Male imago (in alcohol): body length 8.0–10.0 mm, fore-legs 11.0–12.0 mm, fore-wings 9.0–9.5 mm, hind-wings 3.0–3.2 mm, cerci 40.0–42.0 mm, terminal filament 1.0–1.1 mm; body reddish-brown (Fig. 1A and E–F). Antennae brown, but apical half of scape and flagella whitish; antennae narrowed from base to apex (Fig. 2A); vertex white to grey, but with a brown stripe between two compound eyes (Fig. 2A). Ocelli cylindrical, with chocolate basal rings; lateral ocelli distinctly larger than median one (Fig. 2A). Compound eyes divided indistinctly into two parts, shortest distance between two eyes about 0.3x diameter of one eye in dorsal view, just slightly wider than median ocellus, but narrower than lateral ocellus (Fig. 2A). Posterior margin of occiput almost straight (Fig. 2A). Pronotum brown, posterior margin concave (Fig. 1E and Fig. 2A). Mesonotum with four pairs of deep brown dots laterally and one brown median pit (or called medial ocellus, Fig. 1E). Prosternum triangular, mesothoracic furcasternum with almost straight inner margins (Fig. 1F). Fore-wings transparent, but longitudinal veins brown, all crossveins pigmented reddish-brown and surrounding clouds (Fig. 1A and E–F), especially those near margins; MA forked distally to middle point; MP forked at base (Fig. 2B and Fig. 8B). Hind-wings also with various reddish to chocolate dots and markings, length ca. 1.5x width (Fig. 2C); costal projection blunt; R1 straight and zigzag; MA single, MP forked at middle point almost symmetrically (Fig. 2D and Fig. 8C). Femora of fore-legs brown to chocolate, tibiae with brown base and apex, segment I of fore-tarsi dark brown, segments II–V with brown apex, other parts yellow (Fig. 2E). Length ratio of fore-femora: tibiae: tarsi = 1.0: 2.3: 2.7, length ratio of five segments of fore-tarsi = 1.0: 6.3: 4.7: 3.7: 2.3 (Fig. 2E); two claws of fore-legs black and blunt (Fig. 2F). Tibiae of mid-legs with brown base and dark apex, tarsi with dark apex too; length ratio of mid-femora: tibiae: tarsi = 1.0: 0.9: 0.6, length ratio of four movable segments of mid-tarsi = 1.0: 1.0: 0.5: 2.5 (Fig. 2G); claws black, one blunt one acute (Fig. 2H). Hind-legs similar to mid-legs, length ratio of mid-femora: tibiae: tarsi = 1.0: 0.9: 0.6, length ratio of four movable segments of mid-tarsi = 1.0: 0.8: 0.5: 2.0 (Fig. 2I). Mid- and hind-tibiae with patellar-tibial fusion suture (Fig. 2G and I). Terga of abdomen reddish-brown, washed with indistinct pale stripes (Fig. 1E); sterna pale, but with brown lateral margins (Fig. 1F). Tergum IX with distinct posterolateral projections (Fig. 2J–K and Fig. 8D–E). Genitalia: subgenital plate with almost straight posterior margin, width about 3.5x length (Fig. 2J); forceps slightly longer than length of subgenital plate, length ratio of segment I: II = 1.0: 0.6; segment I narrowed slightly from base to apex, apex dark; segment II with slightly expanded apex (Fig. 2J–K). Penes membranous, ventral surface grey, slightly extended out of subgenital plate; gonopore at posterolateral corner of penis (Fig. 2L); a dorsal plate between penes and subanal plate, posterolateral corner of this plate slightly extended outwards forming an acute apex (Fig. 2K). Cerci with two kinds of segments: apical half chocolate segment and basal dark ring segment, two kinds of them arranged alternately (Fig. 1A and E–F). Terminal filament black, with 8–10 segments, distinctly thinner than cerci (Fig. 2M).

Female imago (in alcohol): body length 11.0–12.0 mm, fore-legs 5.0–6.0 mm, fore-wings 11.5–12.0 mm, hind-wings 3.5–4.5 mm, cerci 18.0–21.0, terminal filament 1.8–2.0 mm; body similar to male, but markings of wings slightly paler (Fig. 1B, G, H and Fig. 3B–D). Shortest distance between two compound eyes about 1.8x eye diameter (Fig. 3A). Length ratio of fore-femora: tibiae: tarsi = 1.0: 0.8: 0.8, length ratio of five segments of fore-tarsi = 1.0: 2.5: 2.0: 1.0: 3.5, Fig. 3E–F); length ratio of mid-femora: tibiae: tarsi = 1.0: 0.9: 0.5, length ratio of four movable segments of mid-tarsi =1.0: 1.0: 0.6: 2.5 (Fig. 3G); length ratio of hind-femora: tibiae: tarsi = 1.0: 0.7: 0.4, length ratio of four movable segments of mid-tarsi = 1.0: 1.0: 0.8: 3.0 (Fig. 3H). Two claws of all legs dissimilar, one acute, one blunt (Fig. 3F). Sternum VII slightly thickened and extended posteriorly, forming a triangular lobe (Fig. 3I). Posterior margin of sternum IX slightly extended laterally, median portion slightly concave (Fig. 3I). Terminal filament with 10–12 segments (Fig. 1G and H).

Male subimago (Fig. 1C): body length 9.0 mm, fore-legs 5.0 mm, fore-wings 9.0 mm, hind-wings 3.5 mm, cerci 8.0 mm, terminal filament 0.5 mm. Shortest distance between two compound eyes about 0.67x eye diameter. Length ratio of fore-femora: tibiae: tarsi = 1.0: 1.1: 1.5, length ratio of mid-femora: tibiae: tarsi = 1.0: 0.7: 0.6. Hind-legs similar to mid-legs.

Female subimago (Fig. 1D): body length 11.0 mm, fore-legs 4.0 mm, fore-wings 11.0 mm, hind-wings 3.5 mm, cerci 7.0 mm, terminal filament 1.0 mm. Shortest distance between two compound eyes about 2.0x eye diameter. Length ratio of fore-femora: tibiae: tarsi = 1.0: 1.1: 1.5, length ratio of mid-femora: tibiae: tarsi = 1.0: 0.7: 0.7. Mid- and hind-legs similar to fore-legs, but paler.

Mature nymph (in alcohol): (see Nguyen and Bae (2003) for full description).

Main characters include: body length 8.0–12.0 mm, caudal filaments 3.0–4.0 mm. Body reddish-brown to chocolate (Fig. 4). Mouthparts as in Fig. 5A–F, filtering mouthparts, maxillary and labial palpi elongated and with dense long hair–like setae, especially their apical segments, length ration of three segments of maxillary palpi = 1.0: 1.5: 4 (Fig. 5E), ratio of labial palpi = 1.0: 0.3: 2.0 (Fig. 5F); fore-legs with a row of subapical transverse spines (Fig. 5G and H); fore-tibiae with two rows of long hair-like setae on dorsal surface, one transverse, one at base, one oblique row near middle (Fig. 5G and H); fore-tarsi with a transverse row of long setae at base and two rows along dorsal and inner margins (Fig. 5G). Length ratio of fore-femora: tibiae: tarsi = 1.0: 0.9: 1.0 (Fig. 5G), ratio of mid-leg = 1.0: 0.6: 0.6 (Fig. 5I), hind-leg = 1.0: 0.6: 0.4 (Fig. 5J), claws hooked. Terga I–II, VI–VII with a median tubercle respectively, that of tergum II more distinct than others (Fig. 6A). Gills shown in Fig. 6B–F, gills I two segmented, with tiny hair–like setae (Fig. 6B); both dorsal and ventral surface of gill opercula smooth, posterior inner margins with dense setae, ventral lamella with fringes (Fig. 6C); gills III–V similar, with dorsal and ventral lamellae (Fig. 6D and E); gills VI single (Fig. 6F). Caudal filaments with stout spines on articulations, cerci with additional short spines on outer side and long spines on inner side; terminal filament with long spines on both lateral sides; articulations with very sparse tiny hair–like setae, especially apical half (Fig. 6G and H).

Egg: surface smooth, micropyle at middle (Fig. 7).

The nymph of P. unicutibius can be recognised by its filtering mouthparts and fore-legs: apical segment of maxillary and labial palpi more than 2 times of segment II in length, with dense hairlike setae (Fig. 5E–F); fore-tibiae with two oblique rows of long filtering setae (Fig. 5G-H), fore-tarsi with filtering hairs (Fig. 5G). In addition, their relatively short caudal filaments, smooth body and gill opercula, independent gills II, neither pronotum nor mesonotum with distinct expanded anterolateral corner are useful to their identification (Fig. 4).

In imaginal stage, its relatively short body (10.0–12.0 mm), heavily stained wings (Fig. 1A–B), long fore-legs and its tibiae (longer than 2.0x fore-femora, Fig. 2E) and widely opened compound eyes (Fig. 2A) are good identical characteristics. Its male imago has reduced genitalia, but all components distinct, especially its penes (Fig. 2J–L).

All other congeners in the genus Potamanthellus have no filtering setae on their fore-leg or mouthparts in nymphal stage. In imaginal stage, all other species have no distinct penes, only forceps and dorsal plate are visible. In addition, the dorsal plates of those species are different: P. amabilis and P. chinensis have deeply divided plates and their forceps are three-segmented. Although P. caenoides has two-segmented forceps, its dorsal plate is two semi-circular (see Bae and McCafferty (1998)). In contrast, P. unicutibius has two-segmented forceps and its dorsal plate has two distinct posterolateral processes (Fig. 2L and Fig. 8E).

Egg: surface smooth, without distinct mesh or KCTs (knob-terminated coiled threads), micropyle at equator (Fig. 7).

Yunnan Prov. (China); Lao Cai Prov. (Vietnam).

This nymphs were found in a narrow creek with large or small stones, mountain area. The width of the creek is about 1–2 m and the flow velocity is 1 m/s (Fig. 9). An Isonychia sp. was also found in the same water.