﻿Taxonomy of Homoeusa Kraatz, 1856 (Coleoptera, Staphylinidae) from the East Palearctic: I. Homoeusarufescens (Sharp, 1874) and a new allied species

﻿Abstract There is insufficient information to identify most species of the myrmecophilous rove beetle genus Homoeusa. In this paper, after examining the type material, Homoeusarufescens (Sharp, 1874) is redescribed in detail and its new allied species Homoeusaovatasp. nov. is described. We also observed the behavior of these two species in the field; the behavior was similar to that reported for H.acuminata (Märkel, 1842). A checklist of Homoeusa from the Palearctic and Nearctic is also provided.


Introduction
Nine species of the myrmecophilous genus Homoeusa Kraatz, 1856 (tribe Oxypodini) are known from the Palearctic region, of which eight are recorded from the East Palearctic, i.e., China, Taiwan, Far East Russia, and Japan (Smetana 2004;Pace 2010;Maruyama et al. 2013). Most species have not been redescribed since their original descriptions 90-120 years ago, which are very short and almost useless for identification. This series of papers aims to revise the East Palearctic species of Homoeusa.
Homoeusa is distinguishable from the other myrmecophilous oxypodine genera of the Palearctic by its sub-limuloid body shape and unilobed ligula; the latter distinguishes it from the similar genus Thiasophila Kraatz, 1856, which has a bifid ligula. Maruyama and Zerche (2014) suggested that Thiasophila rufescens Sharp, 1874 described from Japan could be a member of Homoeusa. Probably based on this statement, Schülke and Smetana (2015) listed T. rufescens as a Homoeusa species. Here, we confirmed that T. rufescens should be transferred to Homoeusa. We found that another species allied to T. rufescens is included in the syntypes of T. rufescens and is very common in the nests of Lasius species in the fuliginosus group (formerly members of the subgenus Dendrolasius). It was found to be a new species and is also described here. The taxonomy of Homoeusa is very difficult and some species complexes still require time to work. Therefore, this paper treats these two species, which are very common and need to be identified, prior to the revision of the whole genus. We discuss the feeding behavior of these species and provide a checklist of the world species of Homoeusa.

Materials and methods
The material examined in this study is deposited mostly in the Kyushu University Museum (KUM), and some in The Hokkaido University Museum, Sapporo (HUM), National Museum of Nature and Science, Tsukuba (NSMT), Sagamihara City Museum, Kanagawa, Japan (SCM), and private collections of Hiromu Kamezawa (Saitamaken) (cKam). Type material of Homoeusa rufescens is deposited in the Natural History Museum, London (NHM). Part of the paratypes from the Russian Far East are in the Institute of Biology and Soil Science, Vladivostok, Russia (IBSS).
The morphological observations and measurements were conducted using an Olympus SZX10. On the methods of dissection and preparation of permanent slides, we followed Maruyama (2004). Habitus photos were taken using a Canon 7D camera with a Canon MP-E 65 mm f/2.8 1-5X macro lens, and Neewer TT560/1Y strobe, and image stacking was conducted using Zerene Stacker ver. 1.04 (Zerene Systems LLC). Drawings were made using a microscope Olympus BX50 with an Olympus drawing tube attached. Measurement definitions and abbreviations are shown as follows: BL, approximate body length; AL, antennal length; HW, head width; PL, pronotal length; PW, pronotal width; EL, elytral length (sutural length from apex of scutellum to posterior margin of elytra); EW, elytral width; HTL, hind tibial length. Measurements of dorsal morphology were made on 20 specimens of each species, and then the sexes were identified. Measurements of each segment of the antenna (six specimens of each species) were made on dissected specimens mounted in Euparal.
Symbiotic hosts were mostly identified by MM, F. Ito, and some by TN and K. Kinomura.

Host ants
The scientific names of Lasius ants follow Bolton (1995), Radchenko (2005), and Boudinot et al. (2022). Lasius fuji was formerly recognized as the eastern Palearctic population of L. fuliginosus. Recently, the Japanese population of "Lasius fuji" has been suggested to be a species complex (Maruyama et al. 2013). Therefore, 'Lasius cf. fuliginosus' is adopted as the name of the Japanese population here. Lasius fuliginosus and allied species have long been classified in the subgenus Dendrolasius. However, Boudinot et al. (2022) synonymized all the subgenera of Lasius with the genus Lasius and recognized five species groups, which is adopted here.
The distribution and host ant species of each species in the checklist is a synthesis of information from mostly Schülke and Smetana (2015), Maruyama et al. (2013), and this paper. Diagnosis. This genus is distinguished by the following combination of characteristics: body somewhat sub-limuloid; apex of ligula unilobed and round; antennae not or weekly clubbed; posterior margin of antennal insertion forming distinct latitudinal carina extending medially; spermatheca somewhat S-shaped. Maruyama (2009), this genus is similar to Losiusa and Aspidobactrus, which are called the "Homoeusa genus complex". Together with some other genera, these genera are classified in the subtribe Dinardina of the tribe Oxypodini (Schülke and Smetana 2015). The subtribe Dinardina is defined based on its limuloid body and shield-like pronotum (Seevers 1978). However, the monophyly of Dinardina (sensu Seevers 1978) has been rejected; the Dinarda+Thiasophila clade was distant from Myrmobiota (Osswald et al. 2013), which is a close relative of Homoeusa and often regarded as a junior synonym of Homoeusa. The Homoeusa genus complex is assumed to form a clade with Myrmobiota distant from Dinarda+Thiasophila given their symbiotic hosts and morphological characteristics of the head, ligula, and facial structure of the body. The relatedness of the Homoeusa genus complex and Nearctic genus Decusa Casey, 1900 was also suggested (see also Wasmann 1901). To reveal the relationships among these genera in detail, future phylogenetic research is necessary.
Diagnosis. It is distinguished from the other species of the genus by the following combination of characteristics: body small, slender and subparallel-sided; pronotum less transverse (PW/PL, 1.38-1.47), widest near middle, posterolateral angle obtuse, posterior margin hardly sinuate; apical lobe of male aedeagus thick with round sheetlike projection; apical lobe of paramere straight; velum broad and round.
Head (Fig. 6) relatively large, reddish brown; frontal margin with few long setae; carina of antennal insertion gently sinuate; eye large. Antennae (Fig. 7) stout, as long as head and pronotum combined, reddish brown, but segments I-IV and XI paler; segment I dilated, widest near apex; segment II slightly longer than III, widened apically; segment III widened apically; segment IV-X widened apically, each apical margin fringed with small teeth and longer than wide; segments IV as long as wide; segments V-X slightly wider than long; segment XI oblong oval. Labrum (Fig. 8), apical margin concave, surface with 20 setae; epipharynx with 2 pairs of sensillae on anterior margin, and 3 pairs of micro setae on lateral margin. Mentum ( Fig. 9) with 3 pairs of setae and 1 pair of microsetae; anterior margin deeply concave. Prementum (Fig. 10) with many pseudopores, and with 2-3 real pores and 1 setal pore on each side near middle lateral margin. Ligula elongate, apical margin with 1 pair of spinule and several sensilla. Labial palpus, segment II and III each with 5 setae. Pronotum (Fig. 6) convex, subrectangular, less transverse, widest near middle; posterolateral angle obtuse, posterior margin hardly sinuate, brownish red to brownish black; surface finely covered with setae and punctures. Elytra (Fig. 6) slightly widened posteriorly, posterior margins shallowly notched near lateral corners, brownish red; surface finely covered with setae and punctures, moderately reticulated. Mesoventral processes (Fig. 11) narrow, with medial carina forming Y-shaped, apex rounded; mesocoxal cavities separated; metaventral process weakly produced.
Abdomen elongate, slightly narrowed posteriad; surface sparsely covered with short setae and each posterior margin with long stout setae; polished and weekly reticulated.
Male: 8 th sternite longer than wide, weakly rounded apically. Median lobe of aedeagus (Fig. 12), apical lobe of aedeagus thick with round flattened projection on ventral side; apical lobe of paramere (Fig. 13) short and straight; velum broad and round; ventral margin of paramerite almost straight.
Female: 8 th sternite longer than wide, weakly rounded apically. Spermatheca (Figs 14-16), apical part moderately swollen; apical 3/5 with inner wall densely reticulated. Variation. In most individuals, the pronotum tends to be brownish red, but the color varies from brownish red to brownish black.
Distribution. Japan (Honshû, Shikoku, Kyûshû). Bionomics. From April to July, this species can be found in the trails of the Lasius fuliginosus species group (Fig. 28). When they encounter host ants, they pause briefly until the ants ignore them and then start walking. TN observed that they ate prey that the ants were trying to carry but dropped. This species is sometimes observed to climb on food that the host ant is carrying and eat it (Fig. 29).
Symbiotic hosts. Lasius cf. fuliginosus, L. morisitai, L. nipponensis, L. spathepus. Remarks. The original description suggested that the syntypes included another species (Sharp 1874). Based on our examination of the syntype, it includes two species, as mentioned in the original description. Here, the specimens of the more slender species that more closely match the original description are designated as the lectotype and paralectotype. The specimens that were judged a different species were excluded from the syntypes of H. rufescens and included in the paratypes of the following species. As is often the case with old specimens, the proteins in the lectotype were denatured and potassium hydroxide did not sufficiently dissolve the muscle of the genital organs (Fig. 3), but the morphological characteristics of this species were fully observed.
Abdomen elongate, gently narrowed posteriad; surface sparsely covered with short setae and each posterior margin with long stout setae; gently polished and weakly reticulated.
Female: 8 th sternite longer than wide, rounded apically. Spermatheca (Figs 25-27), apical part weakly swollen; apical half with inner wall densely reticulated. Variation. In most individuals, the pronotum tends to be darker, but the color varies from brownish red to brownish black.
Bionomics. This species behaves almost the same as H. rufescens. It is also observed to feed on prey among ants and sometimes climbs on, and eats food carried by ants (Fig. 31). Symbiotic hosts. Lasius fuji, L. cf. fuliginosus, Lasius morisitai, L. nipponensis, L. orientalis, L. spathepus.

Discussion
As Quinet and Pasteels (1996) reported, Homoeusa acuminata has been observed to climb on prey being carried by ants and feed on it, simultaneously hitchhiking and stealing food. This beetle has also been reported to rarely feed directly on food not yet carried by ants (Quinet and Pasteels 1995). Similar behaviors have been observed in H. rufescens and H. ovata sp. nov., such as stealing food while boarding and being ignored by the ants during the process. However, H. rufescens and H. ovata sp. nov. have also been observed feeding on food that ants are trying to carry and on prey dropped by ants. These two species may have a wider range of foraging strategies than H. acuminata and may also scavenge and not merely be kleptoparasitic. The optimal foraging strategy for these beetles near an ant trail may depend on the level of ant activity. Checklist of the genus Homoeusa Kraatz, 1856 Palearctic