of most important generic characters in the Megasternini, and clearly differentiates both Himalcercyon species from all other members of the genus Cercyon. Both species of Himalcercyon are very similar to each other in all important characters and in the general form of male genitalia, indicating that they are likely closely related. Moreover, both species occur in the Himalayas. All of this supports Himalcercyon as a monophyletic clade that differs from Cercyon, as well as other megasternine genera, in the character currently considered as crucial at the generic level. For this reason, we elevate Himalcercyon to genus rank. See Diagnosis for the characters distinguishing Himalcercyon from other megasternine genera, and the identification key for a comparison of Himalcercyon with other Asian Megasternini. Body broadly oval, elytra combined 1.1× longer than wide (Fig. 1A). Prosternum widely carinate medially (Fig. 2C, D).
Thorax. Pronotum with punctation similar to that on frons, interstices without microsculpture; lateral marginal bead shortly overlapping to anterior margin but not to posterior margin, stopping at posterior angle. Scutellar shield smooth, with three to five punctures. Elytral striae sharply impressed (Figs 1E-F), striae 6, 8, and 9 not reaching base; intervals with much finer and sparser punctures than on pronotum, each interval puncture bearing a fine short seta (Fig. 3G), interstices between punctures smooth. Epipleuron with bare outer and pubescent inner portion delimited from each other by a fine ridge, inner pubescent part narrower than the outer part, reaching the level of posterior part of metaventrite. Mesoventral elevation arrowhead-shaped, ca 2.0× longer than wide, densely pubescent (Fig. 3C). Metaventrite with large median elevation, finely and sparsely punctate (Fig. 3F), interstices without microsculpture; lateral portions microsculptured with sparse coarse punctures and dense pubescence. Legs with trochanters densely pubescent, femora with sparse and moderately coarse punctures, interstice between punctures with fine microsculpture consisting of transverse lines.
Etymology. The species is named after Dr Franz Hebauer, a German taxonomist of the Hydrophiloidea who recognized and described Himalcercyon as a subgenus of Cercyon.
Head. Clypeus with moderately dense fine setiferous semicircular punctures, smooth between punctures. Frons with punctures of the same size and density as those on clypeus, smooth between punctures. Mentum 1.4× wider than long, rugose, with dense punctures (Fig. 2B), slightly concave anteriorly. Antenna with pedicel ca 0.2× as long as scape, pedicel ca. as long as antennomeres 3 and 4 combined, cupule small. Thorax. Pronotum with punctation similar to that on frons, interstices without microsculpture; lateral marginal bead shortly overlapping to anterior margin but not to posterior margin, stopping at posterior angle. Scutellar shield smooth, with five to seven punctures. Elytral striae sharply impressed (Fig. 1A), striae 6, 8, and 9 not reaching base; intervals with finer and sparser punctures than on pronotum, each puncture bearing a fine short seta, interstices between punctures smooth. Epipleuron with bare outer and pubescent inner portion delimited from each other by a fine ridge, inner pubescent part narrower than the outer part, reaching the level of posterior part of metaventrite (Fig. 1A). Mesoventral elevation arrowhead-shaped, ca 1.5× longer than wide, sparsely pubescent (Fig. 2F). Metaventrite with large median elevation, finely and sparsely punctate (Fig. 2E), interstices without microsculpture; lateral portions microsculptured, with sparse coarse punctures and dense pubescence. Legs with trochanters densely pubescent, femora with sparse and moderately coarse punctures, interstice between punctures with fine microsculpture consisting of transverse lines.

Male genitalia.
Middle lobe of abdominal sternite IX narrow, shorter than lateral struts (Fig. 1D). Aedeagus (Fig. 1C) with median lobe ca as long as tegmen; paramere ca 1.5× as long as phallobase. Paramere gradually narrowed from base to apex, obliquely truncate apically, widened inwards to form a process with a few setae. Median lobe ca as wide as paramere, gradually narrowing in apical third, apex narrowly rounded, gonopore subapical.
Distribution. Known from two localities in central Nepal (Fig. 4).

Key to Eastern Palaearctic and Oriental genera of the Megasternini
The following key is mainly based in the ventral characters, namely the form of prosternum and meso-and metaventrite, which are illustrated in Figures 5-8. The concept of some of the genera will likely be modified in the future; the key reflects the current status. The key includes all genera occurring east of Iran, the Black Sea, and the Ural Mountains. (i.e. it does not cover the Near East and the Arabian Peninsula); eastwards it includes all regions west of New Guinea. See Table 1 for the number of described species and references to the most important keys or taxonomic treatments for each genus. Remarks and numbers of species only refer to those from the Eastern Palaeartic and Oriental Regions.

1
Antennal grooves large, reaching to the lateral margin of hypomeron (Fig. 5A, B, D) 2 Metaventrite with complete femoral lines reaching from posteriomesal portion to anterolateral corner (Fig. 5A, D) ..... 3 -Metaventrite without complete femoral lines, at most with short vestiges anterolaterally. (Fig. 5B, C) (Fig. 5A). Mentum with sharply pointed anterolateral corners (Fig. 8D)  Median portion of prosternum roof-like, high (Fig. 5C). Mesoventral plate longer than wide. Metaventrite without any traces of femoral lines (Fig. 5C)  21 Median portion of prosternum with a pair of transverse ridges partly delimiting prosternal process (Fig. 8A)  and Oreocyon Krumbiegel, 1949 (a genus of Canidae described based on fur remains, later renamed to Dasycyon Krumbiegel, 1953 due to homonymy and today considered as a synonym of Canis Linnaeus, 1758). To avoid the homonymy, we are here proposing a new replacement name Oreosternum nom. nov. for Oreocyon Hebauer, 2002. The new name combines the prefix oreo-referring to mountains as used in the original name, and the core sternum, referring to the expected close relationship of this genus to Paroosternum Scott, 1913 exhibited by the prosternal morphology (see the key above). The new name is gender neutral.

Discussion
The genus-level systematics of the tribe Megasternini are currently based on the traditionally understood genera, defined by characters of the prosternum and meso-and metaventrite, i.e. structures which are morphologically very diverse within the clade. Following this approach, it is possible to define small and morphologically rather uniform genera for roughly half of the known species. On the other hand, the remaining half of megasternine species (i.e. ca 270 species) is assigned to the genus Cercyon Leach, 1817 as they are rather uniform in ventral characters. Eleven subgenera are defined inside of Cercyon to facilitate the identification of species, some of which seem to truly group related species (e.g., Arcocercyon Hebauer, 2003, Paracycreon d'Orchymont, 1942, but others very likely grouping unrelated species sharing a single derived character (e.g., Acycreon d'Orchymont, 1942; see Arriaga-Varela et al. 2018b). Preliminary molecular analyses , Arriaga-Varela unpubl. data) clearly indicate that Cercyon as currently circumscribed is a polyphyletic genus which needs to be reclassified in the future. To facilitate future analyses, it is necessary to reexamine Cercyon species and define groups of morphologically similar and likely closely related species. Selected representatives of these groups should later be included in the phylogenetic analysis. To that end, this paper recognizes Himalcercyon as such a group. The phylogenetic position of this clade needs to be tested in future analyses.