Nigrobaetis bilongus Li & Shi & Li & Tong 2023, sp. nov.

Nigrobaetis bilongus sp. nov.

(Figs 1–45, 47–48)

Material examined. Holotype: male mature larva (in ethanol, deposited in SCAU), CHINA, Yunnan Province, Dali City, Mt. Cangshan, Qingbi Stream (2100–2600 m a.s.l.), 2.vi. 2020, collected by Xianfu Li. Paratypes (in ethanol): 15 larvae, 10 male imagos, 3 female imagos reared from larvae and 5 specimens on mounted slides (including 2 mature larvae, 2 male imagos and one female imago), 26.v.–2.vi.2020, same data as holotype; 9 larvae (including one on mounted slides), Heilong Stream (2220 m a.s.l.), Mt. Cangshan, Dali City, 15.v. 2020, coll. Xianfu Li; 3 male imagos, 4 male subimagos and 7 female imagos reared from larvae in Heilong Stream, Mt. Cangshan, Dali City, 1.v. 2018, coll. Xianfu Li; 20 larvae (including one on mounted slides), Ziyang River (2500–2600m a.s.l.), West Slope of Mt. Cangshan, Yangbi County, Dali City, 4.xii.2012, coll. Haomiao Zhang. Sichuan Province: 2 larvae (including one on mounted slides), Pingwu County, Laohegou Natural Reserve, Shabazi (1400 m a.s.l.), 29.v.2012, coll. Weifang Shi.

Diagnosis. Larvae: Body colouration generally brown, immature or female larvae usually predominantly brown dorsally except tergites IX–X yellowish; mature male larvae tergites I–VI light brown or yellowish, tergites VII–X brown (X paler than VII–IX).Antennae extremely long, approximately 3/4–4/5 length of body. Mandible with dense spine-like bristles on margin between incisors and mola. Maxillary palp elongate, much longer than galea-lacinia. Glossae with a cluster of 3–5 dorsal setae subapically. Foreleg tarsi with one row of 24–27 stout pointed setae on ventral margin. Gills on abdominal segments I to VII. Posterior margin of tergites II–X with triangular spines. Inner margin of paraproct with one row of 27–30 pointed spines. Cerci exceptionally long, subequal or slightly longer than body. Male imagoes: Head reddish purple; turbinate eyes cylindrical with high stalk, facetted surface rounded and orange red. Thorax purple or dark brown. Forewing transparent with light brownish longitudinal veins; hind wing with costal projection and three longitudinal veins, second vein bifurcate, third vein relatively short, not exceeding midline. All legs cream-coloured. Abdominal segments I–VI white or transparent, VII–X reddish purple. Terminal segment of genital forceps elongate, ca. 4.5–6.0 times longer than wide. Cerci extremely elongated. Female imagoes: Head creamy yellow. Thorax yellowish brown. Wing coloration and venation as in male. Legs similar to that of male imago. Abdominal segments uniformly purple except tergites I and X slightly paler than other segments; tergites II–VIII each with a pair of oblique paler short stripes and one small paler spot near anterior margin. Cerci as in male.

Description of larvae. Length. Body length 5.5–8.0 mm; antennae:4.5–5.0mm; cerci:5.0– 7.5 mm. Colouration. Head and thorax brown; abdominal tergites tend to be change in colouration depending on the life stage and sexes: immature or female larvae usually predominantly brown dorsally except tergites IX yellowish with a narrow “Tshape” brown marking and X yellowish (Figs 1–3, 5); mature male larvae tergites I–VI light brown or yellowish, tergites VII–X brown (Fig. 4); anterior margin of abdominal tergites II–VIII usually each with pair of yellowish oblique narrow stripes and small spots. Legs largely light brown dorsally, except femora with irregular pale patches on basal and distal areas. Proximal 1/3 of cerci yellowish brown with obscure band, apical 2/3 of cerci brown.

Head. Antennae close to each other, frons with a distinct median carina between bases of antennae (Fig. 6); scape and pedicel without scale-like setae, only bearing sparse hair-like setae. Antennae extremely long, ca. 1/2–3/4 of body length, length of antennae may change depending on the life stage, in mature larvae, antennae tend to shorten as terminal segments becoming as thin as a threadlet which easily lost or broken. Labrum rectangular (Figs 7, 17), ca. 1.5 times wider than long; anteromedian notch deep with a small rounded lobe at base; dorsally in distal half with one long, simple seta near midline and irregular row of 4–5 medium to long, simple setae; in proximal half with fine, simple setae scattered over dorsal surface (Fig. 17); ventrally bordered with feathered setae along anterior margin and 9–10 short, spine-like setae near lateral and anterolateral margin (Fig. 17). Left mandible (Figs 12, 18): incisors fused, with 7 denticles apically; prostheca robust, apically with 3 acute and 6–7 blunt denticles; margin between incisors and mola with dense spine-like bristles (Fig. 18). Incisor of right mandible (Figs 13, 19) fused, with 7 denticles apically; prostheca stick-like with distolateral dentation (Fig. 19); margin between incisors and mola with densely spine-like bristles, inner margin of incisors with one row of very fine setae (Figs 15, 19), tuft of setae at apex of mola present. Hypopharynx apically covered with fine setae, lingua with a median projection (Fig. 8); superlinguae subequal to lingua. Maxillae (Fig. 14): apex of maxillae with three elongated canines, crown of galea-lacinia with three stout feathered dentisetae; base of lacinia with one row of 5–6 simple setae increasing in length upwardly and one short spine-like hump seta. Maxillary palp elongated and 2-segmented, ca. 1.8–2.0 times length of galea-lacinia, segment II apically rounded with hair-like setae and much longer than segment I. Labium (Fig. 16) with glossae basally broad, narrowing toward apex, slightly shorter than paraglossae; inner margin with 12–14 long, simple setae; apex with two long, curved, robust setae and one short, robust, pectinate seta; outer margin with 9–10 long, simple setae; ventral surface of glossae covered with many hair-like setae (Fig. 16), dorsal surface with a cluster of 3–5 fine dorsal setae subapically (Figs 21, 45). Paraglossae sub-rectangular, curved inward, ca. 1.6 times width of glossae; apex with two rows of long, robust setae in apical area; dorsal surface with a slightly curved row of 6–7 long, simple setae near inner margin. Segment I of labial palp shorter than length of segments II and III combined. Segment I ventrally with sparse hair-like setae, dorsal face with many micropores medially. Segment II slightly expanded apico-laterally, dorsal face with one oblique row of 4–5 long pointed setae (Fig. 20), ventrally with many hair-like setae. Segment III truncated apically and slightly expanded inwards; ventrally covered with 12–15 short, simple setae and numerous hair-like setae (Fig. 22), dorsally with 9–12 short, simple setae near apex and many fine hair-like setae.

Thorax. Hind wing pads well developed. Femora of foreleg (Figs 10, 29) dorsally with one row of 10–13 long, stout setae, except proximal part with 4–5 stout setae subparallel to dorsal margin, dorsoapical setal patch formed by 2 stout, bluntly pointed setae; ventrally with many stout pointed setae near ventral margin; anterior face with numerous micropores and scale bases. Tibiae with only sparse hair-like setae on dorsal margin, ventral side with many stout, simple, pointed setae, both faces covered with many scale bases; tibiopatellar suture present (Fig. 29). Tarsi with sparse hair-like setae dorsally, ventral margin with one row of 24–27 stout, simple, pointed setae (Figs 9, 29); both faces with numerous scale bases. Claws (Fig. 29) slender, with one row of 10–12 acute teeth increasing in length toward the apex; subapical setae absent. Middle and hind legs similar to foreleg in structure, except tibiae with 5–6 stout pointed setae dorsally and tarsi with only 12–15 stout, simple, pointed setae along ventral margin.

Abdomen. Dorsal surface of tergites scattered with numerous trapezoidal scale bases. Posterior margin of tergite I without spines, tergites II–X with nearly equilateral triangular spines (Fig. 28) except X with isosceles triangular spines, longer than wide. Sternites surface covered with numerous trapezoidal scale bases, posterior margin of sternites VI–IX with triangular spines. Gills present on segments I to VII (Figs 24–27), long elliptic and transparent; margin with a brown band and small denticles intercalating fine hair-like setae (Fig. 25); tracheation well visible (Fig. 11); gill I slender, similar to gill VII in shape and length, gills II–VI alike in shape (Figs 11, 24–27). Paraproct (Fig. 23) subtriangular, without posterior prolongation, surface with many trapezoidal scale bases, inner margin with 27–30 pointed spines; surface of cercotractor with some trapezoidal scale bases, margin with 21–23 blunt spines. Cerci extremely long, subequal or slightly longer than body length, proximal 1/3 of cerci with swimming setae along inner margin, terminal 2/3 of cerci smooth and extremely prolonged (Figs 1–3); median caudal filament ca. 2/3–3/4 of cerci length. Cerci length can be different depending on the instar examined. In mature larvae (with darker wing pads), terminal segments that lacking swimming setae tend to shrink as thin as a threadlet which easily broken, especially when deposited in ethanol.

Male imago: Body length 5.0– 5.5 mm; forewings: 5.8–6.5 mm; hind wings: 0.8–0.85 mm; cerci: 11.0–12.0 mm. Head reddish purple.Antennal scape and pedicel light brown, flagellum cream-coloured (Figs 32–34); median carina between bases of antennae becoming prominent lamellar (Figs 32, 34). Turbinate eyes with high stalk, cylindrical, slightly widened apically; facetted surface rounded and orange red, stalk 2/3 purple basally and 1/3 orange distally (Figs 30, 32–34). Thorax purple. Forewing transparent with light brownish longitudinal veins (Fig. 30), except pterostigma slightly frosted-white with several incomplete short veins (Fig. 36); hind wing with costal projection and three longitudinal veins, second vein forked at basad midline, third vein relatively short, not exceeding midline (Fig. 37). All legs cream-coloured; tarsi of fore leg 5-segmented, without apical spines on tarsal segments (Fig. 38), tarsi of middle and hind legs 4-segmented, with apical spine on fused 1 st +2 nd and 3rd tarsal segments; all claws with one oval lobe and one pointed curved hook. Ratio of foreleg femur/tibia/tarsus = 1:1.5:2; ratio of foreleg tarsal segments =1:17:14:9:3. Abdominal segments I–VI white, nearly transparent, VII–X reddish purple (Fig. 30). Cerci extremely elongated, white except segment joinings with light brown (Fig. 30). Genitalia light brownish to white, unistyliger nearly rectangular, without apico-internal process; segments I and II of forceps almost completely fused; segment I conical, tapered apically; segment II cylindrical, weakly contracted proximally; terminal segment narrow and elongate, ca. 4.5–6.0 times longer than wide (Fig. 35).

Male subimago (in ethanol). Similar to imago, except following characters: (i) head creamy yellow; (ii) turbinate eyes creamy yellow, stalk shorter with rounded apically; thorax yellowish brown, wings brown with numerous marginal hairs; (iii) all legs except tarsi are covered with microtrichia; in fore legs, tarsal segments I–IV are covered with blunt microlepides and tarsal segment V is covered with pointed microlepides (Figs 39, 42); all tarsal segments of middle and hind legs are covered with pointed microlepides (Figs 40–41, 43–44); (iv) abdominal segments I–VI and X creamy yellow, VII–IX brown.

Female imago (Fig. 31). Body length 5.5–6.5 mm; forewings: 6.0– 6.8 mm; cerci: 7.0–10.0 mm. Head wide, creamy yellow, distance between eyes exceeding eye length. Antennal scape and pedicel light brown, flagellum cream-coloured; interantennal carina raised and lamellar. Thorax yellowish brown. Wing colouration and venation as in male. All legs cream-coloured, similar to that of male imago. Abdominal segments uniformly purple except tergites I and X slightly paler than other segments; tergites II–VIII each with a pair of oblique paler short stripes and one small paler spot near anterior margin. Cerci as in male.

Eggs. Oval, length ca. 200µm, width ca. 120µm; chorion uniformly covered with a small-mesh reticulation and without any ridges when observed under optical microscope (Figs 47–48).

Etymology. The specific epithet is an arbitrary combination of “ bi- ” (derived from Latin, the prefix of two) and “ longus ” (from Latin, meaning long), referring to the larvae having unusually long antennae and cerci, which are conspicuous characters for recognizing this new species in the field.

Distribution. China (Yunnan, Sichuan).

Biology. The larvae of the new species occur in slow-flowing streams or in the margins of pools of high altitude areas (Figs 77–78). They usually climb about surfaces of stones, when disturbed, larvae dart away swiftly by rapidly flipping the long caudal filaments up and down. Emergence occurs in late May and early June in Mt. Cangshan when the average water temperature was around 15 °C. The imagoes have been observed swarming in full sunlight from morning to noon over the pools. The swarms vary in height above the water level from 2.0 m to 3.0 m.

Discussion. The larva of the new species is similar to Nigrobaetis niger (Linnaeus, 1761) and Nigrobaetis gracilentus (Chang & Yang, 1994) (in Kang et al. 1994) in appearance, but it can be distinguished from the latters by the following characters: (1) maxillary palp elongate, ca. 1.8–2.0 times of galea-lacinia while those in N. gracilentus ca. 1.5 times as long as galea-lacinia (Kang et al. 1994); (2) the articulation between segments II and III of labial palp clearly visible (Figs 16, 20, 22) and segment III truncated apically (vs. the articulation unclear in N. niger and N. gracilentus); (3) apex of paraglossae with two rows of long, robust setae (vs. three rows in N. niger and N. gracilentus); (4) dorsal surface of paraglossae with a slightly curved row of 6–7 long, robust setae (Fig. 45), while those in N. gracilentus with a strongly arched row of 9–10 long setae submedially (Fig. 46); (5) dorsal surface of glossae with a cluster of 3–5 dorsal setae subapically (Fig. 21)(vs. 0–1 seta in N. gracilentus and about 15 setae in N. niger); (6) foreleg tibiae with only sparse hair-like fine setae on dorsal margin (vs. foreleg tibiae dorsally with 5–7 stout, pointed setae in N. gracilentus); (7) posterior margins of abdominal tergites with nearly equilateral triangular spines (vs. with long, acute marginal spines in N. niger and N. gracilentus); (8) inner margin of paraproct with one row of 27–30 pointed spines (vs. about 16–19 spines in N. niger and N. gracilentus). In addition, this new species resembles to N. gombaki (Müller-Liebenau, 1984) from Malaysia by sharing with similar number of marginal spines on paraproct (Müller-Liebenau 1984), but it differs from the latter by the following characters: (i) densely spine-like bristles present on margin between incisors and mola of mandible (Figs 12–13); (ii) maxillary palp elongate, approximately 1.8–2.0 times length of galea-lacinia (Fig. 14); (iii) tarsi bearing a close row of 24–27 stout, simple pointed setae on ventral margin (Fig. 9). In general, the number of marginal spines on paraproct is good diagnosis character which has little intraspecific variation (Müller-Liebenau 1984). The members of Nigrobaetis s. str. usually possess a restricted number of marginal spines on paraproct (ranging from 4–16 spines), which is an unique character within the Alainites / Nigrobaetis / Takobia complex (Figs 23, 71, Müller-Liebenau 1984, Novikova & Kluge 1987, Kang et al. 1994, Lugo-Ortiz & de Moor 2000, Gattolliat 2004, Gattolliat & Sartori 2008, Gattolliat et al. 2012, Kluge & Novikova 2014, Kubendran et al. 2015, Martynov & Godunko 2017, Godunko et al. 2018, Sroka et al. 2021, Sivaruban et al. 2022). There are exceptions, however, apart from the new species and N. gombaki above mentioned, N. rhithralis (Soldán & Thomas, 1983) and N. digitatus (Bengtsson, 1912) also have 28–35 and 17–28 spines respectively on inner margin of paraproct (Godunko et al. 2018). Despite that, the number of marginal spines on paraproct in Nigrobaetis s. str. is much less than other members of the Alainites / Nigrobaetis / Takobia complex because Nigrobaetis s. str. has no prolongation of paraproct (Waltz et al. 1994, Kluge & Novikova 2014, Martynov & Godunko 2017, Tong & Dudgeon 2000, Sroka et al. 2021, Phlai-ngam et al. 2022, Yanai et al. 2022). Therefore, the restricted number of marginal spines on paraproct in Nigrobaetis could be considered as one of useful characters to distinguish it from Alainites and Takobia in larval stage.

Additionally, the eggshell structure of the new species is peculiar and characterized by covering with reticular structure (Figs 47–48) which is similar to those of Nigrobaetis digitatus and N. niger (Kopelke & Müller-Liebenau 1981). This type of chorionic structure is called the microreticule-type that was found from Baetis maurus Kimmins, 1938 by Ubero-Pascal and Puig (2007). In male subimago of the new species (female subimago absent in this study), texture of microlepides on subimaginal tarsi is the same with those of N. digitatus and N. acinaciger Kluge, 1983 (Kluge 2022), i.e. fore leg tarsal segments I–IV are covered with blunt microlepides and tarsal segment V is covered with pointed microlepides (Figs 39, 42), all tarsal segments of middle and hind legs are covered with pointed microlepides (Figs 40–41, 43–44). The male imagoes of the new species resemble to those of N. niger, N. ishigakiensis Fujitani, 2017 (in Fujitani et al. 2017) and N. latus Fujitani, 2017 (in Fujitani et al. 2017) in appearance and body colour pattern (i.e., thorax usually dark brown or purple, wings transparent, and abdominal tergites I–VI gray or transparent, VII–X brown) (Fig. 30, Müller-Liebenau 1969, Fujitani et al. 2017), but it differs from the latters by the hind wing with three longitudinal veins and the terminal segment of forceps being slender and elongate (ca. 4.5–6.0 times longer than wide), while those in N. niger, N. ishigakiensis and N. latus each with two longitudinal veins, and the terminal segment of forceps being shorter and stockier than that of the new species (Müller-Liebenau 1969, Fujitani et al. 2017).