﻿A new species of Physomerinus Jeannel (Coleoptera, Staphylinidae, Pselaphinae) from Jiulong National Wetland Park, China

﻿Abstract Physomerinusjiulongensissp. nov. is described from a series of overwintering individuals collected in decomposing wood at Jiulong National Wetland Park, East China. The new species is characterized and separated from related congeners by the unique form of the sexually dimorphic maxillary palpi, greatly swollen male metafemora, as well as by the shape of the genitalia of both sexes. A key to, and a distributional map of, Physomerinus species occurring in China and on the Ryukyu Islands, Japan is provided.


Introduction
The ant-loving beetle genus Physomerinus Jeannel, 1952 (Batrisitae: Batrisini) contains 12 species distributed in the Oriental and East Palaearctic regions (Newton 2022). The genus was placed in Jeannel's fifth division of the Batrisina (Jeannel 1958), or as a member of  'Batrisocenus complex' of genera. Species of this genus have the pronotum with distinct median and two lateral longitudinal sulci, and a complete transverse antebasal sulcus, the sides of the pronotum lack spines, each elytron has two basal foveae, and the first visible tergite (IV) is longest. The males possess variously modified metafemora, and often a constricted basal capsule of the aedeagus. The latter indicates a close relationship to the genus Batriscenaulax Jeannel, 1958 which includes six species from Japan and China Yin 2022). One major taxonomic problem of Physomerinus is that the type species, P. septemfoveolatus (Schaufuss L. W., 1877) from Southeast Asia, has a secretory trichome on the anterolateral margin of the scape, and the aedeagus bears a large basal capsule (Jeannel 1952: fig. 42;Nomura pers. com.), which are congruent with the defining features of a group of genera centered on Batrisiella Raffray, 1904. A revision of the genus is needed to clarify this problem.
Within Physomerinus several species may indeed form monophyletic subgroups by their shared male features. Physomerinus hasegawai Nomura, 1991 from China (Taiwan) and Japan (Ryukyu Islands) and P. clavipes Zhang & Yin, 2022 from China (Guangxi) are linked by the smoothly broadened lateral portions of maxillary palpomeres 4, and greatly swollen metafemora with a large lateral cavity (Nomura , 2010(Nomura , 2012(Nomura , 2013Zhang and Yin 2022).
During December 2022 to January 2023, a team of IZCAS led by Hong-Bin Liang surveyed the beetle fauna of Jiulong National Wetland Park in Zhejiang Province, East China. A series of overwintering pselaphine beetles were collected from decomposing wood along a riverbank and were later sent to me for identification. The result revealed a new species of Physomerinus closely related to P. hasegawai and P. clavipes, which is diagnosed, described, and illustrated in this paper.

Material and methods
The holotype of the new species described here is deposited in the Institute of Zoology, Chinese Academy of Science (IZCAS), Beijing, China, and the paratypes in IZCAS and the Insect Collection of Shanghai Normal University (SNUC), Shanghai, China. The label data of the material are quoted verbatim.
Dissected parts were mounted in Euparal on plastic slides pinned with the specimen. The habitus image of the beetle was taken using a Canon 5D Mark III camera, equipped with a 7.5× Mitutoyo M Plan Apo lens, and two Godox V860III-C TTL Li-Ion flashes were used as the light source. Images of the morphological details were produced using a Canon G9 camera mounted to an Olympus CX31 microscope under reflected or transmitted light. Helicon Focus v. 8.2.0 Pro was used for image stacking. All images were modified and grouped into plates using Adobe Photoshop CC 2020.
Measurements were taken as follows: total body length was measured from the anterior margin of the clypeus to the apex of the abdomen; head length was measured from the anterior margin of the clypeus to the head base, excluding the occipital constriction; head width was measured across the eyes; the length of the pronotum was measured along the midline, the width of the pronotum is its maximum width; the length of the elytra was measured along the suture; the width of the elytra was measured as the maximum width across both elytra; the length of the abdomen is the length of the dorsally exposed part of the abdomen along its midline, the width of the abdomen is its maximum width. Terminology follows Chandler (2001) and Yin (2022). The abdominal tergites and sternites are numbered in Arabic (starting from the first visible segment) and Roman (reflecting true morphological position) numerals, e.g., tergite 1 (IV), or sternite 1 (III). Paired appendages in the description of the new species are treated as singular. Diagnosis. Male. Body length approximately 1.8 mm. Head subtruncate at base; vertex with transverse sulcus posterior antennal tubercles, with short mediobasal carina, vertexal foveae small and asetose; antenna relatively long, antennomeres each elongate, lacking modification; maxillary palpomere 4 protuberant on lateral margin. Discal stria of elytron extending posteriorly to near posterior margin. Metafemur greatly swollen and with large cavity on lateral side. Abdomen dorsally with tergite 1 (IV) longer than 2-4 (V-VII) combined in dorsal view, simple. Aedeagus strongly asymmetric, ventral stalk much shorter than dorsal lobe. Female. Body length slightly over 1.8 mm, maxillary palpus and metafemur lacking modification, genital complex as in Fig. 1G.
Mesoventrite short, demarcated from metaventrite by ridged anterior edges of impressed areas where large, setose lateral mesocoxal foveae situated at mesal portions of impression, with pair of admesal carinae; setose median mesoventral foveae broadly separated, lateral mesoventral foveae large and setose, forked internally. Metaventrite moderately impressed at middle, with pair of setose lateral metaventral foveae, posterior margin with small and narrow split at middle.
Legs elongate; mesotrochanter with tiny, indistinct protuberance on ventral margin; metafemur greatly swollen laterally and with large cavity (Fig. 1D), dorsal side with short sensory setae, with tufts of setae along mesal and posterior margin of cavity.
Comparative notes. This species shares with P. hasegawai from Taiwan and Japan and P. clavipes from Guangxi the sexually dimorphic fourth segments of the maxillary palpi and greatly swollen metafemora of the male, and together these species may form a monophyletic group. The male of Physomerinus jiulongensis sp. nov. can be readily separated by the presence of two protuberances on the lateral margins of maxillary palpomeres 4, and short ventral lobe of the aedeagus. In contrast, both P. hasegawai and P. clavipes have smoothly swollen lateral margins of maxillary palpomeres 4, and the ventral stalks of the aedeagi are as long as or slightly longer than the dorsal lobes. Distribution. East China: Zhejiang ( Fig. 2A). Bionomics. All overwintering adults were collected in decomposing wood near a river bank (Fig. 2B, C).
Etymology. The new species is named after its type locality, i.e., Jiulong National Wetland Park.
Key to males of Physomerinus species from China and the Ryukyu Islands, Japan Hind legs relatively much shorter; aedeagus with ventral stalk as long as dorsal lobe, and slightly broadened from base toward apical portion in dorso-ventral view  comments. Shûhei Nomura also pointed out the distribution of P. hasegawai in Taiwan. The present study was supported by the National Natural Science Foundation of China (no. 31872965).