Osmylus (Plesiosmylus) tessellatus McLachlan, 1875

(Figs 17, 22–23, 26, 29, 34–35, 38, 43–44)

Osmylus tessellatus McLachlan, 1875, 180.

Osmylus (Plesiosmylus) tessellatus: Hayashi, 2005a, 389. Other synonyms see Sekimoto & Yoshizawa (2011).

Description of 3rd instar larva. BL 10.7–15.6 (13.6, n = 20) mm.

Coloration. Head blackish brown except for black stemmata and its surrounding parts; legs right yellow, except for brown upper half of coxae; dorsal side of thorax and abdomen basically blackish gray; peripheries of spiracles pale yellow; anterior and posterior margins of spiracles, anterior and posterior margins of each segment brown to dark yellow; midline in the part from prothorax to 6th abdominal segments right brown to brown; ventral side of prothorax whitish yellow; other thoracic and abdominal segments blackish purple, with reticulation whitish yellow; sclerites blackish brown; 9th and 10th segments and their sclerites whitish brown, subhyaline.

Head (Fig. 17). Mandiblomaxillary stylets rather short, about 2.0 times as long as head capsule length; ML/ CW 2.1–2.4 (2.2, n = 20). Frontal edge of cranium with 4 long setae and 2 pores. Antennae 32 or 33 segmented (n = 2).

Prothorax (Fig. 22): DPc subtrapezoidal, gently tapering anteriorly, fused with DM, its setae in middle part long; 1st and 3rd setae from within in posterior half margin long. Mesothorax (Figs 22–23): DPm1 present; two short seta bearing on DPm2; DPm2 semicircular, its seta on anterior margin long; a seta on DPp1and DPp2 long. Metathorax (Figs 22–23): DAa not expanded; DAm2 and DAm3 fused.

First abdominal segment (Fig. 26). DM1 absent; DM2, DPp1, and DPp4 roughly the size of DPp2; a seta on DM2, DPp1, and DPp4 long; DPa present; a seta in dorso-anterior part of LP long. Second to 5th abdominal segment (Fig. 26): DM3 absent; a seta on LA long. Sixth abdominal segment (Fig. 26):VM3 present. Seventh abdominal segment (Fig. 29): VM3 present; VP1 mostly present or only one side absent. Eighth abdominal segment (Fig. 26): DPp1+DPp2 with 2 pores on anterior margin. Ninth abdominal segment (Figs 34–35): Dc bearing 7 pairs of setae in posterior half margin; outer setal row on VPc composed of 2 setae and a pore in exterior margin.

Material examined. 16 exs. (8 2IL & 8 3IL), Shimashima-dani, Matsumoto-shi, Nagano Pref., 18.IX.2011, M. Hayashi & H. Yoshitomi leg. ; 7 exs. (4 1IL & 3 3IL), Kuriyama-zawa, Yunishi-gawa, Nikkô-shi, Tochigi Pref., 1.vii.2009, H. Kamezawa leg. 5 exs. (1 2IL & 4 3IL), Take-zawa, Yunishi-gawa, Nikkô-shi, Tochigi Pref., 7.ix.2009, H. Kamezawa leg. ; 6 exs. (1 1IL, 3 2IL & 2 3IL), Ôta-gawa, Nakautsunashi, Utsunashi, Togouchi, Akiôta-chô, Hiroshima Pref., 23.x.2010, M. Sakamoto leg. ; 7 exs. (1 1IL, 1 2IL & 5 3IL), Ôta-gawa, Tateno campsite, Yoshiwa, Akiôta-chô, Hiroshima Pref., 30.x.2010, M. Sakamoto leg. ; 6 exs. (3 2IL & 3 3IL), Koitagawa, Mochinoki, Akiôta-chô, Hiroshima Pref., 3.xi.2010, M. Sakamoto leg. ; 1 ex. (2IL), Takiyama-gawa, Ôhirabashi, Kake, Akiôta-chô, Hiroshima Pref., 11.xi.2010, M. Sakamoto leg. ; 2 exs. (2IL), Tsutsuga-gawa, Sakabara, Kamitsutsuga, Togouchi, Akiôta-chô, Hiroshima Pref., 6.xii.2010, M. Sakamoto leg. ; 6 exs. (1 2IL & 5 3IL), Tsutsuga-gawa, Takahashi, Umagoshi, Kamitsutsuga, Togouchi, Akiôta-chô, Hiroshima Pref., 22.xii.2010, M. Sakamoto leg. ; 1 ex. (3IL), Youro-gawa, Todani, Toyohira, Kitahiroshima-chô, Hiroshima Pref., 17.x.2010, M. Sakamoto leg. ; 1 ex. (2IL), same locality, 23.x.2010, M. Sakamoto leg.; 6 exs. (5 2IL & 1 3IL), same locality, 25.x.2010, M. Sakamoto leg.; 1 ex. (3IL), Ôgure-gawa, Shinzan, Ôgure, Geihoku, Kitahiroshima-chô, Hiroshima Pref., 3.xi.2010, M. Sakamoto leg. ; 3 exs. (1 2IL & 2 3IL), Takayama-gawa, Ugakyô, Asakita-ku, Hiroshima Pref., 24.xi.2010, M. Sakamoto leg. ; 1 ex. Omogokei, Kumakôgen-chô, Ehime Pref., 9.v.2009 (collected in 3IL), 17.vi.2009 (PP), 26.vi.2009 (EM), S. Matsuno leg. ; 2 exs. (3IL), same locality, 29.v.2010, S. Matsuno leg.; 3 exs. (3IL), same locality, 11.vi.2010, Y. Minoshima leg.; 1 ex. (1IL), same locality, 14.ix.2010, S. Matsuno leg.; 1 ex. (3IL), same locality and collector, 13.xi.2011.

Remarks. This is a distinct species in having pale yellow spiracles, and easily distinguishable from the other Japanese species (Fig. 38). The conformation of the sclerite of this species is more similar to that of O. (O.) hyalinatus than to that of O. (O.) pryeri . This species also resembles O. fulvicephalus in the absence of DM1 and DM3, and the presence of four long setae in the frontal edge of the cranium (fig. 94 in Rousseau 1921; fig. 28 in Wundt 1961). VP1 of 6th abdominal segment in this species is absent on only one side in 15% of the specimens, that of on the 7th abdominal segment is present on only one side in 90% of the specimens, and is absent in 10% (n = 20).

Biological notes. This species occurs mainly in moss in superaqueous locations and the downstream side of rocks in mountainous streams (Figs 43–44), and sometimes hides in the cracks of rocks. The pupal period is 9 days in a rearing condition under 22? (n = 1).

Distribution. Japan (Hokkaidô, Kunashir, Honshû, Shikoku, Kyûshû).