Rhyacophila arakawa Ito 2021, sp. nov.

Rhyacophila arakawa sp. nov.

(Figs 3, 6B)

Rhyacophila sp. 1: Kuhara & Ito 2017, 16, Yakushima.

Diagnosis. This new species belongs to the R. lieftincki Species Group, and the male is similar to those of R. kando Schmid 1970, found in Nepal and Bhutan (Schmid 1970; Morse 2021), R. claviforma Sun & Yang 1998, found in eastern and southwestern China (Sun & Yang 1998), and R. impar Martynov 1914, found in Japan (Honshu) and Russia (Siberia, Far East) (Hattori 2005; Ivanov 2011), in having a long triangular segment IX in lateral view, an ax-shaped segment X and a sock-like apical segment of each inferior appendage. However, the male of this new species is distinct from the three similar species in the combination of the following characteristics: Segment IX is triangular in dorsal view; apical parts of segment X are strongly broadened with irregularly serrated caudal margin and pair of lateral plates; and parameres are circular-saw-like apically. The female of R. arakawa is similar to that of R. impar (Arefina 2001; Hattori 2005) but is distinguished by the ratio of length to basal width of segment VIII, which is about 1.3 in R. arakawa but lower than 1.0 in R. impar.

Adult. Lengths of forewings and hind wings: Each forewing 8.4–10.3 mm long (mean 9.6 mm, n = 7) and each hind wing 7.1–9.0 mm (mean 8.1 mm, n = 7) in males, 10.7–10.9 mm (mean 10.8 mm, n = 2) and 8.4–9.4 mm (mean 8.9 mm, n = 2) in females, respectively. Head brown, warts light brown with brown setae; antennae brown, scapes thicker and longer than other apical segments; palpi brown to light brown. Thorax brown dorsally. Legs brown, spurs dark brown. Wings brown with darker veins and whitish pterostigmas, light-colored dots scattered on apical 1/3 of forewings. Abdomen with brown tergites and sternites, darker pigments scattered dorsally; small mid-ventral process on sternite VII (or VI and VII) in male and on sternite VI in female, process on sternite VI of male often very small or completely absent.

Male genitalia (Figs 3A–3D). Segment IX (IX) in lateral view subtriangular with dorsum about 5 times as long as venter, in dorsal view subquadrate at basal 1/3, long and triangular at apical 2/3. Segment X (X) arising from mesal base of segment IX, in lateral view ax-shaped with long stem, irregularly serrate caudal margins and pair of thin semicircular lateral plates (la pl) apicoventrally. Apical band (ap ba) slightly sclerotized, round in lateral view, U-shaped in ventral and caudal views. Anal sclerites lacking. Tergal band (t b) long, connected to apical band and phallotheca (ph).

Phallotheca large roundish. Aedeagus (ae) slender, curved dorsoposterad subapically. Parameres (pa) long barlike, posterior 1/10 compressed, circular-saw-shaped apically.

Inferior appendages large. Basal segments (b inf app) in lateral view rectangular, length 1.5 times as long as basal width, dorsal, ventral and caudal margins shallowly concave; apical segment (a inf app) in lateral view subquadrate with enlarged ventro-caudal corner, in dorso-mesal view subtriangular, with numerous spinules mesally.

Female genitalia (Figs 3E–3I). Segment VIII (VIII) large, length 1.3 times as long as basal width, gradually narrowing posteriorly in lateral, dorsal, and ventral views; posterior margin with deep, wide, U-shaped incision dorsally and shallow incision ventrally. Vaginal apparatus slender; processus spermathecae (ps) very weakly sclerotized basically, round in ventral view, directed dorsocaudad in lateral view; posterior process (pp) membranous, subacute apically in ventral view and with thickened end in lateral view.

Holotype. Male, Japan, Yakushima Island, Yakusugi-land, Ara-kawa, Seiryu-bashi (30.304˚N, 130.569˚E, 1030 m a.s.l.), 16.x.2007, TI, S (CBM-ZI 0159997).

Paratypes (SPMN-IS). Yakushima Island: 2 males, 1 female, same data as holotype; 1 male, Yakusugi-land, Ara-kawa, a tributary, 16.x.2007, TI, S; 1 male, Miyanoura-gawa, Yukawa-bashi, 15.x.2007, TI, P; 1 female, Anborindo, Harumaki-rokugo-bashi, 17.x.2007, TI, S; 2 males, Onoaida, Onoaida-gyoko, 24.iv.2003, A. Ishizuka.

Other specimens. Yakushima Island: 1 male, near Yodogawa-goya, small stream, 25.ix.2003, N. Kuhara (N. Kuhara).

Distribution. Japan (Yakushima).

Habitat. Adults of this species were collected beside streams with stony bottoms located at 20 to 1300 m a.s.l. (Fig. 6B).

Etymology. The name “ arakawa ” is a noun in apposition, coined from the name of the type locality.

Japanese name. Arakawa-nagare-tobikera.