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Hydroptila demersa Ito & Sasaki sp. nov. (Figs 3–5) Diagnosis. The male of this species resembles to that of H. ogasawaraensis described from Chichi-jima, Ogasawara Islands, in having a narrow transverse tergal bridge of segment IX, a subtriangular subgenital plate, and an almoststraight phallic apparatus with short titillator, but it is clearly distinguished from the latter by the form of the inferior appendages: Inferior appendages are without any processes in H. ogasawaraensis (Figs 9B, 9D), but each has a dorsolateral hump in this species (Figs 3E, 3G). The larva of this species is distinct from those of other congeneric species by the rather oblong and depressed form of the head capsule. Adult male (Fig. 3). Head: Postoccipital warts large, ellipsoidal, ocelli absent. Maxillary palps each 5- segmented, labial palps each 3-segemented. Mesoscutellum flabellate and without transverse suture. Metascutellum wide, subtriangular. Wings black to dark brown, some light brown patterning in forewings, but often indistinct in alcohol specimens; light brown dots absent in hind wings; forewings each 1.9–2.1 mm in length and hind wings each 1.7–1.9 mm (n = 5). Antennae each 27–29-segmented, length 1.0– 1.1 mm (n = 5); brown with three white bands at antero-middle, postero-middle and apices. Tibial spur formula 0, 2, 4. Pair of anterolateral processes of sternite V each with 2 apical setae, one long and one short. Short, subacute, posteroventral process on sternite VII. Male genitalia (Figs 3D–3H). Segment IX (IX) annular, dorsally with deep and wide circular excision at anterior margin and with shallow and wide excision at posterior margin, resulting transverse tergal bridge narrow, 1/4 as long on midline as segment in dorsal view; no dark and setose plate evident. Dorsal plate (dp) subquadrate in dorsal view with small posteromesal concavity, semi-membranous. Subgenital plate (sp) slightly sclerotized, gradually tapered, directed caudoventrad basally, then curved caudad at apical 1/4 with acute apex in lateral view, blunt in ventral view. Inferior appendages (ia) thick; directed caudoventrad at basal 2/3 and caudodorsad at apical 1/3; each with dorsolateral hump (dh) at middle, acute apically, covered with fine setae and spines. Phallic apparatus (pa) long, almost straight, extending from anterior 1/3 of segment VI when retracted, with short titillator at mid-length. Female. Unknown. Immature stages. Final instar larva (Fig. 4). Body somewhat compressed laterally, length up to 2.5 mm, sclerotized parts light brown, membranes milky white. Head (Figs 4A–4D). Depressed dorsoventrally, subrectangular with almost parallel sides in dorsal view, width up to 0.55 mm; width: length: height = about 1.00: 1.85: 0.70. Dorsal ecdysial lines indistinct, longest setae (setae no. 9, sensu Wiggins 1996) about twice as long as width of head, seta no. 8 (sensu Wiggins 1996) also long, about 1.5 times as long as width of head. Antennae situated near anterolateral corners of head capsule, each unsegmented with short sensilla apically and seta subapically. Small spines (sensu Wells 1985, 1997) absent on dorsum. Thorax (Figs 4A, 4B, 4E). Dorsum of each segment covered with pair of large square sclerotized plates; these plates light brown, with slightly darkened posterior margins; each plate with about 20 setae on pronotum, about 15 setae on each plate of meso- and metanota. Thoracic legs essentially similar in structure and subequal in three legs; foretibiae each with spur having two spines, spines with few fine teeth, spatulate spines indistinct. Foretrochantins and pleura of meso- and metathoraces subrectangular, pleural sutures indistinct. Abdomen (Figs 4A, 4F, 4G). Somewhat compressed laterally, slightly taller at segments III–VI. Tracheal gills, humps, lateral fringes, and lateral tubercles absent. Small, transversely oval chloride epithelia dorsally present on segments I–VI. Paired long sa 1 setae, two short sa 2 setae, and single long sa 3 seta directed laterad on each of segments I –VIII. Tergite IX transversely ellipsoidal with two pairs of long setae and two pairs of short setae. Lateral sclerites of segment X round with one pair of long and two pairs of short setae. Anal claws directed anterolaterad, each with two short and wide accessory hooks. Three anal gills slender, one on middle of dorsum of segment IX behind tergite, pair on posterolateral margins of segment X. Case (Figs 5A, 5B). Case of final instar larva up to 2.9 mm, composed of elongate-ellipsoidal right and left valves, very slightly compressed laterally, made of mineral particles with rather rough surface, never of organic materials. Holotype. Male, Japan, Ogasawara Islands, Chichi-jima: Ishiura-kita River, upper reach (N27.0756, E142.2221, 230 m above sea level), 26.ii.2020, TI, TS & CT (TI et al.), S (CBM-ZI 0184755). Paratypes. 2 males, same data as holotype (CBM-ZI 0184756–0184757). Other specimens. Chichi-jima: 2 males, 2 pupae, 2 larvae, same data as holotype; 1 male, 4 pupae, 4 larvae, type locality, 12–13.iv.2019, TI et al., S & H; 1 larva, Hatsuneura-gawa, 7.iv.2009, G. Yoshinari; 1 pupa, 1 prepupa, same locality, 15.iv.2019, TI et al., H; 2 males, 2 pupae, Fukiage-dani, 14.iv.2019, TI & TS, S & H; 3 males, 1 pupa, 2 larvae, same locality, 2.iii.2020, TI & CT, S & H. Ani-jima: 10 males, Takinoura-gawa, 3.iii.2020, TI et al., S. Remarks and habitat (Figs 5C, 5D). The larva of this species is very distinct in having a rather depressed, oblong head with rather long setae (no. 8) ventrally. As far as we know, such a head has not been known in any congeneric species in North America (Ross 1944; Wiggins 1996), Australia (Wells 1985, 1997), Europe (Wallace et al. 2003; Waringer & Graf 2011), Angola (Wells & de Moor 2020), or Japan (Ito 2021). In caddisflies, the depressed form of the larval head is known for four rhyacophilid species that burrow into sandy stream beds (Taira & Tanida 2011, 2013) and in a philopotamid species inhabiting the hyporheic zone (Torii et al. 2022). Therefore, depressed larval heads, and probably also long setae on the ventral side of the head capsule of H. demersa might be an adaptive form for their microhabitat, the underside or interstices of stony or sandy substrates in shallow water flowing in narrow, steep, mountain streams, up to 5 cm depth and 100 cm width. Absence of small spines on the dorsum of the head is also exceptional, since they are numerous and scattered in many species of Hydroptila larvae (Wells 1985; Ito 2021). The cases of this species, very slightly compressed with rough surfaces, are different from those of other congeneric species (Ross 1944; Wells 1985; Wiggins 1996; Wallace et al. 2003; Waringer & Graf 2011; Ito 2021). This species was sometimes collected with H. ishiura sp. nov. in the same streams, but in fewer numbers than the latter. Distribution (Figs 1B, 1C). Japan (Ogasawara Islands: Chichi-jima, Ani-jima). Etymology. The species epithet (a Latin adjective) refers to the depressed form of the larval head. Japanese name. Muguri-hime-tobikera. |
