Epeorus (Proepeorus) nipponicus (Uéno, 1931) (Figs 20–22)

[ Japanese name "Yumimon-hirata-kagerou"]

Iron nipponicus: Uéno 1931a: p. 97, fig. 4 (male imago) [JAPAN: Nagano]; Uéno & Okamoto 1932: p. 1959, fig. 3860 (male imago) [JAPAN].

Epeorus nipponicus: Imanishi 1934: p. 386 (synonymy only); Ishiwata 2002: p. 23, figs 181–184 (nymph, male imago) [JAPAN: Kanagawa]; Ishiwata & Takemon 2005b: p. 105, figs 45-5, 45-10, 46-6, 47-11, 47-12 (male imago, nymph) [JAPAN]; Maruyama 2016: p. 173, figs 1-171i, 1-180, 1-282, 1-283, 1-284, 1-285 (male imago, female imago, male subimago, female subimago) [JAPAN: Hyogo]; Ishiwata et al. 2018: p. 126, figs 45-5, 45-10, 46-6, 47-11, 47-12 (male imago, nymph) [JAPAN]; Ishiwata & Fujitani 2018: p. 33, pl. 22, fig. 6 (egg) [JAPAN].

Epeorus (Proepeorus) nipponicus: Kluge 2004: p. 204 (name only); Ma et al. 2021: p. 1133, figs 1A–C, 6B, 12C, 13C, 14E–F, 15A, I, 16C (male imago, female imago, nymph) [Northeast CHINA].

Epeorus curvatulus auct., non Matsumura, 1931: Imanishi 1934: p. 392, fig. 3 (male imago, female imago, nymph) [JAPAN: Kyoto]; Imanishi 1940, p. 250, fig. 38 (nymph) [KOREA, Northeast CHINA]; Uéno 1950: p. 123, fig. 307 (male imago, female imago) [JAPAN]; Uéno 1959: p. 54 (nymph) [JAPAN]; Gose 1962: p. 22, fig. 1-11-7 (nymph) [JAPAN]; Gose 1979: p. 45, figs 14, 21 (male imago, nymph) [JAPAN]; Okuma 1980: p. 30, fig. 1 (nymph) [JAPAN: Saitama]; Gose 1985: p. 16, fig. 42 (nymph) [JAPAN]; Bae et al. 1998: p. 91 (nymph, imagines) [KOREA, Far East RUSSIA]; Ishiwata 2000: p. 77, fig. 43 (male imago, nymph) [JAPAN: Kanagawa]; Quan et al. 2002, p. 253, figs 32, 76, 92, 104, 153 (nymph) [Northeast CHINA].

Epeorus (Epeorus) curvatulus: Byong & Bae 1984: p. 6, pl. 3, figs a–e (nymph) [KOREA].

Epeorus (Belovius) curvatulus: Tshernova 1981: p. 326, fig. 8 (male imago) [JAPAN].

Epeorus torrentium (non Epeorus torrentium Eaton, 1881): Uéno 1928: p. 34, misidentification (Imanishi 1940).

Materials examined. Type specimens: We investigated The Kyoto University Museum collection but could not find the type specimen. Probably lost . Other specimens: JAPAN, HOKKAIDO, ISHIKARI: [4.] Sapporo-shi, Minami-ku, Jozankei, Usubetsu-gawa Stream (a tributary of Toyohira-gawa Riv.), 472 m a.s.l. (42°54'33.5"N, 141°07'28.9"E), 2 mature male nymphs, 8.IX.2002, K. Saito; [5.] Sapporo-shi, Minami-ku, Jozankei, Shirai-gawa Stream (a tributary of Toyohira-gawa Riv.), the confluence of Shirai-gawa Stream and Migimata-gawa Stream, 450 m a.s.l. (42°59'39.6"N, 141°04'42.9"E), 1 mature male nymph and 1 mature female nymph, 07.VIII.2004, K. Saito; [15.] Sapporo-shi, Chuo-ku, Minami 13-jo, Nishi 1-chome, Toyohira-gawa Riv., mainstream, 28 m a.s.l. (43°02'35.3"N, 141°21'31.7"E), 2 mature male nymphs and 4 mature female nymphs, 19.VIII.2020, T. Takayanagi; ditto, 1 male mature nymph and 1 female mature nymph, 08.VIII.2021, T. Takayanagi; [16.] Sapporo-shi, Chuo-ku, Kita 1-jo, Higashi 19-chome, Toyohira-gawa Riv., mainstream, 15 m a.s.l. (43°04'04.8"N, 141°23'13.2"E), 1 mature male nymph and 1 mature female nymph, 21.VIII.2021, T. Takayanagi .

Nymph (mature, in ethanol), redescription (Figs 20–22).

Body length 11.0 mm (male), 10.0–12.0 mm (female). Cercus length ca. body length x 1.3–1.4.

Head. Shape trapezoidal. Color dark-brown, with white markings. Compound eyes dark gray. Ocelli dark gray. Antennae white, basally brown or completely brown. Anterior margin densely covered with fine hair-like setae extending to lateral margins. Dorsal surface of head covered with fine hair-like setae (Figs 21a, b). Labrum: anterior and lateral margins evenly convex, anterior margin medially concave. Ventral surface with a row of bristles along lateral margins extending to anterior margins, row of short setae and brush of median fine hair-like setae on each side. Dorsal surface with six long adjacent bristles medially and two long bristles near each side of antero-lateral margin, and scattered variable-length setae (Fig. 22a). Mandibles: each outer incisor with three apical teeth (right mandible) or two apical teeth (left mandible) and serrated margins, inner incisor with two apical teeth (right mandible) or three apical teeth (left mandible) and outer margin of both inner incisors sharply serrated, right inner incisor slender and almost straight. Tuft of long setae on the base of inner incisor of right mandible, tuft of long setae and one plumose seta on the base of inner incisor of left mandible (Figs 22b, c). Maxillae: One penicillate seta at base of apical tooth complex (Fig. 28d). Hypopharynx: superlinguae distally widened, lingua subquadrate. Labium: Labial palps two segmented, outer margin of proximal one with sparse, thick setae. Distal segment with sparse, hair-like setae on outer surface and dense, brush-like setae on distal 1/3 area. Glossae and paraglossae with long, dense setae.

Thorax. Color dark brown dorsally, white ventrally (Fig. 20). Pronotum margin roundly projecting laterally (Fig. 29c). Legs. Each femur with dark brown markings and a hypodermal black spot on middle part of anterior face (Fig. 21c), blade-like long setae on dorsal edge, and short bluntly pointed spines on ventral edge. Femoral setae diversely shaped: semi-oblong, maize-grain-shape or semi-circular (Fig. 22d). Tibia with two dark-brown markings, long hair like setae on its dorsal edge (Fig. 21c). Tarsus darker in proximal area, long hair like setae on its dorsal edge (Fig. 21c). Tarsal claw with four small denticles.

Abdomen. Terga dark-brown, each tergum with a pair of dark spots (Figs 20a, d). Sterna white, each sternum with a pair of slightly blackish spots (Figs 20b, e). Each tergum with a pair of rows of sparse long setae located to each side of midline. Posterior margin of each segment with several long setae and diversely shaped spines which are pointed, blunt or spatulate and arranged in an irregular line, overlapping in 2–3 levels (Figs 22j, 31d). Each of terga I–VII with three postero-lateral projections, ventral and dorsal ones blunt, lateral one short-tipped (Fig. 30d). Posterior margin of sternum IX round with relatively rounded emargination (Figs 22h, i). Dorsal surface of cercus with a row of fine setae. Gills. Costal and posterior half part of each gill brown (Fig. 21d). Gill I not anteriorly elongate (Fig. 22e). Gills II–VII oval (Figs 22f, g). Costal margin of gill I with fine setae. Costal margin of gills II–VII with small spines and fine setae forming a rough surface. Gill II–VII with anal ribs on anal margin. Each gill plate with filaments forming a fan-shape together. Gill VII without longitudinal fold (Fig. 22g).

Imagines of this species were not obtained in this study.

Diagnosis and comparison. Nymph. Gill I is not extended anteriorly, and gill VII lacks a longitudinal fold, in contrast to E. curvatulus and E. aesculus . Immature nymph’s display a blackish streak on the posterior part of each abdominal tergum, similar to E. aesculus, but this streak diminishes in mature nymphs.

Remarks. Nymph. According to Okuma (1980) and Ishiwata et al. (2018), nymphs of this species can be distinguished from the similar species E. ikanonis Takahashi, 1924, by the length of the cerci and seasonal growth patterns. In E. ikanonis, the cerci length is roughly equal to the body length, whereas in E. nipponicus, it is about 1.5 times the body length. Mature nymphs of E. ikanonis are observed from winter to early spring, while those of E. nipponicus appear in autumn. Subgenus attribution. Our observation fully supports the classification by Ma et al. (2021) which assigned this species to the subgenus Proepeorus based on male imaginal characteristics. Nymphs of this subgenus are characterized by the following features: 1) gills II–VII possess an anal rib along the anal margin; 2) gill I is not elongated; and 3) gill VII lacks a longitudinal fold (Kluge 2004). The subgenus Epeorus s. str. (= Epeorus /fg4) also shares these traits, with male imaginal penial structures being the only distinguishing feature between Proepeorus and Epeorus s. str.

The taxonomic status of this species has been ambiguous. Uéno (1931a) first described the species based on material collected from the Nakabusa-gawa River in Nagano, central Honshu. However, the type specimens were likely lost (Author's investigation 2019). Imanishi (1934) included this species in his monograph without any accompanying explanation. Since then, it has been omitted from major works, such as Gose's monograph on Japanese mayflies (Gose 1979). Ishiwata (2001a; 2018) listed the species as valid and described its diagnostic characters (Ishiwata 2002; Ishiwata et al. 2018), which allowed for its identification. The nymphal morphology of this species, in its current sense, closely aligns with Imanishi's (1934) description of E. curvatulus, suggesting that E. nipponicus may have been misidentified as E. curvatulus in Imanishi's work. Ma et al. (2021) published a revision of Epeorus (Proepeorus) in China, providing a detailed description of the morphology of E. nipponicus . The nymphal morphology observed, particularly the structure of the mouthparts and the posterior margin of the abdominal terga, aligns closely with our observations.

A similar species, E. anatolii Sinitshenkova, 1981, was described from Far East Russia based on female nymphs, and the description aligns completely with our observations. Bae et al. (1998) later synonymized this species with E. curvatulus, likely based on Imanishi’s concept, which may have confused E. curvatulus with E. nipponicus . However, the nymphal morphologies of E. curvatulus and E. nipponicus are distinctly different, suggesting that E. anatolii should potentially be synonymized with E. nipponicus . Nevertheless, confirming this requires examination of the type specimens of E. anatolii, as also discussed by Ma et al. (2021).

Emergence periods and habitat. Mature nymphs were obtained after August in the study areas. This species was obtained from a wide area within the Toyohira-gawa River system.

Distribution. Japan (Hokkaido, Honshu, Shikoku, and Kyushu), Korea, China (Northeast), and Russia (Far East).