Rhantus fengi sp. n. from Xizang, China, and notes on Laccoporus nigritulus (Gschwendtner) (Coleoptera, Dytiscidae)

Abstract Rhantus fengi sp. n. from Mount Sejila, Xizang, China is described and illustrated. Laccoporus nigritulus (Gschwendtner, 1936) is redescribed and illustrated; Laccoporus viator Balfour-Browne, 1939, syn. n. is established as its junior subjective synonym.


Introduction
Xizang Autonomous Region is one of the largest provinces of China. The environment varies considerably with different elevation, but rich aquatic resources such as natural lakes and wetlands are abundant up to 5500 meters altitude. Yet, the diving beetle fauna of Xizang remains comparably little known due to the inaccessibility of many areas. Nilsson (1995) reported only eighteen species of diving beetle from "Tibet", or the Tibetan Plateau, an area that includes parts of Sichuan and Xizang as well as Qinghai, Gansu and Yunnan. Subsequently, another six species were reported from "Tibet", mainly from Qinghai and Sichuan (Toledo and Mazzoldi 1996;Toledo 1998;Fery 2003;Brancucci andHendrich 2006, 2008).
In 2009, the first two authors and Shuai Jiang collected a series of specimens of Rhantus Dejean, 1833 in Xizang, close to Mt. Nanjiabawa, which could not be assigned to any known species of the genus. This species is here described as new to science. In addition, the very rarely collected Laccoporus nigritulus (Gschwendtner, 1936) is redescribed based on specimens in the insect collection of Sun Yat-sen University (Guangzhou, Guangdong, China).

Material and methods
Morphological terminology largely follows Nilsson and Holmen (1995), Balke (1993) and Miller and Nilsson (2003). Photographs were taken using a Zeiss Axioskop 40 compound microscope and an Olympus SZX7 stereomicroscope combined with Au-toMontage software; with a Leica M205C equipped with Zeiss Progres Camera (detail of tarsus and detail of surface sculpture of Rhantus fengi presented in the supporting online resources) and with a Leica Photar 25 mm on a bellows attached to Nikon D700 and using Helicon Focus stacking software (dorsal habitus of Rhantus fengi and R. gogonensis). Diagnosis. General appearance of beetle rather dark, epipleuron black. Description. Body elongate-oval ( Fig. 1). Measurements. Body length 13.2-14.1 mm, width 6.2-6.5 mm.

Codens
Colour. Head black, with orange triangular mark on frons and contrasting yellow clypeus (Fig. 3). Scape and pedicel yellow, antennomeres 2-5 piceous with yellow base, antennomeres 6-11 piceous. Maxillary and labial palps piceous. Pronotum yellow to yellowish brown, with a wide transverse black medial maculation that not reaching anterior and posterior margin; anterior and posterior margins with black bands rather broad medially that become lighter laterally, sometimes becoming thin laterally, lateral margin black except anterior angle, near anterior and posterior margins usually with somewhat regular black spots. Elytra yellow to yellowish brown, disc with very dense black speckles giving elytra dark appearance; lateral margin, base of elytra and near suture with yellow band, suture black; epipleura black. Ventral surface and legs black (Fig. 2), trochanters with yellowish base, pro-and mesotarsal claws somewhat dark brown.
Surface sculpture. Head with irregular polygonal meshes and fine punctation; meshes rather elongated and punctures coarser behind posterior margins of eyes; along inner face of eye with series of coarse setiferous punctures. Pronotum with irregular polygonal meshes and fine punctuation; along anterior and posterior margins with rows of coarse punctures, interrupted posteromedially; lateral portion distinctly depressed, without microreticulation, with broad rim (or border) reaching anterior angle; posterior angle broadly rounded. Elytra with irregular polygonal meshes and fine microreticulation best visible laterally and posteriorly (at 50×); each elytron with five rows of coarse setiferous punctures being somewhat irregular except in row 1 (sutural row) and very regular row immediately along lateral margin; elytral disc with similar meshes and fine punctures as on pronotum, mircroreticulation faint, hardly visible. Metaventrite ("metasternal wings") broad laterally. Metafemur without group of setae on posteroexternal angle. Pro-and mesotarsomeres with claws gently curved; outer protarsal claw slightly shorter than inner one (Fig. 5) and inner mesotarsal claw distinctly shorter than outer one; inner hindclaw twice as long as outer one. Abdominal ventrites 4 and 5 with a transverse rows of coarse setiferous punctures; ventrite 6 rugose with stronger punctures and setae laterally, more or less depressed and smooth posteromedially.
Remarks. The elytral sculpture of polygonal meshes and microreticulation as well as the parameres with some trumpet shaped setae, place this species in the Rhantus suturalis group (Balke 1993). The species is well characterized by its black epipleura; this is shared only with the Bhutanese species R. gogonensis Wewalka, 1975 (Fig. 9) which is only known from the female holotype (Bhutan: Sha Gogona; http://www.speciesid.net/wiki/Rhantus_gogonensis). Rhantus fengi differs from the latter by the darker body, pronotum not narrower than the base of elytra; pronotum distinctly impressed laterally; head, pronotum and elytra with very dense fine punctures, and black tarsi and antenna.
Etymology. The species is named in honour of Hsiao-Tang Feng, a pioneer in the Chinese Dytiscidae studies.
Habitat. Collected from a branch of a small stream with fine sand on bottom. The water of the stream was about 10-15 cm deep and slowly flowing.
Colouration (Figs 10, 11, 15). Dorsum uniformly yellow brown. Head yellowish brown, with one short dark brown line near antennae. Antenna with antennomeres 1-4 and base of antennomeres 5-6 yellow brown, antennomeres 7-11 andapical parts of antennomeres 5-6 dark brown. Maxillary palpomeres yellow brown, apical palpomere apically dark. Labial palpomeres yellow brown, apical palpomere dark. Pronotum sometimes paler with darker median, anterior and posterior transverse bands. Elytra clearly darker than pronotum, but with the same color at base as pronotum. Ventral surface of head and thorax black. Abdomen black, ventrites 3-6 yellow brown along posterior margins (e.g. Fig. 15). Legs yellow brown with coxae black, posterior half of metacoxal process yellow structures (see http://www.species-id.net/wiki/Lacco-porus_nigritulus for high resolution images of surfaces). Head with surface sculpture consisting of small irregular polygonal meshes, with some sparse coarse and shallow punctures in posterior half. Meshes somewhat stronger on posterior portion. Clypeus with transverse series of large punctures present anteriorly. Labrum with very deep excavation, surface with short transverse meshes, anterior notch with dense and short white setae. Antenna filiform, not dilated. Pronotum more or less depressed laterally, without lateral bead, with small irregular polygonal meshes, sometimes very tiny punctures detectable. Posterior corner almost rectangular. Elytra with base as wide as posterior portion of pronotum, surface with irregular polygonal meshes in size and shape, sculpture more or less stronger than that on pronotum, tiny punctures very sparse and clearly detectable. Prosternal process thin and long,bisinuate in lateral view, almost reaching posterior margin of mesocoxae. Metaventrite medially with some coarse and strong punctures behind mesocoxae. Metacoxae and metasternum strongly and irregularly scratched, with dense longitudinal sculptures and sparse tiny punctures. Pro-and mesofemora with tuft of long setae at basal portion posteriorly. Metafemora with fine and dense sculpture and tiny punctures. Metatibia with inner spur longer than outer one. Pro-and mesotarsi smooth. Metatarsi smooth, with fine sculpture dorsally. First tarsomere of metatarsi almost as long as following two tarsomeres combined. Claws normally curved, protarsal claws longer than metatarsal claws. Metatarsomere 1 almost as long as metatarsomeres 2-3 combined. Claws regularly curved, proclaws longer than metaclaws.
Male  Gschwendtner, 1936. The latter taxon was transferred to Laccoporus and raised to species status by Vazirani (1970) who studied two syntypes housed in ZSIC. Vazirani (1970) had doubts that L. viator and L. nigritulus were different species, but he could not study Balfour-Browne's types. We have no access to specimens in ZSIC, but Vazirani (1970) provided an illustration of the ventral side of the median lobe of Laccoporus nigritulus, which agrees with our specimens.
The types of L. viator are from Gyangtse (Jiangzi). The types of L. nigritulus were collected from "Te-ring" (Dingri) and "Gompa" (Gangba). The distance between these sites is only about 100-120 km (Fig. 16). The two latter sites are not far (about 90-120 km respectively) from Rikaze where some of our newly studied specimens originate from. The altiude of these three sites is 3900-4300 m. Zeng (1989) described "Laccophilus thibetanus sp. n." from Rikaze, Xizang in her PhD thesis (unpublished, in Chinese) (in the corrected thesis version, the name was changed to L. xizangensis sp. n. by herself, possibly acknowledging that Xizang is a more precise reference to the type locality than "Tibet"). Nilsson (1995: 71) recorded a "Laccophilus sp." and stated that it is based on "Laccophilus tibetanus Zeng, 1989: 4 (nom. nud., Tibet)". We found the specimens that have the same locality data as recorded by Zeng (1989) in SYSU. Although there is no specific name and no type designation attached to these specimens, we are confident that Zeng (1989) based her unpublished Laccophilus xizangensis on these specimens. All the specimens are conspecific with Laccoporus nigritulus.