Holotype: male: China, Jiangxi Province, Jinggangshan Mountain, 1.3 km southwest of Xiping County, 26°33'4"N, 114°12'2"E, 850 m, at the edge of a stream, beneath a stone, caught, 24 June 2011, Fenglong Jia leg. Paratype: five specimens: same data as holotype.

Length 0.9–1.0 mm, broadly oval, strongly convex in form. Dorsal surface smooth and shiny, without any punctation or striae. Each elytron with 8–12 conical and rough asperities. Antennae with last four antennomeres slightly clubbed, but not as strongly dilated as in other species of the genus. Prosternal process T-shaped, strongly elevated, with anterior margin straight and lateral margin smoothly concave. Pro- and mesotibiae each bear a ventral disc at the apex. Median lobe of the aedeagus long and laterally compressed, apex distinctly dilated, apical angle rounded, never prominent; left paramere digitus-shaped, half the length of the median lobe; right paramere petaliform.

Body widely oval, strongly convex in form. 0.9–1.0 mm in length, ca. 1.5× as long as wide, widest in the middle. Dorsal surface uniformly brown to dark brown, smooth and shining, without any punctation (Fig. 1). Ventral surface reddish brown, with front legs and mid legs yellow (Fig. 2). Micro-reticulation distinct on head and pronotum at magnification of more than × 1000, but fainter on elytra. The meshes of reticulation pentagonal or hexagonal (Fig. 5).

Figures 1–9. 

Sphaerius minutus sp. n. 1–3 habitus (1 dorsal view 2 ventral view 3 frontal view) 4 dorsal view of head 5 micro-reticulation on pronotum 6 epipleuron 7 ventral view of head 8 antenna 9 prosternal process.

Head. Short and broad. Frons attenuated anteriorly, with several setae along lateral margin. Clypeus short and broad, with anterior margin straight, fronto-clypeal suture indistinct. Compound eyes developed, strongly protruding (Figs 3, 4). Mentum flat, shovel-like, slightly attenuated anteriorly, anterior margin slightly rounded; lateral margin with two longitudinal ridges on each side (Fig. 7). Antennae inserted near eyes, extending well beyond the posterior margin of prosternum, 11–segmented; antennomere 3 long, about as long as the following 4 antennomeres combined; last 4 antennomeres slightly clubbed, but not dilated as strongly as in other species, at most twice as wide as previous antennomeres, with rough and long setae at apex (Fig. 8).

Pronotum. Attenuated anteriorly, ca. 0.35× as long as wide, lateral margin smoothly rounded; anterior angle produced and acute, posterior angle rectangular, slightly rounded; posterior margin slightly narrower than base of elytra. Scutellum almost equilaterally triangular, small and flat, with length a bit narrower than wide (Fig. 1).

Elytra. Smooth and shining, without any punctation or striae, lateral margin smoothly rounded (Fig. 5). Elytron with 8–12 conical and rough asperities arranged as in Fig. 15, some of which are obsolete or absent in some specimens. Epipleuron developed, constricted posteriorly, with a deep and broad furrow along basal third; outer edge with several round depressions composed of a cluster of micro-holes (Figs 6, 11).

Ventral surface. Prosternal process T-shaped, strongly elevated, ca. 0.6× as long as wide, extending to about half of procoxae; anterior margin straight, while lateral margin smoothly concave (Fig. 9). Mesoventrite short and broad, largely elevated at middle, constricted posteriorly, reaching the middle of mesocoxae, fused with the much larger metaventrite; metaventrite ca. 2.5× as long as mesoventrite; meso- and metaventrite smooth and flat, bearing very sparse bristles (Fig. 2).

Legs. Coxae transverse, procoxae close to each other, mesocoxae widely separated by meso-metaventrite, while metacoxae contiguous, forming a large transverse coxal plate with two setae at outer apical angle and middle of the posterior margin (Fig. 12). Profemur robust and broad, basal half of anterior margin rather concave, forming a cavity for head; mesofemur strongly dilated, short and broad, posterior side margin straight, forming obtuse angle; metafemur hidden under coxal plate. Protibia short and broad, bearing a row of short setae at inner edge and a row of denticles near the apex (Fig. 13); pro- and mesotibiae both bearing some long setae and a ventral disc at the apex (Figs 10, 11). Tarsus 3-segmented, first and second tarsomeres very short, shorter than wide, third tarsomere much longer, more than 3/4 as long as the entire tarsus; some setae on tarsus flattened and broadened, transforming into banding-shaped (Fig. 14). Claws quite well developed, strongly unequal, the smaller one ca. 0.5× as long as the other one in front legs, and much smaller in mid and hind legs (Figs 10, 11, 14).

Figures 10–18. 

Sphaerius minutus sp. n. 10 front leg 11 mid leg 12 hind leg 13 protibia 14 metatarsus 15 asperities on left elytron 16 abdomen 17 lateral view of male genitalia 18 illustration of male genitalia (lateral view).

Abdomen. Abdomen with three visible ventrites; first ventrite large and broad with two large coxal cavities occupying almost the entire ventrite, the basal half was divided into two parts by a longitudinal furrow in the middle; third ventrite shorter than first ventrite; second ventrite extremely narrow, ca. 0.15× as long as the third ventrite, anterior and posterior margins straight and parallel (Fig. 16).

Male genitalia. Median lobe long and laterally compressed, gradually narrowed toward apex, apex distinctly dilated, with apical angle rounded; left paramere digitus-shaped, half the length of the median lobe; right paramere petaliform (Figs 17, 18).

The species name comes from the Latin adjective minutus (= tiny), referring to the minute body size of this species.

Only known from type locality in Jiangxi Province, China.

This species was found under a stone on the muddy shore of a running stream. They were tightly attached to the lower surface of the stone when the stone was removed. Biology of larvae unknown.

The new species closely resembles S. papulosus Lense, 1940 described from Myanmar. These two species share several character states. Besides the similar body shape and size, their antennal clubs are composed of four antennomeres, they both have smooth and shiny dorsal surface and with similar reticulation on pronotum, and they both have some conical and rough asperities on the elytra. However, the new species has a different pattern in the arrangement of the asperities and the metatarsus is 3-segmented instead of 2-segmented.

The new species differs from other species of Sphaerius by its less dilated antennal club. Additionally, the smooth, uniformly brown dorsal surface, the T–shaped prosternal process, the serial denticles on protibiae, and ventral disc on pro- and mesotibiae are also important characters to identify the new species. As for the median lobe, the Himalayan species have a more or less prominent apex, while the new species has a round and more dilated apex.