The new species is characterized by dark coloration of body, bicoloured elytra (yellowish maculation extending from humeral angles to mesal area), absence of a sexual dimorphism of the head, a basally curved and apically obtuse (lateral view) ventral process of the aedeagus, and a pronounced and long crista apicalis of the aedeagus.

(17 ♂♂, 27 ♀♀). Holotype: 1 ♂, labelled ‘China: Zhejiang Prov., Lin’an City, W. Tianmushan (西天目山), nr. Kaishanlaodian (开山老殿), 30°20'45"N; 119°25'34"W, alt. 1200 m, 30.v.2014, Xiao-Bin Song & Liang Tang leg. // HOLOTYPE [red], Pella tianmuensis sp. n., Yan & Li det. 2015, SNUC’. Paratypes: 16 ♂♂, 27 ♀♀, same label data as holotype, all bearing the following label: ‘PARATYPE [yellow], Pella tianmuensis sp. n., Yan & Li det. 2015, SNUC’.

Body (Fig. 1A) length: 4.56–6.60 mm. Coloration: fore body black; elytra bicoloured, with yellowish maculation extending from humeral angles to mesal area; abdomen brownish-black, with posterior margins of segments yellowish-brown; legs and antennae dull-red.

Figure 1. 

Pella tianmuensis sp. n. A Dorsal habitusB Antenna C Male tergite VIII D Female tergite VIII E Male sternite VIII F Female sternite VIII G Spermatheca H Aedeagus, in ventral view I Aedeagus, in lateral view J Aedeagus, in dorsal view. Scale bars: 2.0 mm (A); 0.5 mm (B); 0.2 mm (C–J).

Head (Fig. 1A) widest anteriorly; surface finely reticulate, covered with short golden setae; antennomeres VI–X distinctly transverse (Fig. 1B). Pronotum (Fig. 1A) 1.28 times as wide as long and 1.40 times as wide as head; widest approximately in anterior third, narrowed posteriorly; surface covered with short golden setae; hypomera fully visible in lateral view. Elytra (Fig. 1A) approximately 1.08 times as long as pronotum; covered with short golden setae; humeral angle with one macroseta. Hind wings fully developed. Abdomen (Fig. 1A) widest at segments III–IV; surface with transverse microsculpture.

Male. Tergite VIII (Fig. 1C) with posterior margin slightly emarginate, its emarginated apex weakly serrate; sternite VIII (Fig. 1E) with posterior margin rounded; median lobe of the aedeagus (Figs 1H–J) cone-shaped in ventral view; ventral process of aedeagus curved at base, obtuse at apex in lateral view; copulatory piece as in Fig. 1J.

Female: Tergite VIII (Fig. 1D) with posterior margin truncate and weakly crenate; sternite VIII (Fig. 1F) with 12 or 13 pairs of macrosetae. Spermatheca (Fig. 1G) coiled three times.

BL: 4.56–6.56; FBL: 2.19–2.67; HW: 0.87–0.93; PL: 0.92–1.01; PW: 1.22–1.32; EL: 1.02–1.18; EW: 1.45–1.54; SL: 0.79–0.82.

Most material of the new species was taken by sifting mixed leaf litter around the nest of Lasius (Dendrolasius) spathepus, together with three species of Homoeusa Kraatz, 1858 and with Dendrolasiophilus monstrotibialis (Hlaváč, Sugaya & Zhou, 2002). At least three Pella and some Homoeusa beetles were observed walking along the ant trails. Approximately five Pella individuals were observed eating dead caterpillars together with Lasius workers.

Based on the size of eyes, the shapes of the pronotum, the bicolored elytra, and the morphology of the aedeagal median lobe, Pella tianmuensis belongs to the P. cognata group, of which four species are known from China: P. kishimotoi Maruyama, 2006, P. sichuanensis Zheng & Zhao, 2014, P. puetzi Assing, 2009, and P. maoershanensis Song & Li, 2013. The new species is distinguished from P. kishimotoi by the broader and shorter ventral process of the aedeagus in ventral view; from P. sichuanensis by the darker color of the body, and by the length of the elytra slightly exceeding that of the pronotum (P. sichuanensis: EL/PL= 0.86); from P. puetzi and P. maoershanensis by the absence of a sexual dimorphism of the head and the different shape of the ventral process of the aedeagus, especially in lateral view.

The specific epithet is derived from the type locality.