Rhyacophila yosiiana, Tsuda, 1940

Ito, Tomiko, 2021, The genus Rhyacophila Stephens (Trichoptera, Rhyacophilidae) in Yakushima Island, northern Ryukyu, southwestern Japan, Zootaxa 5023 (1), pp. 44-58 : 54-55

The Rhyacophila yosiiana Species Group (s. str.) and the R. ulmeri Species Group.

The Rhyacophila yosiiana Species Group (s. lato) was first recognized with five East Asian species within Branch 7 of Ross (1956). He showed two characteristics for Branch 7 [in terminology of Schmid (1970); hereafter in same manner]; specifically, the anal sclerites are absent in most species, and the U-shaped apical band is directly attached to the base of the phallic apparatus, so that a tergal band is absent. Moreover, he described four characteristics of the R. yosiiana Species Group: The complex of the apicodorsal lobe of segment IX and preanal appendages is an undivided dorsal structure, wide at the base, and narrow or bell-shaped at the apex; the phallic apparatus has a small base; the aedeagus is divided apically; and the parameres each bears a series of large spines in most species. Ross (1956) recognized three lines in the R. yosiiana Species Group, the R. yosiiana , R. tsudai , and R. ulmeri Lines, based on the following three characteristics: the presence or absence of anal sclerites, the length and shape of the phallic apparatus, and the shapes of the apicodorsal lobe of segment IX and the preanal appendages.

In his revisional study for Rhyacophilidae in Japan, Hattori (2005) distinguished the R. ulmeri Species Group from the R. yosiiana Species Group, mainly based on female and larval characteristics. He did not provide a definite diagnosis for the new species groups, but the following characteristics of the R. ulmeri Species Group are apparent from his figures and description in Japanese: The female (from his figures of a R. kuramana female) segment VIII is cone-shaped and 2 times as long as its basal width and the vaginal apparatus is long and log-like; the larval head (from his figures of five species and associated descriptions) is relatively short, with a length/width ratio of about 1.1–1.3, distinct dark color patterns and dots are present on the head and pronotum, and the posterolateral corners of the pronotum are strongly sclerotized and seem to be different sclerites from the pronotum itself.

In contrast, the diagnostic characteristics for the female and larva of the R. yosiiana Species Group seen in Hattori’s (2005) figures and descriptions are as follows: The female (from his figure of a R. yosiiana female) segment VIII is only as long as the basal width and the vaginal apparatus is semi-membranous, slender anteriorly, and gradually thickened posteriorly with a bilobed apical margin; the larval head (from his figure of a R. yosiiana larva) is long, with a length/width ratio of 1.8, and dark color patterns or dots are absent on the head and pronotum.

Hattori (2005) assigned numerous East Asian species to each of the two groups as follows: Three species in the R. yosiiana Species Group ( R. yosiiana Tsuda 1940 , R. kawaraboensis Kobayashi 1976 , and R. satoi Kuranishi 1997 ; all found in Japan; Taira & Nozaki 2021); 10 species in the R. ulmeri Species Group [ R. ulmeri Navás 1907 (found in China and Japan; Taira & Nozaki 2021), R. diffidens Tsuda 1940 , R. kuramana Tsuda 1942 , R. kohnoae Ross 1956 , R. tsudai Ross 1956 (synonymized with R. diffidens by Kuranishi 2016), R. minoyamaensis Kobayashi 1973 , R. shekigawana Kobayashi 1973 , R. tachikawana Kobayashi 1973 , R. nagaokaensis Kobayashi 1976 , and R. kiyosumiensis Kuranishi 1990 (all found on the main islands of Japan; Taira & Nozaki 2021)].

As discussed above, Hattori (2005) did not provide diagnoses for males of the two species groups. However, based on the male genitalia of known species and the new species in this article, I propose the following male diagnostic characteristics for the two groups. In the R. yosiiana Species Group, segment IX is short ventrally; the apicodorsal lobe of segment IX is large and almost as long as the basal segments of the inferior appendages; the phallic apparatus is small and does not exceed the basal 1/3 of the basal segments of the inferior appendages; the basal segments of the inferior appendages are thick, long, and oblique caudally; and the apical segment of each inferior appendage is thick and long, almost as long as the basal segment, more or less narrowing posteriorly. In the R. ulmeri Species Group, segment IX is short ventrally, the apicodorsal lobe of segment IX is roundish and covered with fine granules mesally in most species, the preanal appendages are closely joined to the apicodorsal lobe of segment IX, the apex of the phallic apparatus exceeds 1/2 of the lengths of the basal segments of the inferior appendages, parameres are present, serrated or each with a spine apically in some species, the basal segment of each inferior appendage is long and thick, and the apical segment is shorter than the basal segment and bifurcated apically in most species.

It is therefore apparent that the R. ulmeri Species Group designated by Hattori (2005) is a taxon that includes the R. tsudai and R. ulmeri Lines of Ross (1956), and Hattori’s (2005) R. yosiiana Species Group is the same as the R. yosiiana Line of Ross (1956). Therefore, according to the definitions based on the revision of Hattori (2005), I recognize that R. miyanoura sp. nov. and R. nagata sp. nov. belong to the R. yosiiana Species Group and the R. ulmeri Species Group, respectively.

Females of R. nagata have sclerotized intersegmental membranes invaginated in the posterior parts of segment VIII. This structure is also observed on the females of three species of the R. ulmeri Species Group, R. diffidens Tsuda 1940 , R. kuramana Tsuda 1942 , and an undescribed species (cf. R. kohnoae Ross 1956 ) (T. Nozaki, personal communication). This structure could be a peculiar characteristic for this species group, although females of other species are not yet known. A similar phenomenon was observed in females of several North American species (Prather et al. 2001).