(25 exx.). Holotype: China: ♂, ‘China: Zhejiang, Lishui, Bihu To., Hongyu Vill., Jiulong Wetland Park, 28.38479°N, 119.8247°E, 60 m, 31.xii.2022, wood, Liang, Qin, Wang leg. [丽水九龙湿地公园, 梁红斌, 秦雨瑶, 王凯 采]’ (IZCAS). Paratypes: China: 9 ♂♂, 8 ♀♀, same collecting data as for holotype; 4 ♂♂, 3 ♀♀, same collecting data as for holotype, except ‘4.i.2023’ (4 exx. in IZCAS, 20 exx. in SNUC).

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.

Figure 1. 

Morphology of Physomerinus jiulongensis sp. nov. (A–F male G female) A habitus B head and pronotum C maxillary palpus D metafemur E, F aedeagus, lateral (E) and ventral (F) G genital complex. Scale bars: 0.5 mm (A); 0.3 mm (B); 0.1 mm (C, E, F, G); 0.2 mm (D).

Male. Body (Fig. 1A) length 1.77–1.81 mm; color reddish-brown, tarsi and mouthparts lighter in color. Dorsal surface of body covered with relatively short pubescence.

Head (Fig. 1B) subtruncate at base, much wider than long, length 0.35–0.36 mm, width across eyes 0.40–0.43 mm; vertex finely punctate, smoothly convex, with small, asetose and broadly separated vertexal foveae (dorsal tentorial pits), with short, straight transverse sulcus posterior antennal tubercles, mediobasal carina extending posteriorly to occipital constriction and anteriorly to level of slightly posterior midlength of eyes, antennal tubercle moderately raised, surrounding area roughly punctate; frons broadly and shallowly impressed medially, confluent with clypeus; clypeus with smooth surface, its anterior margin carinate and moderately raised; ocular-mandibular carina complete. Venter with tiny gular foveae (posterior tentorial pits) originating from shared transverse slit, with distinct median carina extending from foveae anteriorly to mouthparts. Compound eyes greatly prominent, composed of approximately 35 large ommatidia. Antenna 0.97–1.01 mm long, lacking modification; antennomere 1 thick, subcylindrical, 2–7 each slightly elongate, successively longer, 8 shortest, slightly elongate, enlarged 9–11 forming distinct club, 9 and 10 of subequal size, each moderately expanded on mesal margin, 11 largest, 1.7× as long as 10, fusiform, truncate at base.

Pronotum (Fig. 1B) slightly longer than wide, length 0.41–0.44 mm, width 0.40–0.42 mm, widest at middle; lateral margins rounded; disc moderately convex, finely punctate, with median longitudinal sulcus much shorter than semicircular lateral sulci in dorsal view; lacking median antebasal fovea, with complete, deep transverse antebasal sulcus connecting lateral antebasal foveae, with small, blunt antebasal tubercles; outer and inner pair of basolateral foveae small. Prosternum with anterior part as long as coxal part, with small lateral procoxal foveae; hypomeral ridge complete, with lateral antebasal hypomeral pit; margin of coxal cavity thinly carinate.

Elytra much wider than long, length 0.61–0.62 mm, width 0.69–0.72 mm; each elytron with two large, widely separated basal foveae, lacking subbasal fovea; humeral protuberance small, acute; discal stria long, extending from outer basal fovea to approximately apical 4/5 of elytral length; subhumeral fovea small, carinate marginal stria extending posteriorly from fovea to apex of elytron. Metathoracic wings fully developed.

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.

Abdomen much narrower than elytra, widest at basolateral margins of tergite 1 (IV), length 0.44–0.52 mm, width 0.62–0.65 mm; lacking modification. Tergite 1 (IV) in dorsal view longer than 2–4 (V–VII) combined; setose basal sulcus separated by mediobasal and one pair of basolateral foveae, with pair of thin, triangular discal carinae; tergites 2 and 3 (V and VI) each short, lacking fovea, 4 (VII) as long as 2 and 3 combined along middle, with one pair of small basolateral foveae, 5 (VIII) semicircular, posterior margin roundly emarginate at middle. Sternite 2 (IV) with one pair of mediobasal and one pair of basolateral foveae, and large basolateral sockets, with one pair of short and one pair of long lateral carinae; midlength of sternite 2 shorter than 3 (V) and 4 (VI) combined, 3–5 (VII) each short, successively shorter, lacking fovea, 6 (VIII) greatly transverse, posterior margin roundly convex at middle, sternite 7 (IX) membranous, indistinct.

Aedeagus (Fig. 1E, F) 0.21 mm long, strongly asymmetric; median lobe with constricted basal capsule and sub-triangular foramen, with long basoventral projection, ventral stalk short, in lateral view curved and narrowing towards apex; dorsal lobe much longer than ventral stalk, broad, sides incurved, with narrowed apex; parameres fused to form broad, semi-sclerotized plate.

Female. Similar to male in external morphology; antenna slightly shorter; maxillary palpus and metafemur lacking modification; each compound eye composed of approximately 30 ommatidia; humeral protuberance small and weak; metathoracic wings reduced. Measurements (as for male): body length 1.83–1.85 mm; length/width of head 0.35–0.37/0.41–0.43 mm, pronotum 0.40–0.43/0.40–0.43 mm, elytra 0.54–0.57/0.66–0.71 mm; abdomen 0.50–0.54/0.65–0.67 mm; length of antenna 0.92–0.96 mm; maximum width of genital complex (Fig. 1G) 0.29 mm, genital plate much wider than sternite 9, lateral arms broadened distally.

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.

East China: Zhejiang (Fig. 2A).

Figure 2. 

Distribution and habitat of Physomerinus A map showing the distribution of Physomerinus species occurring in China and on the Ryukyu Islands, Japan B, C collecting environment (B) and habitat (C) of P. jiulongensis sp. nov. in Jiulong National Wetland Park, Zhejiang, China. Yu-Yao Qin (left) and Kai Wang (right) were searching for pselaphines from decomposing wood.

All overwintering adults were collected in decomposing wood near a river bank (Fig. 2B, C).

The new species is named after its type locality, i.e., Jiulong National Wetland Park.