Arcobaetis sripadai sp. nov., by present designation.

Species included in Arcobaetis gen. nov.

1. Arcobaetis sumbawensis sp. nov.

2. Arcobaetis sumatrensis sp. nov.

3. Arcobaetis bornensis sp. nov.

4. Arcobaetis sripadai sp. nov.

5. Arcobaetis gracilentus (Chang & Yang, in Kang et al. 1994), comb. nov.

Larva. The following combination of characters differentiate Arcobaetis gen. nov. from all other genera of Baetidae: A) frons with carina-like elevation, slightly overlapping antennal base (Fig. 4e); B) maxillary palp with two segments (Fig. 2g); C) both mandibles with outermost denticle in ventral position, set apart from other denticles; both mandibles with long, slightly feathered setae between prostheca and mola (Fig. 7b, d); D) paraglossae dorsally with an arc of long, spine-like, simple setae in distal area (Fig. 2h, j); E) labial palp segment II with poorly developed distomedial protuberance (Fig. 2h); F) legs very slender; femur length 4–6× maximum width; outer margin of femora with row of short, spine-like setae (Fig. 4a–c); G) claw with single row of denticles, distal denticles larger and directed distad, proximal denticles minute (Figs 3b, 5a, b); H) surface of abdominal terga with fine, longitudinally striated scales situated in angulate nests, whose angles bear opercula (Fig. 3c, d); H) folding of subimaginal gonostyli developing under cuticle of last instar male larva of the “Nigrobaetis-type”.

Larva. Head (Fig. 4e). Frons with carina-like elevation, slightly overlapping antennal base.

Labrum (Fig. 2a). Sub-rectangular, wider than long. Distal margin with medial, shallow emargination and small process. Dorsally with pair of long, simple, sub-median setae, and on each side partial sub-marginal arc of long, stout, simple setae; sometimes with setae between sub-median seta and partial sub-marginal arc; surface scattered with medium to long, simple setae. Ventrally with long long-feathered setae on anterolateral margin and medium short-feathered setae on medial margin; several small, stout setae near anterolateral and sometimes also midlateral margin.

Right mandible (Fig. 2b, c). Incisor and kinetodontium fused; incisor with denticles, outer denticle turned ventrally and much lower than other denticles; kinetodontium with denticles; inner margin of innermost denticle of kinetodontium with row of thin setae; prostheca stick-like, distolaterally denticulate; margin between prostheca and mola straight, with long, slightly feathered setae; apex of mola with tuft of setae.

Left mandible (Fig. 2d, e). Incisor and kinetodontium fused; incisor with denticles, outer denticle turned ventrally and set apart from other denticles; kinetodontium with denticles; prostheca robust, distally denticulate; margin between prostheca and mola straight, with row of long, slightly feathered setae; apex of mola without tuft of setae.

Both mandibles with outer lateral margins almost straight.

Hypopharynx and superlinguae (Fig. 2f). Apex of lingua with poorly developed tuft of setae. Distolateral margin of superlinguae with fine setae.

Maxilla (Fig. 2g). Apically with three slender, pointed canines and three denti-setae; distal denti-seta tooth-like, bent in the same direction as canines; other denti-setae slender, bifid, and pectinate; maxillary palp with two segments, apex rounded.

Labium (Fig. 2h). Glossae basally broad, narrowing towards apex, shorter than paraglossae; inner margin with row of spine-like setae, increasing in length distally; apex with several medium to long, robust setae; outer margin with row of spine-like setae; ventral surface with numerous medium to long fine, simple scattered setae. Paraglossae laterally straight, distally slightly bent inwards; apex with three rows of long, robust, distally pectinate setae; ventrally usually with several short, simple setae in distomedial area; dorsally with distal arc of long, spine-like setae. Labial palp with three segments, segment II with slightly developed distomedial protuberance.

Thorax. Legs (Figs 3a, b, 4a–c). Long and very slender. Femur. Length approx. 4–6× maximum width; outer margin with row of curved, spine-like setae; apex rounded, with pair of curved, spine-like setae; ventrally with stout, pointed, pectinate setae along margin; femoral patch absent. Tibia. Outer margin of fore legs almost bare, with one curved, spine-like seta at apex; middle and hind legs additionally with row of few spine-like setae. Tarsus. Dorsal margin almost bare. Claw. Slender, with long point, and with one row of denticles. Distal denticles larger and directed distad, proximal denticles are minute.

Abdomen. Terga (Fig. 3c, d). Surface with irregular rows of scale bases and with fine, longitudinally striated scales situated in angulate nests, whose angles bear opercula (see also Kluge 2012: 365, fig. 6).

Tergalii. Present on segments I–VII or II–VII.

Paraproct (Fig. 3f). Posterior margin with stout spines. Cercotractor with numerous, small, marginal spines.

Larval protogonostyli (Fig. 4d). Subimaginal gonostyli developing under cuticle of last instar male larvae folded in “Nigrobaetis-type”: all segments (1^st, 2^nd, and 3^rd) directed caudally and compressed in longitudinal direction, base of 2^nd segment deeply inserted into 1^st segment, and 3^rd segment inserted into 2^nd segment (as in Novikova and Kluge 1994: fig. 2: 1, 5, 20).

Imago. Following combination of characters differentiate Arcobaetis gen. nov. from other genera of Baetidae A) forewing with double intercalary veins (Fig. 17a); B) gonostylus segment III extraordinary small, much narrower than apex of 2^nd segment (Fig. 18a); C) gonovectes sharply bent (Fig. 18a).

Male Imago. See description of male imago under A. sripadai sp. nov. below.

Arcobaetis is a combination of Arco-, in reference to the Latin word arcus for arc and the arc of long, simple setae dorsodistally on paraglossae, and baetis, to highlight the similarities with the genus Baetis. The gender is masculine.

(Fig. 19). Indonesia (Sumatra, Sumbawa), Brunei, Sri Lanka, Taiwan.

Holotype. Indonesia • male larva (last instar); Sumbawa, Batu Dulang, Mt. Batu Pasak, forest streams; 08°37'42"S, 117°15'27"E, SUMB09; 1380 m; 17.ix.2011; leg. M. Balke; on slide; GBIFCH00975680; MZB. Paratypes. 9 larvae; same data as holotype; 4 on slides; GBIFCH00692615, GBIFCH00975688, GBIFCH00592652, GBIFCH00592653; 5 in alcohol; GenBank OQ699910, OQ699911; GBIFCH00975695, GBIFCH00975689, GBIFCH00975690, GBIFCH00975694; MZL.

Larva (Table 1). The following combination of characters distinguish A. sumbawensis sp. nov. from other species of Arcobaetis gen. nov.: A) distal margins of segments in middle part of flagellum without enlarged spines; B) labial palp segment III sub-rectangular, at base approx. as wide as distal margin of segment II (Fig. 2h); C) hind protoptera absent; D) claw with single row of denticles, four or five distalmost denticles larger, ca. ten basal denticles small to minute (Figs 3b, 5a); E) tergalii present on abdominal segments I–VII; F) posterior margin of tergum IV with triangular spines, slightly wider than long (Fig. 3c).

Larva (Figs 1–5). Body length 5.7–7.0 mm.

Cuticular colouration (Fig. 1a–c). Head, thorax, and abdomen dorsally brown, thorax darker. Head, thorax, and abdomen ventrally ochre. Legs ochre, femur distally with brown streak; tibia basally along patella-tibial suture darker. Caudalii ochre.

Hypodermal colouration. Abdomen dorsally with narrow reddish transverse stripes on intersegmental membranes (Fig. 1a).

Antenna. Flagellum in middle part without enlarged spines at distal margin of segments.

Labrum (Fig. 2a). Length 0.7× maximum width. Dorsally with sub-median seta, short sub-marginal arc of simple setae, and two setae in between.

Right mandible (Fig. 2b, c). Incisor with five denticles; kinetodontium with three denticles.

Left mandible (Fig. 2d, e). Incisor with five denticles; kinetodontium with three denticles.

Hypopharynx and superlinguae (Fig. 2f). Lingua as long as superlinguae, longer than broad. Superlinguae distally straight; lateral margins rounded; fine, long, simple setae along distal margin.

Maxilla (Fig. 2g). Galea-lacinia ventrally with two simple setae just proximad of canines. Medially with one pectinate, spine-like seta and ca. six short to long, simple setae. Maxillary palp approx. 1.5× as long as galea-lacinia; palp segment II approx. 1.3× as long as segment I; setae on maxillary palp fine, simple, scattered over surface of segments I and II.

Labium (Fig. 2h–j). Inner margin of glossa with ca. 16 spine-like setae, increasing in length distally; apex with two long and one medium robust setae; outer margin with ca. 12 spine-like setae; paraglossa ventrally with ca. four medium, simple setae in anteromedial area; dorsally with arc of ca. 10 long, spine-like setae in distal area. Labial palp with segment I 0.7× length of segments II and III combined. Segment II dorsally with row of ca. four spine-like setae. Segment III sub-rectangular; at base approx. as wide as distal margin of segment II; ventral surface with short, spine-like, simple setae and short, fine, simple setae.

Hind protoptera absent.

Legs (Figs 3a, b, 4a–c, 5a–c). Long and slender, middle, and hind legs slenderer than fore legs. Ratio of leg segments: fore leg 1.3:1.0:0.7:0.2, middle and hind legs 1.2:1.0:0.6:0.1. Femur. Length of fore femur ca. 5× maximum width, outer and inner margins almost parallel; length of middle and hind femora >6× maximum width, outer margins slightly concave. Outer margin with row of ca. 12 short, curved, spine-like setae, on fore leg larger than on middle and hind legs. On ventral side with short, spine-like, pectinate, pointed setae; larger and denser on foreleg, smaller and less dense on middle and hind legs. Tibia. Short, stout, pointed, pectinate setae near inner margin. Inner margin with row of short, curved, spine-like, pectinate setae, on apex two longer, curved, spine-like, pectinate setae. Outer margin with one apical, spine-like seta; on fore leg without other spine-like setae, on middle and hind legs with row of few medium, spine-like setae. Patella-tibial suture on proximal ¹/3 of tibia on all legs. Tarsus. Inner margin with row of short, curved, pectinate, spine-like setae; outer margin bare. Claw with one row of denticles; four or five distalmost denticles larger and directed distad; basally ca. 10 small to minute denticles; two vestigial subapical setae on anterior side, one vestigial subapical seta on posterior side (Fig. 5a, b).

Abdominal terga (Fig. 3c, d). Posterior margin of terga: I smooth, without spines; II–IX with triangular spines, increasing in length toward IX; slightly wider than long on tergum IV; row of spines on tergum IX interrupted by smaller spines in middle part behind bases of submedian setae (similar to Fig. 15j). Posterior margin of tergum X with median concavity with smaller spines (similar to Fig. 15k).

Abdominal sterna. Posterior margins of sterna: I–V smooth, without spines; VI–IX with triangular spines. On sternum IX of male mature larva row of narrow pointed spines between protogonostyli, smaller spines laterad of protogono­styli and larger, pointed spines laterad of them (similar to Fig. 15m).

Tergalii (Fig. 3e). Present on abdominal segments I–VII. Tracheation in tergalius I limited to basal part of main trunk. In nine well preserved larvae, only a few tergalii were found, but insertions were always well developed: two tergalii I in one specimen; one tergalius IV and one tergalius V in another specimen, both very small, looking like replacements of naturally broken tergalii.

Paraproct (Fig. 3f). Posterior margin with ca. 16 stout spines. Surface scattered with scale bases and micropores.

Caudalii. Cerci and paracercus with broad triangular spines on posterior margin of each segment.

The specific epithet refers to the type locality in Sumbawa (Indonesia).

Indonesia: Sumbawa (Fig. 19).

The species was found at an altitude of 1380 m.

Holotype. Indonesia • female larva (premature); Sumatra Barat, Harau Canyon, stream near Ikbal’s cottage, UN11; 00°06'26"S, 100°40'22"E; 520 m; 23.vi.2012; leg. M. Balke; on slide; GBIFCH00592617; MZB. Paratypes. 1 larva; same data as holotype; on slide; GBIFCH00975679; MZL • 1 larva; Sumatra Barat, Bukit Barisan, above Padang, creek, UN3; 00°56'44"S, 100°32'44"E; 1047 m; 8.xi.2011; leg. M. Balke; on slide; GBIFCH00975704; MZL.

Larva (Table 1). The following combination of characters distinguish A. sumatrensis sp. nov. from other species of Arcobaetis gen. nov.: A) distal margins of segments in middle part of flagellum without enlarged spines; B) labial palp segment III sub-rectangular, at base smaller than distal margin of segment II (Fig. 7h); C) hind protoptera absent; D) claw with single row of denticles, ca. six distalmost denticles larger, other denticles minute (Fig. 8b); E) tergalii present on abdominal segments II–VII (Fig. 8c–h; F) posterior margin of tergum IV with triangular spines, wider than long (Fig. 7i).

Larva (Figs 6–9). Body length ca. 4.6 mm.

Cuticular colouration (Fig. 6a–c). Head, thorax, and abdomen dorsally brown, ventrally ochre. Legs ochre, joining of femur and tibia dark brown; middle part of tibia pale brown. Caudalii pale brown.

Hypodermal colouration. Abdomen dorsally with narrow reddish transverse stripes on intersegmental membranes (Fig. 6a).

Antenna. Flagellum in middle part without enlarged spines at distal margin of segments.

Labrum (Fig. 7a). Length 0.7× maximum width. Dorsally with sub-median seta and sub-marginal arc of two simple setae.

Right mandible (Fig. 7b, c). Incisor with five denticles; kinetodontium with three denticles.

Left mandible (Fig. 7d, e). Incisor with four denticles; kinetodontium with three denticles.

Hypopharynx and superlinguae (Fig. 7f). Lingua as long as superlinguae, longer than broad. Superlinguae distally straight; lateral margins rounded; fine, long, simple setae along distal margin.

Maxilla (Fig. 7g). Galea-lacinia ventrally with two simple setae just proximad of canines. Medially with one pectinate, spine-like seta and ca. five short to long, simple setae. Maxillary palp approx. 1.5× as long as galea-lacinia; palp segment II approx. 1.2× as long as segment I; setae on maxillary palp fine, simple, scattered over surface of segments I and II.

Labium (Fig. 7h–j). Inner margin of glossa with ca. ten spine-like setae; apex with three long and one medium robust setae; outer margin with ca. nine spine-like setae; paraglossa ventrally with ca. two medium, simple setae in anteromedial area; dorsally with arc of ca. ten long, spine-like setae in distal area. Labial palp with segment I 0.8× length of segments II and III combined. Segment II dorsally with row of ca. four spine-like setae. Segment III sub-rectangular; at base narrower than distal margin of segment II; ventral surface with short, spine-like, simple setae and short, fine, simple setae.

Hind protoptera absent.

Legs (Figs 8a, b, 9a–c). Long and slender, middle, and hind legs slenderer than fore leg. Ratio of leg segments: fore leg 1.4:1.0:0.8:0.2, middle legs 1.3:1.0:0.6:0.2 and hind legs 1.2:1.0:0.6:0.1. Femur. Length of fore femur ca. 4.4× maximum width, outer and inner margins almost parallel; length of middle and hind femora > 5× maximum width, outer margins slightly concave. Outer margin with row of ca. seven short, curved, spine-like setae, on fore leg larger than on middle and hind legs. On ventral side of fore leg with short, spine-like, pectinate, pointed setae; nearly absent on middle and hind legs. Tibia. Short, stout, pointed, pectinate setae irregularly near inner margin. Inner margin with row of short, curved, spine-like, pectinate setae, on apex two longer, curved, spine-like, pectinate setae. Outer margin with one apical, spine-like seta; on fore leg without other spine-like setae, on middle and hind legs with few medium, spine-like setae. Patella-tibial suture on proximal ¹/4 of tibia on all legs. Tarsus. Inner margin with row of short, curved, pectinate, spine-like setae; outer margin without spine-like setae. Claw with single row of denticles, ca. six distalmost denticles larger and directed distad, other denticles minute.

Abdominal terga (Fig. 8i, j). Posterior margin of terga: I smooth, without spines; II–IV with short triangular spines, wider than long; V with triangular spines approx. as wide as long; VI–IX with triangular spines longer than wide, sharply pointed; row of spines on tergum IX interrupted by smaller spines in middle part behind bases of submedian setae (similar to Fig. 15j).

Abdominal sterna. Posterior margins of sterna: I–V smooth, without spines; VI–IX with triangular spines.

Tergalii (Figs 8c–h, 9d). Present on abdominal segments II–VII. Tracheation partially extending toward inner and outer margins. Margins with minute denticles intercalating short, simple setae.

Paraproct (Fig. 8k). Posterior margin with ca. 13 stout spines. Surface scattered with scale bases and micropores.

Caudalii (Fig. 9f). Cerci and paracercus with small, rather wide, triangular, pointed spines on posterior margin of each segment; on outer side of cerci two somewhat longer spines on each 2^nd segment, no such longer spines on paracercus.

The specific epithet refers to the type locality in Sumatra (Indonesia).

Indonesia: Sumatra (Fig. 12b).

The species was found at altitudes of 520 m and 1050 m.

Holotype. Brunei • female larva (premature); Temburong District, Ulu Temburong National Park, near Kuala Belalong Field Studies Centre (KBFSC); 04°32'55"N, 115°09'27"E; 103 m; May 2014; leg. Kate Baker; on slide; GBIFCH00465126; MZL. Paratypes. 11 larvae; same data as holotype; 5 on slides; GBIFCH00465127, GBIFCH00465128, GBIFCH00465129, GBIFCH00592613, GBIFCH00592645; MZL; 6 in alcohol; GBIFCH00515212, GBIFCH00975703; MZL.

Larva (Table 1). The following combination of characters distinguish A. bornensis sp. nov. from other species of Arcobaetis gen. nov.: A) distal margins of segments in middle part of flagellum with enlarged spines (Fig. 11f); B) labial palp segment III sub-quadrangular, at base narrower than distal margin of segment II (Fig. 10h); C) hind protoptera absent; D) claw with single row of denticles, distalmost denticle larger, other denticles minute (Fig. 11b); E) tergalii present on abdominal segments I–VII; F) posterior margin of tergum IV with triangular spines, wider than long (Fig. 11c).

Larva (Figs 10–12). Body length ca. 3.0 mm.

Colouration . Head, thorax, and abdomen dorsally brown, ventrally ochre. Legs ecru. Caudalii pale brown.

Hypodermal colouration. Unknown.

Antenna. Flagellum in middle part with enlarged spines at distal margin of segments (Fig. 11f).

Labrum (Fig. 10a). Length 0.7× maximum width. Dorsally with sub-median seta, sub-marginal arc of ca. three simple setae, and several setae in between.

Right mandible (Fig. 10b, c). Incisor with five denticles; kinetodontium with three denticles.

Left mandible (Fig. 10d, e). Incisor with four denticles; kinetodontium with three denticles.

Hypopharynx and superlinguae (Fig. 10f). Lingua slightly shorter than superlinguae, longer than broad. Superlinguae distally straight; lateral margins rounded; fine, long, simple setae along distal margin.

Maxilla (Fig. 10g). Galea-lacinia ventrally with two simple setae just proximad of canines. Medially with one pectinate, spine-like seta and ca. four medium to long, simple setae. Maxillary palp approx. 1.4× as long as galea-lacinia; palp segment II approx. 1.2× as long as segment I; setae on maxillary palp fine, simple, scattered over surface of segments I and II.

Labium (Fig. 10h–j). Inner margin of glossa with ca. nine spine-like setae; apex with three long robust setae; outer margin with ca. nine spine-like setae; paraglossa ventrally with ca. four medium, simple setae in anteromedial area; dorsally with arc of ca. seven long, spine-like setae in distal area. Labial palp with segment I 0.8× length of segments II and III combined. Segment II dorsally with row of ca. four spine-like setae. Segment III sub-quadrangular; at base narrower than distal margin of segment II; ventral surface with short, spine-like, simple setae and short, fine, simple setae.

Hind protoptera absent.

Legs (Figs 11a, b, 12a, b). Long and slender, hind legs slenderer than fore leg, middle legs unknown. Ratio of leg segments: fore leg 1.5:1.0:0.9:0.3, hind leg 1.4:1.0:0.7:0.2. Femur. Length of fore femur ca. 3.6× maximum width, outer and inner margins almost parallel; length of hind femur ca. 5× maximum width, outer margin slightly concave. Outer margin with row of ca. 7 short, curved, spine-like setae, on fore leg larger than on hind leg. On ventral side of fore leg with short, spine-like, pectinate, pointed setae; nearly absent on hind leg. Tibia. Few short, stout, pointed, pectinate setae near inner margin. Inner margin with row of short, curved, spine-like, pectinate setae, on apex two longer, curved, spine-like, pectinate setae. Outer margin with one apical, spine-like seta; on fore leg without other spine-like setae, on hind leg with few medium, spine-like setae. Patella-tibial suture on foreleg on proximal ¹/2 (difficult to see), on hind leg on proximal ¹/3. Tarsus. Inner margin with row of short, curved, pectinate, spine-like setae; outer margin without spine-like setae. Claw with one row of denticles, distalmost denticle larger and directed distad, other denticles minute.

Abdominal terga (Fig. 11c). Posterior margin of terga: I and II smooth, without spines; III–IX with triangular spines, slightly increasing in length toward IX; wider than long on tergum IV; row of spines on tergum IX interrupted by smaller spines in middle part behind bases of submedian setae (similar to Fig. 15j). Posterior margin of tergum X with median concavity with smaller spines (similar to Fig. 15k).

Abdominal sternites. Posterior margins of sterna: I–V smooth, without spines; VI–IX with triangular spines.

Tergalii (Fig. 11d). Present on abdominal segments I–VII. Tracheation limited to main trunk. Margin with minute denticles intercalating short, simple setae.

Paraproct (Fig. 11e). Posterior margin with ca. 13 stout spines. Surface scattered with scale bases and micropores.

Caudalii. Spines of cerci and paracercus unknown.

The specific epithet refers to the type locality in Borneo (Brunei).

Brunei (Fig. 19).

(Fig. 12c). The species was found at an altitude of 1380 m in slow current of a small and shallow stream.

Holotype. ♂ imago reared from larva, with its larval and subimaginal exuviae {specimen [XVIII] (5)2020}, Sri Lanka, foot of Sri Pada (Adam’s Peak), Delhausie, river Seetha gangula; 6°50′3.48″N, 80°34′3.36″E; 7.II.2020; leg. N. Kluge & L. Sheyko; SPbU.

Larva (Table 1). The following combination of characters distinguish A. sripadai sp. nov. from other species of Arcobaetis gen. nov.: A) distal margins of segments in middle part of flagellum without enlarged spines (Fig. 14a); B) labial palp segment III sub-quadrangular, at base approx. as wide as distal margin of segment II (Fig. 14i); C) hind protoptera absent; D) claw with single row of denticles, most distalmost denticles larger, basal denticles small to minute (Fig. 15c); E) tergalii present on abdominal segments II–VII; F) posterior margin of tergum IV with triangular spines, slightly wider than long (Fig. 15f).

Larva (Figs 13–18).

Cuticular colouration (Fig. 13a–g). Head, thoracic terga and pleura with diffusive brown, ochre, and colourless areas (Fig. 13d); thoracic sterna colourless. Legs pale ochre with brown apex of femur, base of tibia and band in distal part of tibia (Fig. 13a–c). Abdominal terga I–VII with nearly uniform colour pattern consisting of diffusive ochre and pale brown areas (Fig. 13e–f); tergum VIII nearly uniformly darker brown, with paler ochre anterior margin and anterolateral angles (Fig. 13f); terga IX–X uniformly pale ochre (Fig. 13g); all sterna I–IX and paraprocts pale ochre. Caudalii ochre.

Hypodermal colouration. Unknown.

Antenna (Fig. 14a). Spines at distal margins of flagellum segments not enlarged.

Labrum (Fig. 14b). Length 0.7× maximum width. Dorsally with submedian setae and few long setae, not forming distinct submarginal arcs.

Right mandible (Fig. 14c). Number of denticles of incisor unclear (worn); kinetodontium with three denticles.

Left mandible (Fig. 14d). Number of denticles of incisor unclear (worn); kinetodontium with three denticles.

Hypopharynx and superlinguae (Fig. 14e). Lingua shorter than superlinguae, longer than broad. Superlinguae distally almost straight; lateral margins rounded; fine, long, simple setae along distal margin.

Maxilla (Fig. 14f). Galea-lacinia ventrally with two simple setae just proximad of canines. Medially with one spine-like seta and four simple setae. Maxillary palp approx. 1.1× as long as galea-lacinia; palp segment II slightly shorter than segment I.

Labium (Fig. 14g, h). Inner margin of glossa with 14 or 15 spine-like setae, increasing in length distally; apex with three long and robust setae; outer margin with ca. ten spine-like setae; paraglossa ventrally with ca. four medium, simple setae in anteromedial area; dorsally with arc of ca. ten long, spine-like setae in distal area. Labial palp with segment I 0.8× length of segments II and III combined. Labial palp segment II dorsally with row of three or four spine-like setae. Segment III sub-quadrangular; at base approx. as wide as distal margin of segment II; ventral surface with short, spine-like, simple setae and short, fine, simple setae.

Hind protoptera absent.

Legs (Figs 15a–c). Long and slender; middle and hind legs slenderer than fore legs; hind leg slightly longer than others; on fore leg, tibia and tarsus of subequal length, with patella-tibial suture on proximal ¹/2 of tibia; on middle and hind legs, tibia much longer than tarsus, with patella-tibial suture on proximal ¹/3 of tibia. Segments ratio of fore leg 1.4:1.0:1.0:0.2, middle leg 1.3:1.0:0.7:0.2, hind leg 1.2:1.0:0.7:0.2. Femur. Length 4.6× maximum width. Outer margin with row of 5–7 short, spine-like setae, larger on fore leg and smaller on middle and hind legs; two apical setae larger, equal on all legs (Fig. 15a). Ventral side of femur with smaller pointed spine-like setae, larger and denser on fore legs, smaller and fewer on middle and hind legs. Tibia. Pointed, feathered, spine-like setae irregularly located on inner side of fore tibia (Fig. 15a) and on all sides of middle and hind tibiae (Fig. 15b); one preapical seta on outer side of each tibia. Tarsus. Ventral margin with row of short, curved, feathered, spine-like setae. Claw with one row of ca. ten denticles increasing from basal to distal ones and directed distally (Fig. 15c).

Abdominal terga (Fig. 15d–k). Posterior margin of terga: I smooth, without spines; II–X with triangular spines, increasing in length toward X; slightly longer than wide on tergum IV; row of spines on tergum IX interrupted behind bases of submedian setae (Fig. 15j). Posterior margin of tergum X with median concavity with smaller spines in it (Fig. 15k).

Abdominal sterna (Fig. 15l–m). Posterior margins of sterna: I–V smooth, without spines; VI–VIII with triangular spines (Fig. 15l). On sternum IX of male row of narrow pointed spines between protogonostyli, smaller and narrower spines laterad of protogonostyli and larger pointed spines laterad of them (Fig. 15m).

Paraproct (Fig. 15n). Posterior margin with ca. 16 stout spines. Surface scattered with scales.

Caudalii (Fig. 15o–p). Cerci and paracercus with small, elongate spines on posterior margin of each segment; on outer side of cercus several spines on each 2^nd segment greatly enlarged and pointed; no such enlarged spines on paracercus.

Tergalii (Fig. 13f). Present on abdominal segments II–VII, subequal.

Subimago (Figs 16a–e, 17d, e, I, j).

Cuticular colouration. Head and prothorax mostly brown. Mesonotum brown, medioparapsidal suture contrastingly colourless (Figs 16d, 17e). Meso- and metathoracic pleura and sterna brown with colourless areas (Fig. 17d). Wing membrane colourless with microtrichial circles ring-like, brown (Fig. 16c). Legs mostly colourless with microtricha and microlepides dark brown, with brown markings on femur, at base and apex of tibia and margins of tarsomeres (Figs 16a, b, 17i, j). Abdominal terga I-X and sterna I-VI uniformly brown, with all sigilla of the same colour as background; sterna VII-VIII with submedian sigilla paler than background, sternum IX of male with distal part and gonostyli colourless (Fig. Fig. 16e). Cerci colourless with setae dark brown.

Texture. Mesonotum with cross-striated polygonal areas bordered with microtrichia (Fig. 16d). Abdominal terga and sterna with outlined polygonal areas bearing two or more microtrichia each; sigilla diminished (Fig. 16e). On fore leg of male, 1^st–4^th tarsomeres covered with blunt microlepides, 5^th tarsomere covered with pointed microlepides; on middle and hind legs, all tarsomeres co­vered with pointed microlepides (Fig. 16a, b).

Male imago (Fig. 17a–c, f–h, k). Head ochre with reddish. Antennae with scape and pedicel ochre with reddish-brown apices. Turbinate eyes narrow, cylindrical, red. Thorax ochre-brownish. Fore wings with membrane colourless, proximal portions of C and Sc+RA reddish-brown, other veins ochre-brownish. Pterostigma with ca. three crossveins. Hind wings absent. Legs of all pairs ochre with inner side reddish (Fig. 17f–h). Middle and hind tarsi with two apical spines (on 1^st +2^nd, and 3^rd segments). Abdominal segments I–VI white with yellow stripe on posterior margin of each tergum and blackish spot on each stigma; terga VII-X yellow-ochre. Cerci whitish, with brown marking at each joining (Fig. 17k). Gonostyli whitish.

Genitalia (Fig. 18a–c). Unistyligers widely separated, with shallow, not sclerotised, conic projection between them. Each unistyliger cylindrical, distally widened and projected medially. First segment of gonostylus narrowing toward apex, poorly separated from 2^nd segment. 3^rd (apical) segment extraordinary small, much narrower than apex of 2^nd segment. Gonovectes sharply bent. Sterno-styligeral muscle distinctly developed, but attached far from anterior margin of sternum; in single examined specimen sharply asymmetric.

Dimension. Fore wing length (and body length) 3.5 mm.

Specific epithet refers to the type locality at the foot of the Sri Pada (Adam’s Peak).

Sri Lanka (Fig. 19)

Larva. Following combination of characters distinguish Arcobaetis gracilentus comb. nov. from other species of Arcobaetis gen. nov.: A) distal margins of segments in middle part of flagellum without long spines; B) labial palp segment III sub-quadrangular, at base narrower than distal margin of segment II (Kang et al. 1994: fig. 7E); C) hind protoptera present (Kang et al. 1994: fig. 7G); D) tergalii present on abdominal segments I–VII; E) posterior margin of tergum VI with triangular spines, longer than wide (Kang et al. 1994: fig. 7K).

Taiwan.