Holotype: ♂, CHINA, Sichuan: Chengdu City, Dujiangyan, Zipingpu Town, Lingyanguanyinshan scenic area (灵岩观音山风景区), 29.IV.2016, 31.02956N, 103.61651E, alt. 1210 m, ant colony under deadwood bark, leg. Chao Zhou & Li He (BITS). Paratypes: 1♀, same data as holotype (BITS); 1♂, CHINA, Jiangxi: Yichun City, Mingyueshan (明月山), 27°35'25"N, 114°17'02"E, alt. 1600 m, 22.X.2013, Zhong Peng leg. [from a colony of Pheidole sp.] (SNUC).

This new species is allocated to the subgenus Scaphipaussus Fowler 1912 according to the groups key of Robertson & Moore (2016). Within this subgenus the new species is unique with regard to the combination of the following characters: body lustrous, scatteredly and shortly pubescent, seeming hairless to the naked eyes; head vertex distinctly excavated, laterally bordered by auriculate costae; scape longer than wide, cylindrical; fused flagellum elongated subtriangular, with dorsal margin of outer side with five robust teeth and five deep incisions, while ventral margin of that with five weak protrusions and five weak emarginations; pronotum wider than long, width/length = 1.22, as wide as head, (anterior part width)/(posterior part width) = 1.10, anterior part laterally angulate, posterior part with lateral margins roundly protruded in the apical 3/4 and obliquely substraight in the basal 1/4; elytra not bearing lateral trichome fringes; legs robust; pygidium with posterior dorsal margin distinctly upturned and explanate, marginal trichome fringe dense.

Male holotype. Medium size, body 4.83 mm long. Length (mm) of different body parts: head (0.68) : pronotum (0.91) : pronotal anterior part (0.40) : pronotal posterior part (0.43) : elytra (3.21) : pronotum-elytra (4.12); width (mm): head (1.13) : pronotum (1.11) : pronotal anterior part (1.11) : pronotal posterior part (1.01) : elytra (1.88). (Pronotum-elytra length)/(elytral width) = 2.20.

Body (Figs 1; 2A–C) oblong and overall appearance lustrous due to lack of punctures or microsculptures on the majority of body parts; unicolor, mostly brown, with elytra lighter and pronotal posterior part darker (the body color is much lighter when the species is alive (Fig. 6C–D) or the specimens are newly collected); integument scatteredly pubescent with short, thin, yellowish setae except pygidium, and the setae on pronotum are distinctly thicker and shorter.

Head (Fig. 3A, C) subglobular, width/length = 1.67, vertex high and dorsal parts strongly inclined; clypeus indented, depressed at centre and with frontal ridges well demarked; median frontal suture short and dark, not extending to anterior margin of eye; vertex distinctly and broadly excavated, bordered by a raised, auriculate, double-walled costa at each side; the two costae are separated at their tops by a distinct groove; in addition, one straight and short costa sits in the basal corner of each auriculate costa; eyes reniform, small and less prominent; temples narrow but distinct, slightly projecting laterally beyond eyes in dorsal view; (eye length)/(gena length) = 1.23; head surface rugosely and contiguously punctate, especially along median frontal suture, while central excavation glabrous and smooth; neck narrow, strongly constricted.

Mouthparts of “closed” type, adjacent to the underside of head. Labrum transverse and rectangular. Palpifer exposed. Maxillary palpomere II large, broad and compressed, almost as wide as long, wider than 2× width of palpomere III and longer than palpomeres III and IV combined, with mesal margin more or less rounded; palpomere III only slightly wider than palpomere IV; palpomere IV tapering apically and slightly longer than palpomere III. Palpiger exposed. Labial palpomere II with socket for palpomere III positioned along midline; palpomere III narrow, slender, fusiform and slightly compressed, length/width = 3.43, longer than 2× length of palpomeres I and II combined. Ligula large and broad, with apical margin broadly rounded at middle. Gula with width/length (at narrowest point) = 0.42.

Antenna (Fig. 4A–D): scape longer than wide, cylindrical, surface rugose, without trichome brush on inner anterior margin. Pedicel vestigial ring-shaped. Fused flagellum elongated subtriangular, 2.59 times as long as wide; dorsal surface with a shallow longitudinal furrow, ending at the level of the most apical incision; ventral surface slightly convex; inner side broadly and weakly undulate; apical side obliquely truncate and rounded at tip; both apical side and inner side (especially along apical half) marginate and the marginal band rugosely punctate; outer side longitudinally excavated from outer basal angle to short distance before apex; dorsal margin of outer side with five robust teeth and 5 deep incisions; ventral margin of outer side with five weak protrusions and five weak emarginations; inner basal corner roundly protruded; outer basal angle large, prominent and extended, without trichome; basal side between insertion and outer basal angle simply emarginate without traces of indentations.

Pronotum (Fig. 3A, C) wider than long, width/length = 1.22, as wide as head, widest at lateral angulations of anterior part; transversally cleft, anterior part almost as long as posterior part, (anterior part width)/(posterior part width) = 1.10; anterior part moderately raised, strongly divided by a longitudinal groove, weakly edged behind and laterally angulate; posterior part with lateral margins roundly protruded in the apical 3/4 and obliquely substraight in the basal 1/4, medially with a deep, wide and longitudinal furrow towards scutellum; transverse furrow with trichome-bearing glandular openings at the furthest lateral ends; trichomes dark yellow, not much contrasting with the brown pronotum, (distance between trichomes)/(trichome width) = 3.38; median excavation glabrous and smooth, both sides of excavation equally rugose.

Scutellum ligulate, wider than long, surface densely punctate.

Elytra oblong, length/width = 1.71; humeri hardly demarcated; surface smooth and lustrous, without punctures, merely sculptured with micropores; areas along suture devoid of pubescence. Metathoracic wings fully developed.

Legs robust, with smooth surfaces. Tibiae compressed; protibiae straight, mesotibiae slightly sinuate, metatibiae almost straight; metatibiae broader than meso- and protibiae; tibial spurs absent; pubescence denser on apical part of all tibiae. Tarsi with tarsomeres I–IV subequal in length, with apical margins entire, dorsally straight or inconspicuously emarginate; all tarsomeres without adhesive pads but loosely setose on lateral parts of ventral surfaces.

Stridulatory organ present: scraper as a curved row of transverse spines on abdominal ventrite I, and file present at inner base of metafemur.

Pygidium (Fig. 3E) with disc shining, only sparsely covered with microtrichiae; posterior dorsal margin distinctly upturned and explanate; marginal trichome fringe dense.

Aedeagus as shown in Figs 5A–D: median lobe elongate, slender and arcuate, apex distinctly emarginate; subbasal articulation tubercle well developed; parameres slender, apical parts narrow, apex rounded and devoid of setae; strut present, embedded in membraneous tissue in the basal part of median lobe.

Female paratype (Figs 2D–E; 3B, D). Sexual dimorphism weak, no distinct differences between shape of eyes, shape of costae at head vertex, shape and microstructure of fused flagellum, curvature of metatibiae. The abdomen of the female paratype, including female genitalia, was poured into wash basin when the present author working in a drunken state.

All the type specimens were collected from colonies of the host ant Pheidole sp. (Hymenoptera, Formicidae, Myrmicinae). The male holotype and the female paratype from Sichuan Province were collected from a Pheidole colony under deadwood bark. Two Pheidole sp. minor workers (Fig. 2F) glued to a card and pinned together with the holotype.

Biotope in Lingyanguanyinshan scenic area (Sichuan) as shown in Fig. 6A–B.

The specific epithet is dedicated to Mr. Chao Zhou (Chengdu, Sichuan, China), one of the collectors of this new species and a good amateur obsessing with beetles.

China (Jiangxi, Sichuan).

Maruyama (2016) partially revised the Paussus hystrix group (to which unequivocally the new species belongs) and described 19 species from East Asia and Southeast Asia. Comparing with these species, Paussus (Scaphipaussus) zhouchaoi sp. n. seem to be hairless to the naked eye and it also can be distinguished by the combination of the characters in the above paragraph of Diagnosis.