﻿Six new species of Diostracus Loew (Diptera, Dolichopodidae) from Tibet

﻿Abstract Six species of Diostracus from Tibet are described as new to science: D.concavussp. nov., D.fasciculatussp. nov., D.laetussp. nov., D.polytrichussp. nov., D.strenussp. nov., and D.translucidussp. nov. A key to the species from Tibet of the genus is provided. The distribution of the genus in Tibet is also discussed.


Introduction
Diostracus belongs to the subfamily Hydrophorinae of Dolichopodidae. These flies are usually stout and larger than other dolichopod flies. They prefer to live on the vertical or oblique surfaces of rocks at altitudes 1000 m to 3500 m, with slow water flow, or a thin water layer on the surface, or just wet, but they do not like those rocks behind streams or waterfalls.
Before our study, 101 species of the genus had been reported (Zhu 2006;Zhu et al. 2007a, b;Grichanov 2013Grichanov , 2015Grichanov , 2017Pusch 2014;Wang et al. 2015). The first Diostracus, D. prasinus Loew, was named in 1861 from the Nearctic Region, whereas most species of the genus were reported from the Palaearctic and Oriental realms.
Remarkably, Diostracus shows great diversity in the Himalayas, which is the junctions of the two realms, and 39 species have been reported from this area (Zhu 2006;Wang et al. 2015). Tibet is located in the east of the Himalaya Mountains. However, there were only three species of Diostracus known from Tibet: D. nebulosus Takagi, 1972 is widely distributed in the Himalayas, while D. acutatus Wang et al., 2015, andD. tibetensis Wang et al., 2015 were reported in Nyingchi, Tibet.
Here we provide an investigation of the diversity of Diostracus in Tibet and six new species are reported.

Materials and methods
This work is based on the material collected by sweep netting from Tibet in 2013 and 2018. The main locality is Yadong County (88°52'-89°30'E, 27°23'-28°18'N), located on the southern slope of the Himalaya Mountains. All the altitudes of localities are approximately 3000 m a.s.l. The specimens are deposited in the Entomological Museum of China Agricultural University, Beijing (CAU). The information about the specimens studied in the paper are presented in Table 1. Morphological terminology for adult structures mainly follows Cumming and Wood (2017).
The following abbreviations are used: Each holotype male was submitted to barcode sequencing, using the primers LCO1480/ HCO2198 (Folmer et al. 1994), under the following experimental procedures: 3 min at 95 °C for the first cycle, reactions were amplified through 35 cycles at following paraments, 30 s at 95 °C, 30 s at 55 °C, and 90 s at 72 °C, then elongation for 1 cycle at 72 °C for 10 min. The sequences were uploaded to GenBank (Table 2). Females were also sequenced to pair them to males, in addition to examination of their morphological characters.  Diagnosis. The species belongs to the fenestratus group, characterized by specialized It 1 and It 2 (MSSC). It 1 distinctly shortened, with a nearly acute apicoventral process; It 2 basally bent with a short finger-like ventral process near extreme base. Females of D. acutatus are characterized by the apical or subapical antenna, oblique crossvein, m-cu oblique, and brownish trochanters.

Genus
Male (Fig. 1A). Same as description of Wang et al. (2015). Female (Fig. 1B). Body length 6.0-6.4 mm; wing length 6.9-7.8 mm. Antennal scape with two short dorsal bristles, first flagellomere somewhat prolonged, 1.2 × longer than width, arista apical or subapical, 4.6 × longer than first flagellomere (Fig. 31A, B). Proboscis yellowish brown with blackish edge; palpus relatively smaller than males, not reaching apex of proboscis. six dc, anterior four short and weak, 5 th long and weak. Propleuron with a single pale hair on upper portion, and group of 5-7 long pale hairs on lower portion. Legs black, except trochanters yellowish brown. CI without distinctive bristle or hair, but with short pale anterior hairs on lower portion; FI without distinct bristles; TI with an ad at basal 1/4, four pd, apically with one bristle and comb of anterior bristles; FII with two anterior bristles on apical 1/3; TII with three ad, two pd, apically with five bristles; FIII with three anterior bristles on apical 1/3; TIII with four  ad, two pd, 4-6 weak v, apically with two bristles. Wing (Fig. 32A, B): m-cu somewhat curved, forming right angle with CuA 1 . Halter yellow.
Female terminalia (Fig. 2): Abdominal tergite VIII split into pair of sclerites; epiproct split into pair of triangular hemitergites, apically with row of seven strong curved spines; dorsal lobes of cercus rounded in lateral view, with yellow bristles; ventral lobes of cercus membranous.
Remarks. Diostracus acutatus is similar to D. nishidai Saigusa, in that they both have acute apico-ventral corners of It 1 and It 2 and the shapes of wings and the appendages on abdominal sternite IV are nearly identical. But for males, they are different in the shapes of the main lobe of surstylus, and the apicoventral corner of It 1 in D. acutatus is sharper. Diagnosis. MSSC: first flagellomere 1.5 × longer than wide; propleuron with group of seven long pale hairs on upper portion and group of ~ 20 long pale hairs on lower portion; It 1 shortened and expanded, concave ventrally, forming a hollow with an expanded It 2 . Wing ( Fig. 4B) hyaline, m-cu acutely and deeply arched to vein M 1 , forming an 'h'-shaped hairpin with a slender jet-black mark inside.
Thorax dark metallic green with pale gray pollinosity. Hairs and bristles on thorax black; six mostly hair-like dc except posterior most one dc longest and thick; acr absent; two h, one ph, two npl, one sa, one psa; scutellum with pair of long sc. Propleuron with group of seven long pale hairs on upper portion and group of nearly 20 long pale hairs on lower portion.
Legs nearly entirely black except fore trochanter dark yellow; claws well developed, empodium and pulvilli reduced. Hairs and bristles on legs black except those on coxae pale. CI with cluster of anterior dense, erect, long, pale hairs on apical 1/4 (nearly as long as CI) and comb of pale hairs along anterior margin; CII and CIII nearly bare. Fore trochanter with rows of tiny ventral spines and a hook-like posterior process. FI distinctly thickened, with cluster of four or five erect ventral bristles at extreme base; TI distinctly thickened, weakly curved, with 3 ad, two pd, row of six long pv on apical 1/4; It 1 shortened and expanded, concave ventrally, forming a hollow with expanded It 2 , anterior margin expanded into two dentiform lobes with row of four or five bristles, ventral margin expanded into a lobe at base (corresponding to the lobe of It 2 ), with a subapical pv (Fig. 4C, D); basal half of It 2 expanded and concave ventrally, anterior margin expanded into a pale dentiform lobate and a rectangular lobate, posterior ventral margin expanded into a finger-like lobe at extreme base, apical half with rows of erect ventral bristles, apical half of It 2 , It 3-5 with a ventral suture. FII somewhat flattened laterally, with three ad on apical 1/3; TII with three weak ad, three weak pd, apical 1/5 with two rows of long pale anteroventral hairs (longest ones nearly as long as 1/4 of TII) and row of erect short pv (nearly as long as TII depth), and row of long pale posteroventral hairs along whole length (nearly as long as FII depth), apically with three long bristles. FIII with an anterior bristle and two curved av subapically; TIII with four ad and four pd, apically with two strong long bristles. Relative lengths of tibia and five tarsomeres: LI 5.0: 0.7: 2.5: 2.0: 1.0: 1.3; LII 9.0: 6.1: 2.2: 1.3: 0.7: 1.0; LIII 10.3: 5.2: 3.0: 1.5: 0.7: 1.0.   Abdomen nearly as long as head and thorax combined, dark metallic green with pale gray pollinosity. Abdomen with pale pubescence. Sternite IV medially with an obtuse anterior process and a tubercle bearing bundle of brown bristles, each lateroposterior corner with a tubercle bearing bundle of brown bristles (Figs 5A, 6A). Sternite V split into pair of sclerites, each sclerite ginkgo leaf-like in shape (Figs 5A, 6A).
Male genitalia (Figs 5B, C, 6B, C): Epandrium slightly longer than wide. Epandrial lobe long, wavy, band-like, with an acute basal process, a short bristle at tip. Surstylus thick, lamellated, with two short spines. Hypandrium short thick, apically with a shallow, V-shaped apical incision. Cercus rather short (1/3 as long as epandrium), spoon-shaped, with dark yellow hairs on outer surface, apical one long (nearly as long as cercus), and subapically with group of dense erect dark yellow bristles on inner surface.

Distribution. China (Tibet).
Remarks. The new species belongs to the flexus subgroup of D. fenestratus group. This new species has wing characteristics similar to that of D. strenus sp. nov., but the latter can be separated from D. concavus by It 3-5 , which is normal and has no ventral suture (MSSC).
Etymology. New species name refers to the concave It 1 of males. Diagnosis. MSSC: posterior margin of wing somewhat prolonged along vein CuA 1 . TII and TIII prolonged; apex of TII swollen with two rows of narrow, flat, willow leaf-like ventral bristles, row of long av and pale curve ventral hairs, row of erect pv along whole length, apically with three long bristles; It 1 swollen at extreme base, with rows of long curved posterior bristles.
Thorax (Fig. 8B) dark metallic green with pale gray pollinosity. Hairs and bristles on thorax black; six mostly hair-like dc except posterior most one dc longest and thick; acr absent; one h, one ph, two npl, one sa, one psa; scutellum with pair of long sc. Propleuron with two or three sparse short pale hairs on upper portion and one or two short pale hairs on lower portion.
Legs nearly entirely black except fore trochanter dark yellow; claws well developed, empodium and pulvilli reduced. Hairs and bristles on legs black except those on coxae pale. CI without distinctive bristle, but with dense erect anterior pale hairs on apical 1/4; CII nearly bare; CIII with blackish bristle at extreme apex. Fore trochanter elongated, with hook-like posterior process (Fig. 8D). FI distinctly thickened (Fig. 8D); TI slightly thickened, weakly curved, with one ad at basal 1/3, two pd (one at apical 1/3 outstanding), row of weak pv along whole length; It 1 shortened, concave ventrally, anterior ventral margin expanded into a lobe and recurved, with short bristles apically  8D); It 2 with a finger-like lobe at extreme base, corresponding to the cavity of It 1 , apical half thickened with short dense pv. FII somewhat flattened laterally, with three ad on apical half; apex of TII swollen with two rows of narrow flat willow leaflike ventral bristles, row of long av (anterior ones somewhat curved), rows of pale long ventral hairs (2-3 × longer than TII depth, curved), row of erect pv along whole length (as long as TII depth), apically with three long bristles (Fig. 8E); It 1 swollen at extreme base, with rows of long curved posterior bristles (Fig. 8E). FIII with two av on apical 1/3, rows of sparse pale ventral hairs along whole length (less than FIII depth); TIII with four ad, three pd, without outstanding ventral bristle, apically with two strong long bristles. Relative lengths of tibia and five tarsomeres: LI 5.3: 0.8: 1.5: 1.8: 0.7: 0.75; LII 7.3: 2.9: 1.4: 0.85: 0.45: 0.75; LIII 8.9: 3.3: 2.3: 1.2: 0.65: 0.85.
Abdomen (Fig. 9A) nearly as long as head and thorax combined, dark metallic green with pale gray pollinosity. Abdomen with pale pubescence. Sternite IV medially with pair of obtuse anterior process (very close), bearing bundle of brown hairs (Fig. 10A). Sternite V split into pair of sclerites, each sclerite ginkgo leaf-like (Fig. 10A).
Female (Fig. 7B). Body length 4.9-6.7 mm; wing length 6.1-7.9 mm. Nearly as same as male, but: antenna (Fig. 31C) scape with two short dorsal bristles, first flagellomere somewhat prolonged, 1.2 × longer than width, arista apicobasal, 4.6 × longer than first flagellomere. Proboscis yellowish brown with blackish edge; palpus relatively smaller than males, not reaching apex of proboscis. Six weak dc, except posterior most dc longest and thicken. Propleuron with one or two pale curved hairs on lower portion. Legs black, except trochanters yellowish brown. CI without distinctive bristle or hair, but with short pale anterior hairs on lower position; FI without distinct bristles; TI with one ad at basal 1/4, four pd, apically with a bristle and comb of anterior bristles; FII with two anterior bristles on apical 1/3; TII with three ad, two pd, apically with five bristles; FIII with three anterior bristles on apical 1/3; TIII with four ad, two pd, 4-6 weak v, apically with two bristles. Wing (Fig. 32C): m-cu straight, forming acute angle with CuA 1 . Halter yellow.
Female terminalia (Fig. 11): Abdominal tergite VIII divided into two sclerites; epiproct split into pair of triangular hemitergites, apically with row of nine strong curved spines; dorsal lobes of cercus triangulated, with dark yellow bristles; ventral lobes of cercus short, apex rounded, with long yellow hairs.
Distribution. China (Tibet).  Remarks. The new species belongs to the flex subgroup D. fenestratus group. The species is unique for the shape of wings and It 1-2 , and the prolonged TII and TIII which have relatively short IIt 1 and IIIt 1 . Females are characterized by an arched crossvein m-cu, and the crossvein vertical adjunct to vein CuA 1 ; the trochanters are black.
Etymology. The new species name refers to the cluster of bristles on apex of TII of males. Diagnosis. MSSC: Wings with dark and yellow markings at middle. CI with a brown curved anterior spine at extreme apex; CII with row of four brown anterior spines along apical edge; fore trochanter with row of upwards curved bristles along basal edge. FI with short upwards curved ventral bristles on basal 1/4; TI thickened, with rows of pale ventral hairs on apical 3/4, and apically with two long wavy posterior bristles; FII with row of long av and posteroventral hairs.
Description. Male (Fig. 12). Body length 7.4 mm; wing length 8.2 mm. Head (Fig. 13A) dark metallic green with pale brown pollinosity. Eyes separated; face widened towards clypeus. Hairs and bristles on head black; lower postocular bristles including posteroventral hairs pale. Ocellar tubercle distinct, with pair of oc (broken), pair of posterior hairs; vt weak, slightly shorter than pvt. Antenna black; scape without dorsal bristle; first flagellomere subrectangular, 1.3 × longer than wide; arista apicodorsal, 5.3 × as long as first flagellomere, nearly bare. Proboscis yellowish brown with blackish edge; palpus lobate, 2.3 × as long as broad, blackish with a purple luster, without distinctive bristle. Thorax dark metallic green with pale brown pollinosity. Hairs and bristles on thorax black; six mostly hair-like dc except posterior most one dc longest and thick; acr absent; one h, one ph, two npl, one sa, one psa; scutellum with pair of long sc. Propleuron with two or three sparse short pale curved hairs on upper portion and two long pale curved hairs on lower portion.
Legs nearly entirely black except extreme apexes of coxae and trochanters brownish yellow; claws well developed, empodium and pulvilli reduced. Hairs and bristles on legs black. CI with short sparse pale hairs on anterior surface, and a brown curved anterior spine at extreme apex (Fig. 13D); CII with row of four brown anterior spines along apical edge; CIII nearly bare. Fore trochanter with ridge and row of upwardly curved bristles along basal edge (Fig. 13D). FI distinctly thickened, with short upwards curved ventral bristles on basal 1/4; TI thickened, with four ad, one pd at middle, rows of pale ventral hairs on apical 3/4, and apically with two long wavy posterior bristles (Fig. 13D); It 1 shortened, apex expanded, concave ventrally, anterior ventral margin expanded into a lobate, with comb of bristles on anterior surface (Fig. 13D); It 2 with a finger-like lobe at extreme base, with a posteroventral ridge on basal half (Fig. 13D). FII thickened, with row of seven short av spines, row of three long anteroventral hairs   and row of long posteroventral hairs (as long as FII depth, somewhat curved on basal 2/3) (Fig. 13C); TII with two ad, apically with two long bristles. FIII without conspicuous hairs and bristles; TIII with three ad, two pd, without outstanding ventral bristle. Relative lengths of tibia and five tarsomeres: LI 5.5: 1.2: 3.0: 3.3: 1.8: 1.0; LII 10.2: 6.4: 3.2: 1.8: 0.9: 1.0; LIII 11.8: 6.2: 4.2: 1.8: 0.9: 1.0.
Wing (Fig. 13E) hyaline, anterior half and area around crossvein m-cu yellowish, with dark cloud on middle of cell R 2+3 , and a stripe of dark cloud along M 1 and expanding along posterodistal corner of distal cell, dark cloud in subapical portion of distal cell prominent; veins dark brown, R 4+5 curved at middle, M 1 curved at apical 1/3; crossvein m-cu acutely and deeply arched to vein M 1 , forming a hairpin curve, with a slender jet-black brand inside hairpin curve, accessory cellula 1.3 × longer than width. Squama brown with brown hairs. Halter yellow with blackish apex.
Abdomen (Fig. 13B) nearly as long as head and thorax combined, dark metallic green with pale brown pollinosity, except edge of sternites, apex of epandrium and base of cercus pale. Abdomen with short sparse pale pubescence. Posterior edge of sternite IV forwards recurved with row of short curve spines (Figs 14A, B, 15A). Lateroposterior corner of tergite V elongated into a triangular process. Sternite V split into pair of sclerites, each sclerite ginkgo leaf-like (Figs 14A, B, 15A).
Male genitalia (Figs 14C, D, 15B, C): Epandrium slightly longer than wide. Epandrial lobe and surstylus pale and lamellated. Epandrial lobe short, band-like, apically with one long and one short bristles. Surstylus irregular in shape, with irregular processes and bristles. Process of subepandrial sclerite exceeding epandrium margin, with short fine pubescence. Hypandrium thick, apically with a shallow, U-shaped incision. Cercus band-like, somewhat bent, with long dark yellow hairs around cercus.

Distribution. China (Tibet).
Remarks. The new species belongs to pulchripennis subgroup and is quite similar to D. emotoi. Both species have same chaetotaxy on FII and they are similar in wing style. However, the new species has no posterior bristles on FI, long ventral hairs and two wavy bristles on TI, and relatively smaller wing accessory cellula (2.5 × as long as wide).
Etymology. The name of new species refers to the bright coloration of male wings. Description. Male (Fig. 16A). Body length 6.6-7.0 mm; wing length 9.4-9.8 mm.
Thorax dark metallic green with pale gray pollinosity; mesoscutum with two dark brown longitudinal stripes. Hairs and bristles on thorax black; six weak dc except posterior most one dc longest and thick, occasionally with excess dc; acr absent; one weak and one strong h, one ph, two npl, one sa, one psa; scutellum with pair of long sc. Propleuron with one or two sparse long pale hairs on upper portion and group of long pale hairs on lower portion.
Legs nearly entirely black except fore trochanter dark brown; claws well developed, empodium and pulvilli reduced. Hairs and bristles on legs black except those on coxae pale. CI with group of pale curved anterior hairs on basal 1/3, upper ones long, and cluster of erect bristles on apical 1/4; CII nearly bare; CIII with blackish bristle at extreme apex. FI thickened, with two rows of ventral hairs (as long as FI depth), basal ones pale, and one posterior bristle at extreme base; TI with two pd, three d, row of six or seven long pv on apical half, apically with two bristles and comb of anterior bristles; It 1-2 with rows of pd and pv, ventral surface with short dense fine hairs, It 1 with row  of av spines, extending to It 2 (Fig. 17D); FII with two rows of av on basal half, one distinct v at middle, row of short av on apical 1/3, row of pale long pv on basal 2/3 (longest ones 2.5 × longer than FII depth); TII with two ad, two pd, apically with four bristles. FIII with four av on middle 1/3, apically with one av; TIII with four ad, four pd, four short ventral bristles, apically with three long bristles and comb of short anterior bristles. Relative lengths of tibia and five tarsomeres: LI 6.9: 3.1: 3.2: 1.7: 1.1: 1.0; LII 5.8: 3.2: 1.4: 0.9: 0.5: 0.8; LIII 7.8: 3.4: 2.5: 1.5: 0.8: 0.9.

Male genitalia
Female (Fig. 16B). Body length 5.4-6.0 mm; wing length 6.8-7.9 mm. Nearly as same as male, but: ocellar tubercle without posterior hairs, antenna (Fig. 31D) scape with two short dorsal bristles, first flagellomere semicircular, nearly as long as width, arista apicobasal, 5.6 × longer than first flagellomere. Proboscis blackish; palpus relatively smaller than males, not reaching apex of proboscis. Seven weak dc, except posterior most dc longest and thicken. Propleuron with two or three sparse short pale curved hairs on upper portion and group of five or six sparse long pale curved hairs on lower portion. Legs black. CI with erect pale hairs on anterior surface, ones on lower portion black and thick; FI with one preapical pv; TI with four pd, two pv, apically with three bristles and comb of short anterior bristles; FII with one strong preapical av and one weak preapical pv; TII with three ad, two pd, apically with four strong bristles; FIII with one strong preapical av and one weak preapical pv; TIII with three ad, apically with three bristles. Wing (Fig. 32E): m-cu straight, forming obtuse angle with CuA 1 ; area around m-cu tingled with blackish ash. Halter yellow with blackish apex.
Female terminalia (Fig. 20): Abdominal segments VII and VIII slender; tergite VIII divided into two sclerites; epiproct split into pair of hemitergites, apically with row of five strong curved spines; dorsal lobes of cercus finger-like, somewhat elongated, with dark yellow bristles; ventral lobes of cercus short, apex rounded.
Distribution. China (Tibet). Remarks. The new species is quite similar to D. tibetensis, but the cerci of new species are lamellate with broad base. Females of the new species are characterized by the semicircular first flagellomere of antenna, the straight crossvein m-cu, and the blackish apex of halter.
Etymology. The name of the new species refers to the dense abdominal ventral hairs. Diagnosis. MSSC: dark and robust fly; FI and TI distinctly thickened; posterior ventral margin of It 1 and anterior ventral margin of It 2 expanded into auriform lobes; crossvein m-cu acutely and deeply arched to vein M 1 , forming a 'h'-shaped curve, with a jet-black mark inside curve. (Fig. 21). Body length 6.0 mm; wing length 8.0 mm. Head (Fig. 22A) dark metallic green with pale brown pollinosity. Eyes separated; face widened towards clypeus. Hairs and bristles on head black; lower postocular bristles including posteroventral hairs pale. Ocellar tubercle distinct, with pair of strong oc, without posterior hairs; vt short, 0.7 × as long as oc, nearly as long as pvt. Antenna black; scape with a weak dorsal bristle at basal 1/3; first flagellomere subtriangular, 1.5 × longer than wide; arista apicodorsal, 3 × as long as first flagellomere, nearly bare. Proboscis blackish with pale hairs; palpus lobate, 3.5 × as long as broad, blackish with a purple luster, without distinctive bristle.

Description. Male
Thorax dark metallic green with pale brown pollinosity. Hairs and bristles on thorax black; six mostly hair-like dc except 1 st and 6 th dc long and thick; acr absent; one h, one ph, two npl, one sa, one psa; scutellum with pair of sc. Propleuron with two or three sparse, short, pale hairs on upper portion and one or two short pale hairs on lower portion.
Legs nearly entirely black except fore and mid trochanters dark yellow; claws well developed, empodium and pulvilli reduced. Hairs and bristles on legs black except those on coxae pale. CI without distinctive bristle, but with dense, erect, pale, anterior hairs on apical 1/4; CII with cluster of black bristles at extreme apex; CIII nearly bare. Fore trochanter elongated, with lobate posterior process. FI distinctly thickened, with group of pale hairs on apical 1/5 (less than FI depth) (Fig. 22D, E); TI distinctly thickened, curved, with three ad on basal half, two pd on apical 1/3, row of five long pv on apical 1/3, apically with comb of short pale av spines (Fig. 22D, E); It 1 shortened, concave ventrally, posterior ventral margin expanded into a auriform lobe (Fig. 22D, E); It 2 thickened and recurved, somewhat flattened dorsoventrally, with anterior ventral margin at extreme base expanded into an auriform lobe, corresponding to the auriform lobe of It 1 , a spine-like lobe at basal 1/4, and two rows of short pv spines (Fig. 22D, E). FII with rows of pale postoventral hairs (as long as FII depth), and one ad at apical 1/8; TII with rows of curved ventral hairs on basal 2/3 (longest ones 2 × longer than TII depth), three weak ad, two weak pd, apically with two long bristles. FIII with two ad on apical 1/6, rows of sparse pale ventral hairs on basal half (less than FIII depth); TIII with five ad, three pd, without outstanding ventral bristle, apically with two bristles. Relative lengths of tibia and five tarsomeres: LI 5.7: 0.6: 2.8:    Wing (Fig. 22C) hyaline, indistinctly tinged grayish; veins dark brown; crossvein m-cu acutely and deeply arched to vein M 1 , forming a 'h'-shaped curve, with a jet-black mark inside curve. Squama brown with brown hairs. Haltere blackish with pale knob. Abdomen (Fig. 22B) nearly as long as head and thorax combined, dark metallic green with pale gray pollinosity. Abdomen with pale pubescence. Sternite IV medially with an obtuse process, and one tubercle bearing bundle of brown bristles, lateroposterior corner with brown bristles (Figs 23A, 24A). Sternite V split into pair of sclerites, each sclerite ginkgo leaf-like (Figs 23A, 24A).

Distribution. China (Tibet).
Remarks. The new species belongs to D. fenestratus group. It looks like D. flexus, but can be separated from the latter by the following features of males: the weak acute ventral process near extreme base of It 2 , the swollen apex of It 2 , the row of erect dense strong long posterior, and the anterior ventral bristles on apex of TII.
Etymology. The name of new species refers to the strongly thickened legs. Diagnosis. MSSC: palpus normal, not reaching apex of proboscis. Scutellum with pair of sc and four or five pairs of marginal hairs; CI with row of anterior hairs and two strong recurved spines at extreme apex; FI with a deep hollow at base; wing indistinctly tinged grayish; FII with row of dense ad on apical 2/5; wing apically with three translucent windows between vein C, vein R 2+3 , vein R 4+5 , and vein M 1 .
Description. Male (Fig. 25A). Body length 5.8 mm; wing length 6.0 mm. Head (Fig. 26A) dark metallic green with pale gray pollinosity. Eyes separated; face widened towards clypeus. Hairs and bristles on head black; lower postocular bristles including posteroventral hairs pale. Ocellar tubercle distinct, with pair of oc (lost), without posterior hairs; vt rather short, 0.2 × as long as pvt. Antenna brownish black; scape prolonged, without dorsal bristle; first flagellomere subtriangular, 1.3 × longer than wide; arista apicodorsal, 4.8 × as long as first flagellomere, nearly bare. Proboscis huge and brown, apex blackish with pale hairs; palpus lobate, not reaching apex of proboscis, without distinctive bristle. Thorax dark metallic green with pale gray pollinosity; mesoscutum with pair of dark brown longitudinal stripe. Hairs and bristles on thorax black; six mostly hair-like dc except posterior most one dc longest and thick; acr absent; one h, one ph, one npl, one sa, one psa; scutellum with pair of sc (lost) and four or five pairs of marginal hairs (MSSC). Postnotum well developed, and convex. Propleuron with two or three sparse short pale hairs on lower portion.
Legs nearly entirely black except fore trochanter dark yellow, mid-and hind trochanters brownish black; claws prolonged (MSSC), empodium and pulvilli present. Hairs and bristles on legs black. CI depressed laterally, without distinctive bristle, but with row of anterior hairs and two strong recurved spines at extreme apex (MSSC); CII and CIII with clusters of anterior bristles at extreme apex. FI distinctly thickened, with a deep hollow at base, and row of three or four curved ventral spines and row of dense anterior bristles long the edge of the hollow (MSSC), basal 2/3 with row of four long yellow av (nearly as long as FI depth) (MSSC), apically with long thin pale hairs (MSSC) (Fig. 26C); TI slightly thickened, with row of ad along whole length, ventral surface nearly bare (MSSC); It 1 somewhat thicken, with row of long ad and pd along whole length (nearly as long as It 1 depth), apical half flattened ventrally with two rows of short curved spines and rows of bristles along the edge, apically with a strong curved pv spine (MSSC) (Fig. 26C); It 2 with row of long curved anteroventral hairs along the     whole length, apical half with two rows of long posterior bristles (MSSC); It 3 elongated, base and apex somewhat swollen, nearly bare, except with row of three or four posteroventral hairs at base and four or five dorsal bristles at apex (MSSC); It 4 with long ventral bristles apically (MSSC). FII with row of dense ad on apical 2/5 (MSSC); TII with two ad, 8 pd. FIII without outstanding bristle; TIII with row of long thin ad and long erect thin pd; IIIt 1 with row of thin ad long whole length. Relative lengths of tibia and five tarsomeres: LI 5.3: 1.2: 2.2: 2.6: 0.4: 0.7; LII 7.3: 5.0: 2.4: 1.2: 0.6: 0.8; LIII 8.5: 4.8: 3.4: 1.6: 0.8: 0.8. Wing (Fig. 26B) hyaline, indistinctly tinged grayish, apically with three translucent windows between vein C, vein R 2+3 , vein R 4+5 , vein M 1 (MSSC); vein M with brown strip on middle section (MSSC); veins dark brown; crossvein m-cu somewhat curved. Squama yellow with yellow hairs. Halter brown.
Abdomen (Fig. 26D) nearly as long as head and thorax combined, dark metallic green with pale gray pollinosity. Abdomen with sparse pale pubescence. Tergites II-V Male genitalia (Figs 28B, C, 29B, C): Epandrium slightly longer than wide. Epandrial lobe long, wavy, band-like, with an acute basal process, one short bristle at tip. Surstylus thick, lamellated, with two short spines. Hypandrium short thick, apically with a shallow, V-shaped apical incision. Cercus rather short (1/3 as long as epandrium length), spoon-shaped, with dark yellow hairs on outer surface, apical ones long (nearly as long as cercus length), and subapically with group of dense erect dark yellow bristles on inner surface.

Remarks.
The new species is unique. It has prolonged scapes, small palpus, and convex postnotum. But the huge proboscis, stout body and specialized structures of legs indicate that the new species belongs to Diostracus.
Etymology. New species name refers to the translucent windows on male wing.

Discussion
Including the species described in this work, the number of worldwide species of Diostracus has increased to 107, of which nine species occur in Tibet (Zhu 2006;Zhu et al. 2007a, b;Grichanov 2013Grichanov , 2015Grichanov , 2017Pusch 2014;Wang et al. 2015). The six new species of Diostracus were found in Nyingchi and Shigatse areas of Tibet. Most of them were found at Yatung in Shigatse, which is located on the southern slope of the Himalaya Mountains (Fig. 33). As well as these two sites, we performed a five-year survey in Lhasa, Nyingchi, Shannan, and Qamdo, but there were no Diostracus found, although it does not mean Diostracus only occur in Nyingchi and Shigatse in Tibet. Due to the special habitat of the genus, it can only be collected with sweep nets after finding the flies using the naked eyes. Malaise traps collect the majority of insects in Tibet, as its environment is often too complex to walk through, but no Diostracus were found in them during these surveys.
Diostracus in the Oriental realm shows great diversity in MSSC, especially in modified FI and wing. Complex structures of FI are usually associated with modified wing. The most bizarre MSSC was shown in D. fenestratus group, with It 1 distinctly shortened, almost triangular in shape, expanded portion concaved, and It 2 sinuous, with a basal denticle (Saigusa 1984). The wing in this group is always ornamented, crossvein m-cu either in S-shape or running posteriorly parallel to M 1 . This species group includes 14 species all of which are distributed in the Himalayan Mountains. The species groups with partially thickened It 1 and modified wings (mainly the D. unisetosus group) and the species groups with simple FI and modified wings (with a jet-black nodule besides crossvein m-cu, mainly the D. unipunctatus group) are distributed also mainly in the Himalayan Mountains. The species groups with partially thickened It 1 and simple wings, mainly the D. nebulosus group, are distributed in the Himalayan Mountains and the Chinese mainland. Finally, the species groups with simple FI and simple wing is found in the Chinese mainland and Taiwan. Himalayan Mountains is the diversity center for the genus Diostracus, and species in this area show great diversity (53 species out of 107) and abundant specialized characters. The species in the Chinese mainland and Taiwan usually have low diversity (20 species of 107) and simple structures.
Besides the morphological characters, mitochondrial COI genes of females have also been sequenced to pair them with males. As a result, some females could not be matched to males, and the characters of these females are obviously different from known species found in our investigation. Therefore, we believe that more new species will be discovered in the Himalayan region in the future.