Two new species of the genus Diostracus Loew from Tibet, with a key to the Himalayan fauna (Diptera, Dolichopodidae)

Abstract Previously only one species of the genus Diostracus was known to occur in Tibet. Here the following two new species are added to the fauna of Tibet: Diostracus acutatus sp. n. and Diostracus tibetensis sp. n. Their relationships with similar species are discussed. A key to the species of Diostracus from the Himalayas is presented.


Introduction
The genus Diostracus is a large genus in the subfamily Hydrophorinae and includes dolichopodids living on wet rocks and stones in mountain streams (Saigusa et al. 1997). It is distributed in the Holarctic and Oriental regions with 83 known species. Among them, three species are known to occur in the Nearctic region, 21 in the Palaearctic and 59 in the Oriental (Yang et al. 2006). The major references dealing with this genus are as fol-lows: Takagi (1968Takagi ( , 1972, Negrobov (1980), Saigusa (1984), Saigusa et al. (1997), Yang (1998), Masunaga (2000) and Yang et al. (2011). Up to now, 23 species are recorded from China (Takagi 1968;Wei and Liu 1996;Yang 1998Yang , 1999Yang and Saigusa 2000;Zhang et al. 2003;Yang et al. 2011). The Chinese species were revised by Yang et al. (2011).
Only one species, Diostracus nebulosus Takagi, of the genus Diostracus was known to occur in Tibet (Yang et al. 2011). Here two new species are added to the fauna of Tibet, based on material collected by Dr. Zhaohui Pan and the junior author with Malaise traps (Map 1). Nine species groups for the species of Diostracus from the Himalayas were proposed by Saigusa (1984), and the three species from Tibet are placed within these groups. A key to the species of Diostracus from the Himalayas is presented.
Abdomen distinctly longer than head and thorax combined, metallic green with pale gray pollen. Abdomen with pale pubescence. Tergite 5 with lateral portion slightly extended downward. Sternite 1 with a nearly acute process at middle; sternite 4 medially with an obtuse anterior process and 2 short thin, contiguous posterior processes bearing bundle of brown hairs. Hypandrium not distinctly swollen.
Male genitalia (Fig. 6): Epandrium slightly longer than wide. Epandrial lobe short thick, finger-like, weakly bent, with an acute basal process; 1 slightly long bristle present at middle and 1 short thick bristle at tip. Surstylus short thick, apically furcated, with 1 very short apical denticle bearing 1 very short spine-like bristle. Hypandrium short thick, apically with a shallow, V-shaped apical incision, subapically with 2 small acute processes. Cercus slightly bent, nearly finger-like in lateral view, with long dark yellow hairs.
Female. Unknown.   Takagi and D. nishidai Saigusa from Nepal by the fore tarsomere 1 with a nearly acute apicoventral process and fore tarsomere 2 with a short finger-like ventral process near the extreme base (Figs 2, 5). In D. flexus and D. nishidai, the fore tarsomere 1 has an obtuse apicoventral corner, and the fore tarsomere 2 has no finger-like process near the extreme base (Saigusa 1984, figs 7-8).
Etymology. The specific name refers to the fore tarsomere 1 with a nearly acute apicoventral process. Takagi, 1972 Diagnosis. First flagellomere (Yang et al. 2011, fig. 199b) 1.3 times longer than wide, obtuse apically; arista dorsal. Wing (Yang et al. 2011, fig. 199a) with an obscure spot at anteroapical corner of discal cell; crossvein m-cu straight. Male cercus (Yang et al. 2011, fig. 199d)  Head metallic green with pale gray pollen. Eyes widely separated; face widened towards clypeus. Hairs and bristles on head black; lower postocular bristles including posteroventral hairs pale, mostly very long. Ocellar tubercle distinct, with pair of strong oc, without posterior hairs; vt rather short and weak, 0.4 times as long as oc, 0.7 times as long as pvt. Antenna (Fig. 9) black; scape without any dorsal hairs; first flagellomere short, somewhat quadrate, 1.3 times longer than wide; arista subapical, 3.9 times as long as first flagellomere, nearly bare. Proboscis blackish with pale hairs; palpus lobate, smoky black with black hairs. Thorax metallic green with pale gray pollen; mesoscutum with two pairs of dark brown longitudinal spots (middle pair strip-like). Hairs and bristles on thorax black; 6 slightly long dc, posteriormost dc longest; acr bristles absent; 1 h and 1 short bristle, 1 ph, 2 npl, 1 sa, 1 psa; scutellum with pair of long sc and 4 very short marginal hairs (2 hairs between 2 sc). Propleuron with short pale hairs on upper portion and mostly long pale hairs on lower portion. Legs entirely black; claws well developed, empodium and pulvilli distinct. Hairs and bristles on legs black except those on coxae pale; fore coxa with bundle of short dense black anterior hairs bristle-like at extreme tip; hind coxa apically with 4 long blackish anterior hairs bristle-like. Mid and hind femora with some pale ventral hairs. Fore femur with two rows of black ventral bristles (longest ones slightly shorter than femur thickness), and with 3 long posterior bristles at extreme base. Mid femur basally with nearly two close rows of long pale ventral hairs (longest ones about 3 times as long as femur thickness), subbasally with 4 black short thick av. Hind femur with about two close rows of long pale ventral hairs (longest ones about 3 times as long as femur thickness) and with 5 black short thick av. Fore tibia with 4 ad and 4 pv on apical half; apically with 3 bristles. Mid tibia with 3 ad and 2 pd; apically with 3 bristles. Hind tibia with 4 ad, 5 pd, 3 av and 6 pv; apically with 3 bristles. Fore tarsomere 1 with row of short dense erect av spines and one row of dense thin pv (longer than av). Relative lengths of tibia and five tarsomeres: LI 3.3 : 1.6 : 1.6 : 0.8 : 0.55 : 0.7; LII 5.8 :2.9 : 1.1 : 0.75 : 0.5 : 0.75; LIII 7.1 : 3.2 : 1.9 : 1.2 : 0.6 : 0.8. Wing (Fig. 8) hyaline; veins dark brown, R 4+5 and M convergent apically; crossvein m-cu medially distinctly bent with small round black nodule located at middle of crossvein. Squama brown with pale hairs. Halter brown to dark brown.

Diostracus nebulosus
Abdomen rather short, nearly as long as head and thorax combined, metallic green with pale gray pollen. Abdomen with pale pubescence except dorsum with some black hairs at middle. Tergites 4 distinctly and tergite 5 weakly with lateral portion extended downward; lateral portion of tergite 4 with very long hairs apically bent, slightly shorter than those on sternite 3, but lateral portion of tergite 5 only with short hairs.
Male genitalia (Fig. 10): Epandrium relatively short, slightly longer than wide. Epandrial lobe weak, with 2 long bristles. Surstylus enlarged, with three acute denticles at apical margin. Hypandrium narrowed, bent; apically with a shallow, V-shaped apical incision and lateral lobe curled; basally with a hook-like process. Cercus straight, long finger-like, with long yellow hairs.

Remarks.
The new species belongs to the unipunctatus-group. It is somewhat similar to D. parvipunctatus Saigusa from Nepal in the shape of the first flagellomere and fore and mid femora with long ventral hairs, but may be separated from the latter in the following points: vt is shorter and weaker than pvt; the first flagellomere is shorter, 1.3 times longer than wide, and the abdominal tergite 5 has the short lateral hairs. In D. parvipunctatus, vt is as strong as pvt or stronger; the first flagellomere is 1.5 times longer than wide (Saigusa 1984, fig. 35), and the abdominal tergites 4-5 has the long yellow lateral hairs (Saigusa 1984).
Etymology. The specific name refers to the type locality Tibet.