Holotype: macropterous ♂: ‘China: S. Zhejiang, Longquan (龙泉市), Fengyang Shan (凤阳山), creek valley nr. hotel, 27°54'42.85"N, 119°11'52"E, leaf litter, wood, sifted, 1190–1250 m, 28.iv.2014, Peng, Song, Yan, Yin, & Yu leg. / HOLOTYPE [red] ♂, Acetalius grandis sp. n., det. Z.-W. Yin, 2016’ (SNUC). Paratypes: 1 apterous ♂, 1 apterous ♀, same collecting data as the holotype (SNUC); 1 apterous ♂, 3 apterous ♀♀: ‘China: S. Zhejiang, Longquan, Fengyang Shan, Da-Tian-Ping (大田坪), 27°54'36"N, 119°10'20"E, leaf litter, moss, ferns, sifted & beating, 1320 m, 27.iv.2014, Peng, Song, Yan, Yin, & Yu leg.’ (SNUC). Each paratype bears a yellow type label similar to holotype except for ‘PARATYPE, ♂ (or ♀)’.

Body large-sized, 1.85–2.23 mm; frons with a Y-shaped carina extending anteriorly to reach clypeal anterior margin; each eye of macropterous male with about 55 facets, that of apterous male with about 25–30 facets, and apterous female about 12 facets; antennomeres III elongate, IV–X distinctly transverse; abdominal tergite IV with three pairs of marginal carinae, discal carinae parallel; sternite IV with long median and two shorter admesal carinae.

Apterous male (Fig. 1B). Length 1.91–2.03 mm; head (Fig. 2A–C) roundly rectangular, HL 0.34–0.36 mm, HW 0.43–0.46 mm; with small, nude vertexal foveae (Figs 2A, 3A; vf) situated at dorsal level of midline of eyes, lacking sulcus connecting vertexal foveae; antennal tubercles moderately prominent, connected by Y-shaped frontal carina (Figs 2A, 3A; fc) that extends anteriorly to meet anterior margin of clypeus, area between antennal tubercles depressed; with pair of subantennal carinae (Fig. 3A; sac) just posterior of antennal bases; each eye composed of about 25–30 facets; mandibles exceptionally elongate; antennal clubs (Fig. 3B) formed by apical two antennomeres, antennae with large scapes and pedicles, antennomeres III elongate, IV–X strongly transverse, successively wider and larger, XI largest, nearly oval, apex forming thumb-like protuberance surrounded by ring of thick setae; ocular-mandibular carinae (Fig. 2B; omc) present; gular foveae (Fig. 2C; gf) in single pit, situated in strongly depressed area, median gular sulcus (Fig. 2C; mgs) thin; apicolateral genal projections (Fig. 2C; agp) present.

Pronotum (Fig. 2D–F) as long as wide, PL 0.35–0.37 mm, PW 0.35–0.36 mm, laterally rounded at apical half, sides of basal half successively constricted toward base; broad antebasal sulcus connecting nude lateral antebasal foveae (Fig. 2D–E; laf); lacking median antebasal fovea; disc moderately convex, with antebasal tubercles (Fig. 2D–E; at) in sulcus, mediobasal carina (Figs 2D, 3A; mbc) extending from anterior margin of antebasal sulcus to pronotal base, with pair of short basolateral carinae (Fig. 2D; blc); median longitudinal sulcus (Fig. 3A; ms) thin, lacking lateral longitudinal sulci; paranotal carinae (Fig. 2E–F; pc) sinuate, complete. Prosternite with distinct median carina (Fig. 2F; mc); lateral procoxal foveae small and close (Fig. 2F; lpcf).

Elytra (Fig. 2G) rounded laterally, distinctly constricted at base, EL 0.62–0.64 mm, EW 0.66–0.71 mm; each elytron with two basal foveae (Fig. 2G; bef); with one pair of short discal striae extending from inner margins of outer basal foveae posteriorly to less than one-fourth of elytral length; sutural striae (Fig. 2G; ss) complete; lacking subhumeral foveae, marginal sulci, and apicolateral cleft. Metathoracic wings absent.

Mesoventrite with two widely separated median foveae (Fig. 2H; mmsf); lateral mesoventral foveae (Fig. 2H; lmsf) simple, slanted anteriorly; with lateral mesocoxal foveae (Fig. 2H; lmcf); metaventrite lacking lateral metacoxal foveae, posterior margin with narrow split at middle.

Legs with profemora simple ventrally, protibiae slightly expanded at apices.

Abdomen (Fig. 3C–E) widest at middle, AL 0.60–0.66 mm, AW 0.61–0.67 mm, with tergite IV (first visible tergite) longest, V–VIII successively shorter. Tergite IV with two small basolateral foveae (Fig. 3D; blf) in deep basal sulcus (Fig. 3D; bs) which is separated into three parts by pair of short, parallel discal carinae (Fig. 3D; dc), with short pair of short carinae between long inner and outer marginal carinae (Fig. 3C–D; imc, omc); V–VIII each with one pair of small basolateral foveae; tergites IX (Fig. 4A) paired, weakly sclerotized. Sternite III (first visible sternite) short, with arrow-like protuberance at middle; IV longest, with two large basolateral foveae (Fig. 3E; blf) in basal impression, with single long median carina (Fig. 3E; mc) and pair of shorter admesal carinae (Fig. 3E; amc); sternites V–III each short; sternite IX (Fig. 4B) nearly oval.

Length of aedeagus (Fig. 4C–D) 0.25–0.26 mm; aedeagus weakly sclerotized, parameres fused with median lobe; endophallus composed of one large, curved and several smaller sclerites.

Macropterous male (Fig. 1A). Measurements: BL 1.85 mm, HL 0.32 mm, HW 0.42 mm, PL 0.32 mm, PW 0.32 mm, EL 0.58 mm, EW 0.66 mm, AL 0.63 mm, AW 0.58 mm. Length of aedeagus 0.24 mm. Each eye composed of about 55 facets; base of elytra more expanded than that of apterous male, each elytron with two discal striae extending from lateral and mesal margin of outer basal foveae, respectively. Metathoracic wings fully developed.

Apterous female. Measurements: BL 2.19–2.23 mm, HL 0.40–0.42 mm, HW 0.48–0.50 mm, PL 0.39–0.42 mm, PW 0.36–0.38 mm, EL 0.66–0.72 mm, EW 0.76–0.80 mm, AL 0.70–0.71 mm, AW 0.70–0.75 mm. Each eye composed of about 12 facets. Elytral base constricted as that in apterous male. Metathoracic wings absent. Apices of protibiae not expanded.

Acetalius grandis can be readily separated from both A. dubius (1.1–1.4 mm) and A. pilosus (1.4–1.6 mm) by the distinctly larger body size (1.85–2.23 mm), and presence of a frontal carina on the head, which is lacking in the other species. The new species shares with A. pilosus the presence of three pairs of marginal carinae on tergite IV, and similar structure of the aedeagus, but differs also by the elongate antennomeres III, which are as long as wide in A. pilosus. Acetalius dubius has the smallest body size, and there are only two pairs of marginal carinae on tergite IV.

Eastern China: Zhejiang.

The specific epithet implies that Acetalius grandis is the largest species in the genus.

(modified from Nomura 1988a)