Length: 11.5–12.5 mm; width: 3.6–3.7 mm. Habitus as in Figure 1. Body extremely slender and elongate, with very long antennae, legs and mouthpart palps.

Black, but ventral surface, femora (except basal and apical tips), clypeus, labrum and apical half of mandibles dark brown, legs including basal and apical tips of femora, antennae, palps, and basal half of mandibles yellow to yellow brown; elytra with indistinct purplish metallic sheen.

Macrosculpture: Strongly shining, surface glabrous, polish and smooth, but base of pronotum, mesosternum, meso- and metepisterna coarsely and sparsely punctate.

Microsculpture: Engraved meshes moderately transverse on labrum, clypeus and base of frons, and base of pronotum; strongly transverse on head; striate on disc of pronotum and elytra, but clearly isodiametric on scutellum.

Head very long and narrow, strongly elongated, much longer than wide, HL/HW = 2.04–2.15 (mean 2.11); widest at level of eyes, and gradually narrowed backwards to neck constriction which is short but distinct, tempora almost straight but slightly curved just before neck constriction; eyes rather flat, more or less depressed; ventral margins of eye well separated from buccal fissure; supraorbital areas with two pairs of setiferous pores, anterior closer to margin of eye than posterior; posterior pore at about middle of head from clypeal suture to neck; interspaces between anterior pores distinctly wider than that between posterior ones; distance between anterior and posterior pores slightly longer than diameter of eye; distance between eye to buccal fissure distinctly shorter than that between posterior pore to eye, but slightly longer than that between anterior pore to eye; frontal impressions rather shallow and wide, ending before anterior supraorbital pores; frons and vertex moderately convex; clypeus moderately transverse, bisetose, labrum subquadrate, almost straight at front, sexsetose; mandibles elongate, gently and gradually narrowed towards apex, apical teeth slightly hooked; right mandible with a small but distinct anterior retinacular tooth at about median portion which is far from other teeth; both right and left mandibles with short and blunt terebral and posterior retinacular teeth near base; ligula broad, bisetose at apex; labial suture well developed; mentum bisetose, apical margin biconcave, with a long and simple median tooth which is widened at apex, epilobes evenly rounded; submentum bearing two pairs of setae, inner ones much long the outer; palps long and slender, subcylindrical, glabrous and asetose except for labial palpomere 2 which is bisetose on inner margin; labial palpomere 2 distinctly longer than 3; maxillary palpomere 3 slightly longer than 4. Antennae long and slender, filiform, extending at about apical 1/5 of elytra in female, but 1/6 in male; antennomeres 1–3 glabrous, antennomere 1 with a long subapical seta, 2 with a short subapical seta, 3 with several apical setae; finely pubescent from antennomere 4, each of 4 to 11 with several apical setae; antennomere 2 the shortest, half as long as 1, antennomeres 3–5 longer than other, each about twice as long as 1; gradually shortened from antennomeres 6 to 11, antennomere 11 almost as long as 1.

Pronotum narrow and strongly elongate, barrel-like, distinctly shorter than head, almost as wide as head including eyes; much longer than wide, PL/PW = 1.59–1.62 (mean 1.60) in male, 1.40–1.43 (mean 1.41) in female; front slightly narrower than base, PWB/PWA = 1.11–1.14 (mean 1.13) in male, 1.09–1.12 (mean 1.10) in female; widest at about middle, gently narrowed towards both fore and hind angles; front and base finely bordered, lateral margins unbordered, but with evenly and distinctly explanate-reflexed areas throughout, marginal setae absent; basal foveae wide and long; fore angle nearly rectangular, hind angle broad though somewhat rectangular; both front and base almost straight; median line deep and long; disc slightly convex, basal area rather flat, with lateral areas of basal parts evidently depressed; propleura faintly tumid, faintly visible from above, at least at the widest part; prosternal process unbordered at apex; scutellum moderately sized.

Elytra very slender, elongate-ovate, much wider than head and pronotum; well bordered at base, base small, shoulders indistinct; widest at a little behind middle, EL/EW = 1.62–1.78 (mean 1.74), more contracted towards apices than towards base; disc moderately convex though rather flat in basal 1/4; striae very deep, continuous and smooth, weakly punctured; intervals strongly convex, stria 3 with three dorsal setiferous pores, basal one close to stria 3, both middle and subapical ones close to stria 2; other intervals without pore; subapical sinuation rather straight, apex broad; preapical and two apical pores present; marginal series of umbilicate pores not aggregated, which are composed of about nineteen pores, denser in subhumeral and subapical areas, sparser in middle portion; three pores (one at subhumerus, other two at subapical area) bearing much longer setae than others, which is distinctly longer than metatarsomere 4; scutellar pores present; scutellar striae deep and rather long. Hind wings reduced.

Legs very long and slender; fore leg short and stout (Figs 2–5); procoxa asetose, meso- and metacoxae bisetose, inner seta of metacoxa absent; trochanters unisetose; femora very slender, profemora unisetose ventrally, meso- and metafemora with three and two ventral setae respectively; tibiae and tarsomeres 1–3 longitudinal bisulcute dorsally; protarsomeres 1–3 slightly dilated in male, with two rows of short and sparse spongy setae ventrally (Fig. 3), while much narrower and without spongy setae in female (Fig. 5); protarsomeres 4 shortly but distinctly emarginate at apex, with lobes nearly symmetric, each with a row of three setae ventrally; meso- and metatarsomeres 4 without subapical setae (Figs 6–10); tarsomeres 5 glabrous ventrally; claws smooth.

Each of abdominal ventrite IV–VI with pair of paramedian setae in both sexes; ventrite VII with two pairs of marginal setae in female, but only one pair in male.

Male genitalia (Figs 11–13): The median lobe of aedeagus rather stout, basal bulb large, strongly arcuate in middle portion in lateral view, gently and gradually narrowed towards apex, blunt at tip; dorsal opening wide, nearly as half as whole length, reaching 1/3 from base; apical lamella rather long, nearly twice as long as wide, not parallel-sided, rounded at apex; internal sac with a long copulatory piece covered with scales, and a strongly sclerotized spine dorsally; left paramere styloid, not elongated, smaller and shorter than the right.

Female reproductive tract (Fig. 14): Ventrite X sparsely setose; gonosubcoxite bearing about a dozen fringe setae along apical area, gonocoxite strongly curved, sharp at apex, bearing three lateral and one dorsal ensiform setae; bursa copulatrix wide, with middle part evidently folded, basally narrower; spermathecal gland very large, elongate ovate; spermathecal gland duct thin and long, connected below base of spermathecal reservoir, which is shorter than spermathecal duct.

Apart from protarsomeres 1–3 and ventrite VII, sexual differences are also shown on antennae and pronotum: a little longer or more elongate in male than in female.

In general, maxillary palpomere 3 distinctly longer than 4, but reverse in a male specimen which left maxilla with palpomere 3 shorter than 4.

Within platynines, the chaetotaxy on pronotum and meso- and metatarsi are important characters (Habu 1978; Liebherr 1991; Schmidt 2001; Liebherr and Schmidt 2004). We treat this peculiar species a member of the genus Morimotoidius Habu, 1954 due to the fact that it shares the following combined generic characters with other species of this genus: pronotal setae absent, submentum quadrisetose, meso- and metatarsomeres 4 without subapical setae, meso- and metafemora bearing two or three setae, and tarsomeres 5 glabrous ventrally. Certainly, other remarkably modified characters such as the very elongate and rhadinoid body shape, the slender and markedly porrected mandibles, the very thin palps, reduced eyes, the markedly prolonged temples and the very small elytral base must be autapomorphies to adapt the subterranean habitat. Morimotoidius zhushandong sp. n. is the first member of Morimotoidius found in mainland China. Other species of the genus are living either in Japan (two species) or in Taiwan Province of China (one species) (Habu 1954, 1978; Terada et al. 2005).

This new species is named after its type locality. In Chinese, “Zhushan” means a mountain or hill which is covered with bamboo forest, “Dong” means cave.

Holotype: male, Zhushan Dong II, Dongkou Cun, Mabu Xiang, Wanzhai Xian, western Jiangxi, China, 28°02'880"N, 114°22'216"E, 142 m, 2–X-2012, Mingyi Tian & Jingli Cheng leg. in the insect collections of South China Agricultural University, Guangzhou, China (SCAU); paratypes: 6 males and 9 females, ibid. in SCAU, except one male and one female in Muséum National d’Histoire Naturelle, Paris, France, and one male and one female in Coll. J. Schmidt (Admannshagen, Germany), respectively.

China (Jiangxi) (Fig. 15).

There is unknown about the biology and ecology of Morimotoidius zhushandong sp. n. The beetles move quickly on walls and ceilings, feeding on other small arthropods, probably including eggs of the crickets which are common in the cave. The extremely modified troglomorphic characters mentioned above reveal that this beetle has ability to adapt underground environment and probably a troglobite though it has reduced eyes and pigmented body.

Zhushan Dong is a touristic cave in western Jiangxi Province, located at Dongkou Cun, Mabu Xiang, Wanzai Xian, not far from the border between Wanzhai Xian and Yichun Shi (=City) (Fig. 15). Actually, there are two limestone caves in the Zhushan Dong scenic areas. Zhushan Dong I is a well-developed touristic cave, as long as 3985 m, with an underground river throughout the main passage. Zhushan Dong II is about 50 meters far from Zhushan Dong I. It is a small cave, about 30 m in length, with a small streamlet moving out at about 5 m inside of the entrance (Fig. 16, indicated by arrowhead). It is still closed for visitors. The beetles were collected by hands on walls and ceilings of the cave (Figs 17–19).