Body length (both sexes): 13.5‒14.4 mm (13.5 mm in holotype); width at humeri (both sexes): 1.9‒2.0 mm (1.9 mm in holotype).

Male (Fig. 1A). Colouration. Body black, antennomere I yellow ventrally; prothorax yellow, pronotum with a small black marking at middle of anterior margin; metaventrite each side with a longitudinal yellow marking; metanepisternum and scutellum yellow; abdominal segments II‒VII narrowly yellow at lateral and posterior margins.

Eyes large, interocular distance about half of maximum eye diameter. Antennae reaching posterior margin of abdominal tergite III, antennomeres II about 1/3 of the length of I, III‒XI subequal in length and about 4.0 times longer than II.

Pronotum 1.1 times wider than long, anterior margin strongly arcuate and lateral margins slightly arcuate, posterior margin feebly bisinuate, anterior angles rounded and posterior angles nearly rectangular.

Leg slender, mesotibiae each present with a spur at inner apical angle (Fig. 1a).

Elytra 1.3 times longer than humeri width of conjoint elytra, twice longer than pronotum, with lateral margins sinuate, sutures moderately dehiscent at posterior 2/3 part, distance between sutures twice wider than width of an elytron, apices rounded.

Terminal abdominal tergite (or tergite VIII) (Fig. 2A and B) with lateral projections long and cylindrical, about 3/5 of the length of the tergite. Terminal abdominal ventrite (or sternite VIII) (Fig. 2D) widened apically, saddle-shaped and deeply cleft in middle of apical 2/3 part, with lateral margins feebly arcuate, inner apical angles largely and rectangularly emarginate, outer apical angles feebly widened and truncate at apices.

Abdominal sternite IX completely concealed underneath terminal ventrite (Fig. 2A), axe-like (Fig. 2G), with posterior right angle triangularly projecting posteriorly, posterior left angle rectangular, anterior right angle strongly and narrowly protruding anteriorly, rod-like and rounded at apex.

Proctiger (Fig. 2F) completely surrounded by paraproct, strongly sclerotised on both sides and almost membranous in middle, with lateral margins converging posteriorly. Paraproct (Fig. 2F) well-developed and tubular, feebly shrunk apically, shallowly emarginate in middle of dorso-posterior margin, present with a median longitudinal ridge on ventral side, surface coarsely punctate and covered with long and stout setae, tergal flange weakly sclerotised, strongly expanded at base and narrowly protruding apically.

Aedeagus (Fig. 4A‒D): left and right parameres both slender and nearly straight, subequal in length (Fig. 4A, D), rounded at apex; right paramere covered with a few short setae at outer side (Fig. 4B); setifore extension strongly sclerotised, nearly as long as parameres, with a cluster of long setae at apex (Fig. 4B and C); median lobe medium-length, about twice longer than parameres, apex rounded.

Female (Fig. 1B). Similar to males, but mesotibial spur absent (Fig. 1b), terminal abdominal tergite with lateral projections feebly stouter and horn-shaped, progressively thinned apically (Fig. 2C); terminal abdominal ventrite (Fig. 2E) parallel-sided and arcuately narrowed at posterior 1/3 part, present with a pair of small protuberances in middle of posterior margin.

Reproductive system (Fig. 4E‒F): coxites long and fused medially, styles thin and short; vagina elongate, with median oviduct situated in middle of ventral side; bursa copulatrix arising from apex of vagina, swollen and suddenly thinned apically; accessory gland opening at dorsal base of bursa copulatrix, long and slightly expanded at apical 3/4 part; a spermatheca in the form of two short canals and arising from ventro-basal part of bursa copulatrix.

Variation within type series. Sometimes pronotum with a large black marking at anterior half part of the disc (Fig. 1C) or almost black with narrow yellow lateral margins (Fig. 1D). Their aedeagi (Fig. 3) same to the holotype.

It can be easily distinguished from all Chinese species by the uniformly black elytra (Fig. 1), as opposed to the bicoloured and mixed with black and yellow in others. Meanwhile, each mesotibia is present with an apical spur in male (Fig. 1a), while absent in the other species or, at most, each protibia with an apical spine in I. vandepolli Gestro, 1892. Additionally, its terminal abdominal tergite is characterised by long lateral projections, approximately 3/5 of the length of the tergite (Fig. 2A‒B), whereas in other species, it is, at most, half as long. Furthermore, its aedeagus bears a long setifore extension nearly equal in length to the parameres (Fig. 4A and D), unlike other species where it is much shorter than the parameres. Moreover, it can be readily differentiated from I. davidi Gestro, 1892 and I. vandepolli by its slender legs in male (Fig. 1A, C and D) (while meso- or profemora are at least moderately expanded in the latter species) and from I. bourgeoisi 1892 and I. savioi Pic, 1928 by its terminal abdominal tergite with simple lateral projections in male (Fig. 2A‒B) (as opposed to being excavated ventrally in the latter species).

The specific name is derived from the Latin longus (long), referring to its long setifore extension.

China (Chongqing, Shannxi, Gansu, Hubei, Guangxi, Guizhou) (Fig. 5).