Rheocricotopus (P.) masarui sp. nov. urn:lsid:zoobank.org:act: AA635D3F-1D0D-4EC0-9CEA-976DC1B173F1 Fig. 20, Table 18 Rheocricotopus (Psilocricotopus) glabricollis [nec Meigen, 1830] – Makarchenko & Makarchenko 2005: 126. Diagnosis Male Abdomen brown, with faint, median, dark I- or inverted T-shaped markings on tergites I–IV. AR high (ca 1.5). Acrostichal setae long (15–25 μm). Humeral pit large, quadrate. Wing with a well-developed anal lobe, a distinct costal extension and a well-developed squamal fringe. Superior volsella thin, roundly protruded. Inferior volsella triangular, with beak-like apex. Crista dorsalis distinct, tooth-like. Pupa Exuviae small-sized (3.0–4.0 mm long). Frontal setae bifid on prefrons. Etymology Named in honor of the late Dr Masaru Yamamoto (Shimonoseki, Japan) in recognition of his contribution to taxonomic studies on the East Asian chironomid fauna. Type material Holotype CHINA – Sichuan • ♂; Liangshan, Puge, Qiaowo; 27°30′ N, 102°27′ E; 1447 m a.s.l.; 3 May 2021; J.Y. Shi leg.; JNU, JNU 211013H 03B. Paratypes CHINA – Jilin • 1 ♂ with Pe; Antu, Mt Changbai; 42°03′ N, 128°03′ E; 1835 m a.s.l.; 11 Aug. 2022; H.Q. Tang leg.; JNU, JNU 22082001. – Inner Mongolia • 2 ♂♂; Arxan National Forest Park, site 11; 47°33′ N, 120°52′ E; 958 m a.s.l.; 9 Sep. 2023; Z.Y. Ni leg.; JNU, JNU 23102801. – Guangdong • 1 ♂ (pharate), 1 Pe; Shaoguan, Renhua, Hongshan; 25°15′ N, 113°36′ E; 500 m a.s.l.; 16 Feb. 2023; Y.R. He and X. Dong leg.; JNU, JNU 23021901. – Yunnan • 1 ♂; Lyuchun, Mt Huanglian NNR; 22°50′ N, 102°14′ E; 1355 m a.s.l.; 22 Apr. 2017; H.Q. Tang leg.; JNU, JNU170521006. – Sichuan • 1 ♂; same data as for holotye; 23 Apr. 2021; JNU, JNU 21101204 B. Description Male (n = 3) Total length 3.2–3.4, 3.3 mm. COLOURATION. Body mostly dark brown. Head dark brown on vertex. Thorax dark brown on 3 scutal vittae, Pe, Sct and Pe. Abdomen (Fig. 20A–C) brown with faint, dark markings; T I with I-shaped dark marking or more extensively dark brown; II–IV or II–V each with median I- or inverted T-shaped marking, occasionally these markings very faint; T V or V–VI with broad, triangular marking; T VII–IX entirely dark brown. All legs uniformly brown to dark brown. HEAD. Temporals 4–6, 5. AR 1.4–1.6, 1.5. Clypeus with 13–17, 15 setae. Lengths (μm) of Pm 1–5: 34–44, 41; 64–74, 67; 111–128, 121; 155–167, 160; 229–258, 245, respectively. Pm 4 /Pm 3 1.3–1.4, 1.3; Pm 5 / Pm 4 1.5–1.6, 1.5. Pm 3 with 4 SCl. THORAX. Lateral Aps 4–5, 4; occasionally one weak seta present medially. Ac 8–14, 12, longest 17–25, 22 μm long; Dc 15 (2), including 3–6, 5 H, usually biserial anteriorly; Pa 4–6, 5; Scts 6–12 (2). HP (Fig. 20D) large, nearly quadrate, 118–137, 127 μm long, 108–137, 121 μm high. WING (Fig. 20E). Length 2.0–2.3, 2.2 mm. C extension well developed, 64–79, 74 μm long. VR 1.0–1.1 (2). R with 5 setae, R 1 and R 4+5 bare. AnL well developed. Sq with 12–14, 13 setae. LEGS. P 1 with single ti spur 59 μm long; P 2 with 2 ti spurs of equal length, 20–22 (2) μm; P 3 with 2 ti spurs 54–59, 56 and 22–25, 23 μm long. P 2 with 5–8, 6 Sch on ta 1; P 3 with 0–3, 2 Sch on ta 1. P 3 with ti comb of 11–12, 11 bristles. Lengths and proportions of legs as in Table 18. ABDOMEN. T II–VIII (Fig 20F–G) each with setae separated in 3 groups. Hypopygium (Fig. 20H) with AnP (Fig. 20I) pointed apically, bearing 5–6, 6 lateral setae. Gc 229–239, 233 μm long with SVo thin, roundly protruded; IVo triangular with beak-like apex. Gs (Fig. 20J) 93–98, 97 μm long, roundly convex in outer margin, tending to project outwards as weak subapical heel; CD triangular, distinct. HR 2.33– 2.47, 2.41. Pupa (n = 4) Total length 3.2–4.3, 3.8 mm. COLOURATION. Exuviae brown with dark Apo and posterior spines on abdomen. CEPHALOTHORAX. FA (Fig. 20K) weakly rugose with FS bifid, 25–42, 33 μm long. Scutum extensively granular. Thoracic horn (Fig. 20L) 357–431, 408 μm long, 3.4–3.8, 3.5 × as long as broad. Lengths (μm) of Pc 1–3: 140–167, 151; 197–221, 212; 93–111, 100, respectively. Lengths (μm) of Dc 1–4: 69–101, 84 (3); 52–74, 62 (3); 44–49, 47 (3); 62–98, 76, respectively. Distances (μm) of Dc 1 –Dc 2, Dc 2 –Dc 3, Dc 3 –Dc 4: 22–106, 68; 57–96, 76; 30–86, 51, respectively; these setae linear arranged; distance ratio of Dc 1 –Dc 2 to Dc 2 –Dc 3 0.23–1.87, 1.05. ABDOMEN (Fig. 20M–N). Length 2.7–3.3, 3.0 mm. T I–II without spinulation; T III–VIII each with sporadic, weak posteromedian spinulation; T IX with anterior spinulation. S I and IX without spinulation. S II–III with relatively strong, anterior spinulation; S IV with anterolateral spinulation; S V–VI with more or less extensive spinulation; S VII–VIII with anterolateral spinulation. Spine rows on posterior margins of T II–VIII; some posterior spines recurved on T II; posterior spines on T VIII long and strong, longest 17–27, 24 μm long. Anteriorly directed spinules present, posterior to spine rows on T II–V. S IV– VII with rows of spines on posterior margin; posterior spines small, sporadic on S VII. PSB present on A II–III. Segment I with 2 L-seta; II–VI each with 3 L-setae, VII with 4 LS-setae, VIII with 5 LS-setae; some L-setae apically split on A II–VI. AL 294–363, 338 μm long, 2.1–2.5, 2.3 × as long as broad, with 12–22, 18 lateral taeniae on posterior 3 / 5 –¾; male genital sac nearly as long as AL. Female and larva Unknown. Distribution The new species is known from Oriental and Palaearctic China, and the Russian Far East. Remarks The male will key to that of R. (P.) glabricollis (Meigen, 1830) in Lehmann (1969, as Rheocricotopus gouini (Goetghebuer, 1936)), Saether (1986) and Langton & Pinder (2007), but differs in faint dark markings on the tergites I–IV, and the distinctly beak-like apex of inferior volsella. Under the name Spaniotoma (Trichocladius) glabricollis, Edwards (1929: 330) wrote briefly: “abdomen rather dull blackish brown”. The male of R. (P.) glabricollis possesses a simple, triangular inferior volsella, according to Edwards (1929: 329 fig. 5b, as Spaniotoma), Goetghebuer (1932: 57 fig. 95, as Trichocladius), Lehmann (1969: 366 fig. 2, as R. gouini), Saether (1986: 87 fig. 11d) and Langton & Pinder (2007: 80 fig. 189b). The pupa will key to that of R. (P.) atripes (Kieffer, 1913) in Langton (1991) and Saether (1986), but differs in the bifid frontal seta and a larger thoracic horn (357–431 μm in length). According to Langton (1991: 108 fig. 46b), in the latter, the frontal seta is simple and the thoracic horn is 300–350 μm long. Makarchenko & Makarchenko (2005: 126) redescribed R. (P.) glabricollis based on a single male from Sakhalin Island, Russia. However, the shape of inferior volsella (Makarchenko & Makarchenko 2005: 127 fig. 5) is more similar to that of R. (P.) masarui sp. nov. than to that of R. (P.) glabricollis. Further, according to E. Makarchenko (pers. comm. 2023), each of tergites II–IV have a faint dark marking and possibility tergite V has a very faint marking. Therefore, the Russian species is conspecific with R. (P.) masarui sp. nov.

Published as part of Niitsuma, Hiromi & Tang, Hongqu, 2026, Taxonomic review of Rheocricotopus Brundin, 1956 (Diptera: Chironomidae: Orthocladiinae) from East Asia, with descriptions of twelve new species, pp. 1-114 in European Journal of Taxonomy 1037 on pages 68-71, DOI: 10.5852/ejt.2026.1037.3157, http://zenodo.org/record/18507203