Acentrella fimbriata sp. nov.

(Figs 1–11)

Holotype. Final nymphal instar in ethanol [PERC], Nan Province, Mang River, Bo Kluea district (19°09’25.53”N, 101°09’30.37”E, alt. 693 m a.s.l.), 23.xii.2019, Sirikamon Phlai-ngam.

Paratype. One nymph in ethanol [KKU-AIC] same data as holotype .

Additional materials. Chiang Rai Province: 20 nymphs in ethanol [KKU-AIC], one nymph in ethanol [PERC], Khun Korn stream (19°59’19.80”N, 99°37’19.80”E, alt. 610 m a.s.l.), 3.ii.2015, Sirikamon Phlai-ngam; 12 nymphs in ethanol [KKU-AIC], one reared male subimago in ethanol, one nymph in ethanol [KKU-AIC], Khun Korn stream (19°59’19.80”N, 99°37’19.80”E, alt. 610 m a.s.l.), 6.iii.2020, Sirikamon Phlai-ngam; five nymphs in ethanol [KKU-AIC] and one nymph in ethanol [PERC], Pha Kong cave, Phan district (19°31’12.14”N, 099°39’12.54”E, alt. 287 m a.s.l.), 2.ii.2015, Sirikamon Phlai-ngam .

Descriptions.

Final nymphal instar (Fig. 1). Body length 4.3–5.2 mm, head width 0.9–1 mm, head length 0.5–0.6 mm, antennal length 1.0– 1.2 mm, lengths of forefemora, midfemora and hindfemora 0.63–0.65 mm. Length of cerci 1.8–2.2 mm. Body short and ventrally flattened. Thorax apparently enlarged.

Head. Capsule light brown with lighter and darker areas on vertex; frontal suture yellowish with brown markings extending from behind lateral ocelli to occiput. Head with hypognathous orientation; width greater than length. Compound eyes black; ocelli dark brown; orange turbinate eyes of male subimago visible through cuticle of final nymphal instar. Margins of capsule relatively densely covered with fine, long setae, with greatest concentrations on genae and clypeus. Long and translucent scales scattered along capsule margins (Figs 2A, 3A, 6A). Antenna length approximately 2 times head width; scape and pedicel brownish, slightly paler than flagellum; scape broader than pedicel, subequal in length. Inner margins of scape and pedicel densely covered with long, fine, simple setae (Figs 3B, 6B). Labrum (Fig. 3C) approximately 1.5 times wider than long, broadly rounded anteriorly; anteromedian notch shallow, each side with one regular row of brush-like setae, arranged in pairs along anterior margin; dorsal surface with one median pair of long, simple setae, and lateral row of six smaller setae extending to margin; dorsal surface otherwise with scattered simple, hairlike setae. Left (angulate) mandible (Figs 3D, 3F) incisors not fused, with seven bluntly pointed teeth; prostheca comb-shaped; margin between mola and prostheca smooth, without setae; molar surface with one enlarged and blunt tooth at innermost edge, followed by about seven smaller molar teeth. Right (planate) mandible (Figs 3E, 3G) incisors not fused, with seven bluntly pointed teeth; prostheca slender, with numerous sharp teeth; edge between mola and prostheca smooth, without setae; molar area with numerous small, round teeth. Maxilla (Figs 4A–B) with four large teeth on galea-lacinia, rounded at tips; row of five long, complex setae, and row of eight smaller simple setae along margin. Maxillary palp 2-segmented, with scattered small, blunt setae; proximal segment length approximately 3.0 times width, longer than distal segment; articulation between segments distinctly narrowed; distal segment with distinct, nipple-like tip at apex (Figs 4C). Labium (Fig. 4D): Glossae (Fig. 4E) with five acute, serrate setae apically, and row of six acute, robust setae along inner margin, paraglossae (Fig. 4F) width approximately 1.3 times width of glossae, and length approximately 1.3 times length of glossae; apical part of paraglossa with 3 regular rows of long, serrate setae, and row of six submarginal acute setae. Labial palp (Fig. 4G) 3-segmented, articulation between second and third segments indistinct; third segment rounded; ventral surface with sparse hairlike setae on first and second segments, but longer stout setae on third segment. Hypopharynx (Fig. 4H) nearly subrectangular; lingua apically rounded, covered with fine setae especially on apex; superlingua slightly rounded with small hair-like setae along apical margin.

Thorax (Fig. 2B). Coloration light brown to brownish with dark brown patterns. Surface of pronotum (Fig. 2A) brownish with indistinct brown spots. Mesonotum light brown with submedian dark brown stripes and brown longitudinal stripes on half of anterior margin. Metanotum brown, with tiny posterolateral projection on each side. Plural sclerite brown. Sterna mostly pale. Furcasternal projections present on prosternum, but absent on meso- and metasterna. Legs (Figs 5B) yellow-brown but slightly paler than thorax; tarsal claws darker brownish. Femora broad, subequal to length of tibiae; dorsal margins of femora each with one regular row of long feathered bristlelike setae; length of longest setae approximately 2/3 width of respective femur (Fig. 6C); ventral edge with dense irregular row of long, simple submarginal setae (Fig. 6D); femoral villopore present, usually surrounded by rounded flattened setae (Fig. 6E). Tibiae with two rows of long, feathered, bristlelike setae dorsally and shorter, robust, feathered bristlelike setae ventrally. Tarsi with regular row of long, feathered bristlelike setae dorsally and short, robust, feathered bristlelike setae sparsely interspersed with fine, simple setae ventrally. Tarsal claw with row of 7–9 denticles increasing in length towards apex. All tarsal claws lack subapical setae (Figs 5C, 6F).

Abdomen (Fig. 2C). Ventrally somewhat flattened. General coloration of terga yellowish-brown. Abdominal terga I–V light brown, each with transverse red brown stripe on posterior margin; faint transverse stripe on terga IV–V along median line. Terga I–III with indistinct patterns; terga IV–VIII each with large, dark brown, roundish medioanterior sigilla (cf. Kluge & Novikova 2011: Fig. 99) decreasing in size backwards; anterior margins of terga IV–IX with transverse dark brown stripes; terga IX–X light brown without patterns. Surfaces of terga with scattered fine setae, sparse scale-like setae along margins. Posterior margins (Fig. 2D) of terga each with long translucent scales, more abundant posterolaterally; these margins with small, poorly-developed, acute denticles (Figs 5D, 10A) and long-oval translucent scales with increasing number in posterolateral area; truncate, translucent scales sparsely located along posterior margin (Figs 10A–B), with scattered long-oval, translucent scales, increased in number on tergum X. Gills (Figs 2E, 5E) on segments I–VII, simple, with tracheation, though indistinct on segments I and VII; margins smooth with scattered fine setae; coloration white. Gills on segments II–VII with numerous long, fine, simple setae apically (Figs 5F, 10C). Sterna (Figs 7–9) generally yellowish-white; posterior margins without marginal spines; sterna I–VII (Figs 7–8) without multidentated scales; sterna VIII–X (Figs 8C, 9) with multidentated scales; sterna VIII & IX (Figs 7, 8C, 9) with pair of anterior patches of scales and scales on posterior margins. Paraproct (Figs 5G, 8C, 9F) brown, without posterolateral projection; surface scattered with numerous pores and fine, short setae and small, blunt denticles on inner margin. Cerci yellowish-brown; length approximately 3/4body length. Posterior margins of segments with short fine spines on each segment, inner margins of cerci each with row of swimming bristles on posterior half, increasing in length towards apex. Median caudal filament reduced, composed of approximately 5–7 segments, without swimming setae (Figs 5H, 10D).

Male subimago (Fig. 11). Body length 5.3 mm; forewing length 5.2 mm. Head. Hypognathous, dark brownish. Upper portion of compound eye orange. Lower portion of compound eye slightly darker than upper portion. Inner margins of turbinate eyes (Fig. 11B) with space between them. Antennae: Coloration of middle and distal segments of flagellum darker brown than scape and pedicel. Thorax. Coloration dark brown. Mesothoracic projection moderately pointed (Fig. 11B). Legs: Coloration pale brownish to light brown. Leg lengths. Foreleg: femur 0.34 mm, tibia 0.37 mm, tarsi 4 segments (proximal to distal, excluding claw) 0.08 mm, 0.06 mm, 0.04 mm, 0.07 mm; midleg: femur 0.33 mm, tibia 0.30 mm, tarsi 4 segments (proximal to distal, excluding claw) 0.05 mm, 0.03 mm, 0.02 mm, 0.05 mm; hindleg: femur 0.34 mm, tibia 0.36 mm, tarsi 4 segments (proximal to distal, excluding claw) 0.05 mm, 0.04 mm, 0.02 mm, 0.05 mm. Forewing (Fig. 11D) translucent, cloudy in costal and subcostal margins; marginal intercalary veins of first few interspaces distinctly different in length; marginal intercalary veins of middle part of wing more or less subequal; marginal intercalary veins of posterior part of wing tiny. Hind wing absent. Abdomen (Fig. 11C). Abdominal tergum I dark brown with red brown line along posterior margin; terga II–III light brown with red brown line along each posterior margin; terga IV–V light brown, each with pair of red brown areas on posterolateral margins; terga VI–X dark brown. Sterna light brown without distinct markings. Genitalia (Fig. 11E). Forceps with hair-like setae present on inner margin towards base; basal segment subquadrate, with length subequal to width; segment II with overall curved appearance, subcylindrical basally, strongly constricted medially, and slightly flared; segment III length near twice its width, rounded apically. Caudal filaments translucent.

Imago. Unknown.

Egg (dissected from last nymphal instar). Oval shaped. Length about 103–115 µm, width about 70–75µm (Fig. 10E). Extrachorion with transverse striation completely surrounding egg. Chorionic sculpturing with longitudinal striation on surface of entire egg (Fig. 10F).

Nymphal diagnosis. The new species, Acentrella fimbriata sp. nov. is distinguished from all other congeners by a combination of morphological characters (Table 1): (i) body covered with long dense setae; (ii) the proximal segment of the maxillary palp approximately two times longer than the distal segment, and the distal segment with a nipple-like tip; (iii) margins of femora with irregular rows of dense setae; (iv) metatibia with two rows of feathered bristlelike setae on dorsal margin; (v) the absence of preapical setae on the tarsus; (vi) subapical setae of tarsal claw vestigial or absent; (vii) abdominal terga scattered with numerous long, translucent scales, at least some of which appear truncated; and the posterior margins of abdominal terga generally smooth, but sparsely with tiny, blunt denticles; (viii) margins of gills II–VII with densely covered with long, hairlike setae; and (ix) median filament reduced in length, being comprised of approximately 5–7 segments.

Etymology. The specific epithet is from the Latin word for fringed, an allusion to the setose margins of various body parts. The common name of this species is the fringed small minnow mayfly.

Remarks. Acentrella fimbriata sp. nov. has a curious maxillary palp morphology. In at least preserved specimens, the distinctive terminal segments of the maxillary palps are held against the body in a way that nearly mimics the placement and orientation of the procoxal osmobranchiae that are found on some other related baetids, such as Baetis sabahensis Müller-Liebenau, 1984, Barbaetis benfieldi Kennedy, 1985 (in Waltz et al. 1985), and four species of the North American genus Heterocloeon McDunnough, 1925 . This may suggest some analogous or additional functions of these different structures.

Ecology and Distribution. The nymphs of Acentrella fimbriata sp. nov. always were collected from slowflowing streams with a substrate mainly consisting of cobbles and pebbles covered by sediment. They were found under the cobble and covered by much sediment (Fig. 12). The microhabitats where we found them were usually in the upper stream, with relatively cool water temperature. Water temperature was 25ºC at time of sampling; dissolved oxygen was 7.58–7.64 mg /L; pH was 8.1; the electric conductivity of the water was 155–158 µS/cm, and total dissolved solid in the water was 205–209 ppm.

The microhabitat of our new species in slow flow areas laden with sediment is unusual among Acentrella species. Most species are found on the tops and sides of objects in riffles and rapids, in areas of the stream that are relatively free of sediments (McCafferty et al. 1994; LMJ, pers. obs.).

Acentrella fimbriata sp. nov. has been found at only three sites in northern Thailand (Chiang Rai and Nan provinces) (Fig. 18).