Holotype : Male mature larva in ethanol, Thailand, Nan Province, Bo Kluea District, Mae Nam Wa stream, 19°16'22.6"N, 101°10'48.2"E, 848 m, 26.XI.2019, C. Auychinda leg. [ZMKU]. Paratypes: 30 larvae in ethanol, one on slide, same data as holotype [ZMKU]; 4 larvae in ethanol, same data as holotype [MZL GBIFCH00977588].

Mature larva (in alcohol, Fig. 1; living, Fig. 6c). Body length (without cerci) 5.5–6.0 mm; cerci 6.0–8.5 mm; body brownish-black with a conspicuous dorsal median pale line from the head to tergum X (Figs 1a, 6c).

Figure 1. 

Cincticostella ebura sp. nov. a larval habitus in dorsal view b in lateral view and c in ventral view. Scale bars: 1 mm.

Head. Black without tubercles, prominent bright ocelli; antennae three times longer than head length. Labrum densely covered with long fine setae, apicolateral angles rounded; apicomedially with deep emargination; ratio of emargination length to maximum labrum length = 1: 4.7 (Fig. 2a). Mandibles stout with numerous, hair-like setae on 2/3 proximal of dorsal and lateral surfaces (Fig. 2b, c). Left mandible: outer incisor composed of three acute teeth; inner incisor with one main stout and one inner vestigial tooth; prostheca with a bunch of hair-like setae on the inner side (Fig. 2b). Right mandible: outer incisor composed of two pointed teeth; inner incisor composed of two apically pointed teeth, orientated perpendicularly to the outer incisor; prostheca consisting of numerous hair-like setae (Fig. 2c) Hypopharynx: lingual surface covered with short setae, most abundant in apical part; superlinguae with numerous hair-like setae, apices rounded, posterolateral part concave (Fig. 2d). Labium with narrow elliptical glossae, almost four times longer than broad and covered with numerous short fine setae; paraglossae broad, semicircular, with surfaces covered with numerous simple setae. Labial palp three-segmented; segments I and II stout and equal in length, outer margin covered with hair-like setae, segment III spine-like in shape, 2.5 times longer than broad at the base (Fig. 2e). Maxillae slender; maxillary palpi long (0.46 mm), covered with tiny setae and three-segmented, length ratio from basal to apical segments = 4: 4: 1 (Fig. 2f), apex of segment II with long hair-like setae, segment III cone-shaped and with tiny short setae apically (Fig. 2h); apex of maxilla widened, surface with numerous long, hair-like setae; maxillary canine reduced to a small denticulated blade and less than half as long as crown, inner margin of galea-lacinia with 3–4 rows of simple setae (Fig. 2f, g).

Figure 2. 

Cincticostella ebura sp. nov. a labrum b left mandible c right mandible d hypopharynx e labium f maxilla g galea-lacinia h segment III of maxillary palp. Scale bars: 0.2 mm (a–f); 0.05 mm (g); 0.035 mm (h).

Thorax. Black with distinct white median line. Pronotum rectangular without clear anterolateral projections. Mesonotum with rounded anterolateral projections, outer margins not notched (Fig. 1a); mounted on slide, this character looks more angular (Fig. 4a); a pair of sub-median tubercles in the middle, a single posterior prominent median tubercle (Fig. 1b), posteriorly between fore wing pads with a pair of well-developed projections, angular with deep cleft (Figs 1a, 4a–b). Prothoracic sternum trapezoidal, mesothoracic basisternum rectangular, mesothoracic furcastemum broader than basisternum, oval transversely (Fig. 3a). Forefemora moderately dilated, ventral margin with fine setae, dorsal margin with spatulate setae most abundant in distal part, distal part of the dorsal surface with a transversal discontinuous row of 6–8 spatulate setae perpendicular to the femur (Fig. 4 c, i, j). Midfemora moderately expanded, dorsal margin smooth and with a row of short stout setae abundant in distal part (Fig. 4e). Hind femora moderately expanded, longer than mid femora, dorsal margin smooth, with a row of short stout setae from median to distal part (Fig. 4f). All claws similar, strongly hooked without apical setae, each with an acute basal and subapical tooth (Fig. 4d).

Figure 3. 

Cincticostella ebura sp. nov. a prosternum and mesosternum b pairs of tubercles (arrow) on mesothorax of early stage; c–f gills of segment III–VI g ventral lamella of gill of segment VII h ventral lamella of gill of segment VI. Scale bars: 0.2 mm.

Figure 4. 

Cincticostella ebura sp. nov. a thorax in dorsal view, a pair of tubercles was indicated by arrows b posterior projection of mesonotum c foreleg d foretarsal claw e mid-femur f hind-femur g abdominal terga V–VII h abdominal terga VIII–X i, j setae on apically dorsal forefemoral surface k setae on posterior margin of abdominal terga. Scale bars: 0.2 mm (a, b, c, e, f, g, h); 0.05 mm (d, j); 0.01 mm (k).

Abdomen. Terga I–X each with a pair of posteromedian projections, well developed into strong tubercles of terga IV–VIII (Figs 1b, 4g); posterolateral projections of tergum VIII less developed (Fig. 4h); posterior margins of each tergum with bifurcate stout setae (Fig. 4k). Gills present on segments III–VII (Fig. 3c–h), all gills consistent with the diagnostic character of the genus Cincticostella: gill III without medial transverse band of weakened membrane; ventral lamella of gills III–V bifurcated (Fig. 3c–e), gill VI–VII non-bifurcate with marginal processes (Fig. 3f–h). Caudal filaments with whorls of dense setae on each segment.

Eggs. Dissected from mature larva (Fig. 5). Ovoid, length ca 125 µm, width ca 110 µm; one pole covered with a dome-shaped polar cap, chorionic surface reticulated, almost hexagonal in formation, with a central spot (Fig. 5a, b). Equator with 4–6 micropyles, sperm guide circular and smooth (Fig. 5c). Rounded knob terminated coiled threads (KCT) especially abundant at the part opposite the polar cap (Fig. 5d).

Figure 5. 

Cincticostella ebura sp. nov. a an overview of egg b chorionic surface c chorionic surface with micropyles d chorionic surface with KCT. Scale bars: 0.02 mm (a); 0.01 mm (b–d).

Adults. Unknown.

The pair of sub-median tubercles in the middle of mesonotum of early stages is prominent and variable in number, 2 or 4 tubercles (Fig. 3b) which is similar to other Ephemerellidae such as Notacanthella commodema (Allen, 1971) in which the tubercle numbers reduce and are more flattened in later stages (Auychinda et al. 2020b). On the contrary, the posterior median tubercle is distinct in all larval stages of the new species. Although, C. funki has no distinct prominent tubercle on their posterior median mesothorax in later stages, this tubercle is distinct in the small larval stages (A. Martynov, pers. comm.)

The larva of Cincticostella ebura sp. nov. has a well-marked white median line along its body that can be used to separate it from other Cincticostella species. However, this pattern is also present in C. nigra (Uéno, 1928) and C. funki Martynov, Selvakumar, Palatov & Vasanth, 2021, and the body shape is quite similar (Uéno 1928; Ishiwata 2003; Martynov et al. 2021). Although, claws of C. ebura sp. nov. and C. funki are hooked with an acute basal and subapical tooth, this character is absent in C. nigra, where a row of 6–8 teeth of unequal size can be found (Uéno 1928, fig. 9h–i) or 5–8 denticles of tarsal claws (Ishiwata 2003). In addition, the dorsal surface of the mid- and hind femora of C. ebura sp. nov. possess clavate setae while in C. nigra, these setae are absent (Ishiwata 2003, figs 48, 52). Furthermore, C. ebura sp. nov. can be distinguished from C. nigra and C. funki based on the combination of following characteristics: 1) small denticulate blade maxillary canine; 2) maxillary palp segment III cone-shaped; 3) all abdominal terga with long pairs of tubercles, especially on terga IV to VIII, on tergum X small and pointed; 4) anterolateral projection of the pronotum absent; 5) mature larvae length is almost less than two times of C. funki; 6) mesonotum with single prominent median posterior tubercle and posteriorly with a pair of well-developed angular projections; and 7) a transverse discontinuous row of stout setae and without extra setae on surface of forefemora.

The specific epithet ‘ebura’, which means ivory, is a reference to the pairs of long and curve tubercles on the abdominal posteromedian margins.

The type locality of Cincticostella ebura sp. nov. is the Mae Nam Wa stream, Nan Province, Thailand (Fig. 6a). The larvae were collected by handpicking and D-frame net kicking methods from cobble and pebbles within moderate- to fast-flowing current of run/riffle areas (Fig. 6b). This study site also shows a high taxa richness of Ephemerelloidea larvae, as other species, including C. insolta (Allen, 1971), Notacanthella quadrata (Kluge & Zhou, 2004), N. commodema (Allen, 1971), Dudgeodes sp. and Vietnamella nanensis Auychinda, Sartori & Boonsoong, 2020, co-occurred with the larvae of C. ebura sp. nov.

Figure 6. 

a The Mae Nam Wa stream, Bo Kluea district, Nan Province b microhabitat of the larvae of Cincticostella ebura sp. nov. c Cincticostella ebura sp. nov. larva (living).

Nan Province, northern Thailand.