Brachypodus quadratus Deng, Zheng, Gong & Zhou, 2026, sp. nov.

Brachypodus quadratus sp. nov. Materials examined. Holotype: China • male imagos; Bajiao River, Lushui, Nujiang, Yunnan Prov; alt. 1130 m; 26°13.57'N, 99°24.31'E; 3. IV. 2025; D. W. Gong, X. H. Y. Zheng leg.; NJNUMayfly 110101; Paratypes: China • 26 nymphs; same data as the holotype; NJNUMayfly 110102; 7 male imagos, 2 female imagos, 4 male subimagos (all adults reared from mature nymphs); same data as the holotype; NJNUMayfly 110103. Description. Mature nymph (in alcohol, Figs 1, 2, 3, 4): body length 9.5–10.5 mm, caudal filaments 10.0–12.0 mm, body generally light yellowish, with brown stripes and spots (Fig. 1 A). Head: width 3.0–4.0 mm; antennae half width of head capsule; generally brown, marginal area and vertex pale; three pale patches on anterior half of head, one median, two beside compound eyes (Fig. 1 C). Mouthparts: labrum ca 2 / 3 width of anterior margin of head capsule, lateral portion extended into curved lobe, anterior margin slightly concave; both dorsal and ventral surface with dense, hair-like setae, an additional spine-like setae at median notch (Fig. 2 A). Lingua bell-shaped, lateral margins of superlinguae strongly concave; apical margins of lingua and superlinguae with a row of fine, hair-like setae (Fig. 2 B). Apical margins of segment II of labial palpi with dense, hair-brush; glossae and paraglossae with dense, hair-like setae on dorsal surface and apical margins; glossae subcircular; paraglossae expanded into long heart-like shape (Fig. 2 C). Both mandibles with dense, hair-like setae on outer margin; prostheses consist of 7–8 bristles, upper margin near molar with a row of hair-like setae (Fig. 2 D, E); both outer and inner incisors of left mandible with three apical denticles (Fig. 2 D); outer incisor with three apical denticles, inner one with two of right mandible (Fig. 2 E). Maxilla with 18–20 comb-like setae on crown; segment I: segment II of maxillary palpi = 1: 1.6, segment I distinctly wider than segment II; outer margins of maxillary palpi with hair-like setae, setae on segment II longer and denser than on segment I; segment II apex with a row of brush-like setae (Fig. 2 F). Thorax: pronotum expanding laterally, slightly wider than head capsule (Fig. 1 A). Colour pattern of three legs similar: femur with one bigger median pale patch and additional three others, apex of tibia pale, apex of tarsus darker than other parts (Fig. 4 A – C). Apical half of fore- and midfemora slightly expanded (Fig. 4 A, B). All legs with similar setal pattern: femur with strong, hair-like setae on outer margin; tiny spatulate setae on dorsal surface (Fig. 4 D); tibiae with tiny, sparse, spine-like setae and hair-like setae on anterior, posterior, and dorsal margins; tarsus with tiny, spine-like setae on inner margin and tiny, hair-like setae on surface, denser on apical margin (Fig. 4 A – C); claw with three subapical denticles and a basal one (Fig. 4 E). Length ratio of forefemur: tibia: tarsus ca 10.0: 9.0: 3.5 (Fig. 4 A); that ratio of midleg ca 8.3: 7.3: 2.5 (Fig. 4 B); ratio of hindleg ca 13.0: 11.0: 3.0 (Fig. 4 C). Abdomen: terga dark brown to light yellow, terga II – III, VI – VII usually darker than others; each tergum with three pairs of pale dots: submedian pair, posterior one and anterolateral one (Fig. 1 A). Both dorsal and ventral lamellae of gills with clear dark tracheae; dorsal lamellae of gill I leaf-shaped, others oval (Fig. 4 F – I). Terminal filament subequal to cerci, all of them with spine whorls on articulations (Fig. 4 L). Male imago (in alcohol, Figs 5, 6): body length 11.0–12.0 mm, forewing 11.0–12.0 mm, hindwing 2.5–3.0 mm, caudal filaments 20.0–22.0 mm. Compound eyes divided into grey upper half and black lower half. Thorax: mesonotum deep reddish brown (Fig. 6 A, B). Forewings transparent, only areas between C and R 1 pale yellow with light brown stigma. C, Sc, R 1 yellow, crossveins between them distinct, similar to crossveins between R 1 and Rs. Other veins dark brown. Hindwings transparent, crossveins with brown pigments (Fig. 6 D). Legs yellow to pale yellow; femora with medial brown band; apexes of femora, tibiae, and every tarsal segment brown (Fig. 6 E – G). Length ratio of forefemur: tibia: tarsus = 1.0: 1.0: 2.0, tarsal segments from base to apex = 2.6: 3.1: 2.8: 2.0: 1.0 (Fig. 6 E); length ratio of midleg is femur: tibia: tarsus = 1.5: 1.0: 1.6, length of tarsomere I> II> III> V> IV (Fig. 6 F); Length ratio of hindfemur: tibia: tarsus = 3.6: 2.8: 2.9, length of tarsomere I – V ca 1: 0.7: 0.4: 0.3: 0.5 (Fig. 6 G). Abdomen: terga I – IX with a thin, pale, longitudinal midline; posterior margins of them with brown transverse bands. Terga pale to yellowish, with two pairs of brown to black stripes: one sub-medial straight pair and an oblique pair; tergum X generally reddish brown to black. Sterna pale but with grey to black median longitudinal stripe (Fig. 6 A). Genitalia grey to brown; forceps with two pale apical segments; penes with a shallow, V-shaped medial notch (Fig. 6 H, I). Female imago (in alcohol, Figs 5, 6): body length 12.0–13.0 mm, forewing 12.5–13.5 mm, hindwing 4.0– 4.5 mm, caudal filaments 21.5–22.5 mm. Body colour pattern similar to male, but slightly darker, abdominal markings more distinct. Forewings and hindwings semitransparent, veins clear (Fig. 5 B). Length ratio of forefemur: tibia: tarsus = 1.0: 0.7: 1.0, length ratio of tarsomere I – V ca 2.0: 1.5: 1.2: 0.2: 1.0; length ratio of midfemur: tibia: tarsus = 1.3: 1.0: 1.1, length ratio of tarsomere I – V ca 1.0: 0.9: 0.7: 0.5: 0.3. Length ratio of hindfemur: tibia: tarsus = 3.8: 1.3: 1.0, length ratio of tarsomere I – V ca 1.0: 0.7: 0.5: 0.3: 0.7. Posterior margin of subgenital plate smoothly convex, posterior margin of anal plate with median notch, bottom of notch straight (Fig. 6 J). Male subimago (in alcohol, Fig. 5 C): body length 9.5–10.5 mm, forewing 11.0–12.0 mm, hindwing 2.5–3.5 mm, caudal filaments 17.0–18.0 mm. Body brown, with indistinct patterns on abdomen terga. Length ratio of femur: tibia: tarsus ca 1.8: 1.0: 1.7 in foreleg, 1.4: 1.0: 1.1 in midleg, 1.7: 1.1: 1.0 in hindleg; length ratio of tarsomere I – V ca 1.5: 1.5: 1.5: 1.3: 1.0 in foreleg, 2.0: 1.3: 1.1: 0.7: 1.0 in midleg, hindleg tarsomere I> II> V> III> IV. Egg (Fig. 7): 145.0–147.0 µm in length and 101.0–103.0 µm in width. Irregularly shaped; chorion with dense, small KCTs (knob-terminated coiled threads). Biology. The nymphs were found on moist rock surfaces beneath a small waterfall, as well as on the damp surrounding ground (Fig. 8). The nymphs are highly agile and swiftly retreat when disturbed. We sampled several streams connected to the waterfall but did not find the species in any of them, which suggests a specific adaptation to special habitats such as thin water films and wet rock faces. A rearing cage was positioned directly under the waterfall, maintaining conditions of high humidity and minimal water flow inside. Several mature nymphs were introduced into the cage, and subimagos emerged in the following days. The subimago stage lasted approximately 36 h.