﻿The genus Hercostomus Loew (Diptera, Dolichopodidae, Dolichopodinae) from Inner Mongolia, China, with the description of two new species

﻿Abstract Previously, only two species of Hercostomus Loew were known to occur in Inner Mongolia. Here two species from Inner Mongolia are described as new to science, namely Hercostomuschifengensissp. nov. and Hercostomustriangulatussp. nov. Three new records of Hercostomus in Inner Mongolia are added. A key to the species of Hercostomus in Inner Mongolia is provided.


Introduction
Hercostomus Loew is one of the largest genera in the family Dolichopodidae with 475 known species worldwide, of which 300 species have been recorded from China (Yang et al. 2006;Yang et al. 2011;Qilemoge et al. 2017Qilemoge et al. , 2020Grichanov 2020). Members of Hercostomus can be identified by the following features: eyes separated at the lower margin; thorax lacking a distinct dark spot above the notopleuron, pleural surface in front of the posterior spiracle bare; mid femora with an anterior preapical bristle; hind femora with the anterior bristle positioned at the apex, usually slightly flattened laterally, or not; fore tarsus usually simple; wing rarely darkened in the anterior half; vein M 1+2 weakly sinuate, flexion at the basal third or at the middle of the distal part and sometimes with subapical flexion; sometimes the basiventral epandrial lobe of the epandrium and hypandrium forming complex entangled asymmetrical lobes (Brooks 2005;Grichanov 2011;Yang et al. 2011).
Inner Mongolia is located in a narrow region extending northeast to southwest in northern China. The climate of Inner Mongolia is temperate continental with greater precipitation in the northeast compared to the southwest and higher temperatures in the southwest compared to the northeast. Natural vegetation types range from forests, meadow steppe, typical steppe, desert steppe and the Gobi Desert from the northeast to the southwest, respectively.
Previously, only two species, Hercostomus neimengensis Yang, 1997 andH. sinicus Stackelberg, 1934, were recorded from Inner Mongolia (Yang et al. 2011). Here two new species of Hercostomus are described from Inner Mongolia, namely H. chifengensis sp. nov. and H. triangulatus sp. nov. The following three species are newly recorded from Inner Mongolia: Hercostomus beijingensis Yang, 1996, Hercostomus dilatitarsis Stackelberg, 1949 and Hercostomus shennongjiensis Yang, 1997. A key to species of Hercostomus in Inner Mongolia is provided. All of the updated records are distributed in mountains of nature reserves in Inner Mongolia: Jiufeng Mountain, Helan Mountain, Daqinggou. Hercostomus neimengensis Yang, 1997 is also distributed in grasslands of Keerqin. We discovered that all Hercostomus species in Inner Mongolia are distributed in grasslands near creeks and damp areas of mountains (Fig. 1). Currently, the genus comprises 300 species in China and is distributed widely around China. The low level of diversity of the genus in Inner Mongolia is probably the result of few investigations (Yang et al. 2011). Thus, it is promising to find more Hercostomus species in Inner Mongolia, especially in forests of northeast part of Inner Mongolia.

Materials and methods
The specimens on which this study is based were collected from Inner Mongolia in 2013 and 2014 by sweeping net. All specimens are deposited in the Entomological Museum of China Agricultural University (CAU), Beijing. Morphological terminology follows Cumming and Wood (2017). The following abbreviations are used: acr = acrostichal bristle (s), ad = anterodorsal bristle (s), av = anteroventral bristle (s), dc = dorsocentral bristle (s), sc = scutellars, pd = posterodorsal bristle (s), v = ventral bristle (s), LI = fore leg, LII = mid leg, LIII = hind leg, CuAx ratio = length of dm-cu / length of distal portion of CuA.

Taxonomy
Key to species (males) of Hercostomus from Inner Mongolia Diagnosis. Antenna entirely black; postpedicel 1.8 times longer than wide, blunt at tip; basal segment of arista 0.55 times as long as apical segment. Legs entirely black. Wings slightly tinged brown. Male cercus nearly lobate, distinctly longer than wide, with short finger-like marginal processes. Description. Male (Fig. 3). Body length 3.1-3.2 mm, wing length 3.5-4.1 mm. Head metallic green with pale grey pollinosity. Hairs and bristles on head black, but middle and lower postocular bristles and posteroventral hairs yellow. Ocellar tubercle with 2 strong oc and 2 short posterior hairs. Antenna (Fig. 9) black; postpedicel 1.8 times longer than wide, blunt at tip; arista black, basal segment 0.55 times as long as apical segment. Proboscis brownish with black hairs; palpus black with black hairs and 1 black apical bristle.
Thorax metallic green with pale grey pollinosity. Hairs and bristles on thorax black; 7-8 irregularly biseriate acr short hair-like, 6 long strong dc. Scutellum with 2 pairs of sc and several short marginal hairs, basal pair hair-like.
Legs entirely black. Hairs and bristles on legs black. Mid and hind coxae each with 1 outer bristle; mid and hind femora each with 1 preapical bristle; fore tibia with 3 short ad, 2 short pd and 2 apical bristles; mid tibia with 4 ad, 2 pd, 1 av and 3 apical bristles; hind tibia with 3 ad, 2 pd, 1 short av and 3 apical bristles; hind tarsomere 1 with 1 short v at base. Relative lengths of tibia and 5 tarsomeres of legs LI: Abdomen metallic green with pale grey pollinosity. Hairs and bristles on abdomen black. Male genitalia (Fig. 10): Epandrium distinctly longer than wide, narrowed at tip; inner epandrial lobe relatively small, outer epandrial lobe long finger-like with somewhat swollen tip. Subepandrial process with two long processes branched, one blunt at tip, one sharp at tip. Male cercus large, lobate, slightly shorter than epandrium, distinctly longer than wide, with several short finger-like marginal processes. Hypandrium tubular at tip, with a hook-like projection near middle.
Female  Zhang, Yang & Grootaert, 2008 from Xinjiang of China, but can be distinguished from the latter by the arista located at middle of the dorsal margin of the postpedicel and the male cercus long and narrow. In H. subrusticus, the arista is located at the apical one-third of the dorsal margin of the postpedicel, and the male cercus is relatively short and wide (Zhang et al. 2008).
Thorax metallic green with pale grey pollinosity. Hairs and bristles on thorax black; 6 irregularly biseriate acr slightly long and strong, 6 long strong dc. Scutellum with 2 pairs of sc, basal pair hair-like. Propleuron with yellowish hairs and 1 bristle on lower portion.
Female. Body length 3.2-3.5 mm, wing length 3.0-3.2 mm. Specimens examined. Diagnosis. Antenna mainly dark yellow; postpedicel blackish with basal ventral surface dark yellow, 1.2 times longer than wide, obtuse at tip; arista black, basal segment 0.25 times as long as apical segment. All coxae dark yellow. Male cercus nearly triangular with weak denticles and 3 relatively long finger-like processes.
Description. Male (Fig. 8). Body length 3.7-4.1 mm, wing length 3.3-3.6 mm. Head metallic green with dense pale grey pollinosity. Hairs and bristles on head black, middle and lower postocular bristles and posteroventral hairs yellow. Antenna (Fig. 12) dark yellow except scape blackish at base and postpedicel blackish with base and ventral surface dark yellow; postpedicel 1.2 times longer than wide, somewhat acute at tip; arista blackish with short pubescence, basal segment 0.25 times as long as apical segment. Proboscis brownish yellow with black hairs; palpus brownish, with dark yellow hairs and 1 dark yellow apical bristle.
Thorax metallic green with pale grey pollinosity. Hairs and bristles on thorax black; 6~8 irregularly biseriate acr short hair-like; 6 long strong dc. Scutellum with 2 pairs of sc, basal pair short hair-like. Propleuron with yellowish hairs and 1 black bristle on lower portion.
Legs yellow; all coxae yellow; all tarsi brown to dark brown from tip of tarsomere 1 onwards. Hairs and bristles on legs black; mid and hind coxae each with 1 outer bristle; mid and hind femora each with 1 preapical bristle; fore tibia with 1 ad, 2 pd and 2 short apical bristles; mid tibia with 4 ad, 2 short pd, 1 av and 4 short apical bristles; hind tibia with 3 ad, 3 pd, 4 short av (2 inner bristles thin, 2 outer bristles thick) and 3 apical bristles. Hind tarsomere 1 with 1 short ventral bristle at base. Abdomen metallic green with pale grey pollinosity. Hairs and bristles on abdomen black; tergite 1 with several short yellow hairs; sternites 2-3 with short yellow hairs. Male genitalia (Fig. 14): Epandrium distinctly longer than wide, narrowed at tip; epandrial lateral lobe relatively short and thick. Subepandrial process (Fig. 13) with two processes separated, narrowed at tip. Male cercus nearly triangular with some weak denticles and 3 relatively long finger-like processes bearing long bristles on apical margin. Hypandrium somewhat acute at tip.
Female. Body length 3.0-3.6 mm, wing length 3.6-3.7 mm.  Remarks. The new species is somewhat similar to the members of H. crassivena group, but the veins of H. triangulatus are not thickened (Zhang and Yang 2007).
Etymology. This species is named after the triangular cercus.

Discussion
Hercostomus Loew is probably polyphyletic and not a monophyletic genus as it is poorly defined (Brooks 2005;Yang et al. 2011 Stackelberg, 1949, H. neimengensis Yang, 1997, H. sinicus Stackelberg, 1934, and H. triangulatus sp. nov. were not assigned to any species group. Further studies are necessary in order to clarify their systematic placement.