Mae-nang Songsri cave, Hin Tok Subdistrict, Ron Phibun District, Nakhon Si Thammarat Province, southern Thailand: 08°14'45"N, 99°52'01"E, 45 m altitude, 29 October 2015; Payanakarat cave, Tham Thong Lang Subdistrict, Thap Put District, Phang Nga Province, southern Thailand: 08°31'11"N, 98°33'57"E, 140 m altitude, 5 November 2014; Khao Chakan cave, Khao Chakan Subdistrict, Khao Chakan District, Sa Kaeo Province, eastern Thailand: 13°39'36"N, 102°05'04"E, 120 m altitude, 1 September 2017; Plub Pleung Thong cave, Wang Mai Subdistrict, Wang Sombun District, Sa Kaeo Province, eastern Thailand: 13°26'50"N, 102°13'03"E, 223 m altitude, 31 August 2017. All samples were collected by the author.

Huang Po cave, Thung Na Lao Subdistrict, Khon San District, Chaiyaphum Province, north-eastern Thailand: 16°35'25"N, 101°49'28"E, 384 m altitude, 16 October 2017; Pra Hor cave, Tham Yai Subdistrict, Thung Song District, Nakhon Si Thammarat Province, southern Thailand: 08°06'49"N, 99°43'59"E, 101 m altitude, 29 October 2015. Samples were collected by the author.

The specific name of the new species, ‘thailandica’, refers to Thailand, where the species was collected.

Holotype : one adult female dissected and mounted on one slide, NHMUK 2020.56; Allotype: one adult male dissected and mounted on one slide, NHMUK 2020.57; Paratypes: three adult females and three adult males not dissected and stored in a 1.5 ml microtube with 70% ethanol, NHMUK 2020.58-63; one adult female dissected and mounted on one slide, NPU 2020-003; one adult male dissected and mounted on one slide, NPU 2020-004; three adult females and three adult males not dissected and stored in a 1.5 ml microtube with 70% ethanol, NPU 2020-005.

(holotype). Body (Fig. 2A) cylindrical, with mean length 510 µm, measured from rostrum to distal rim of caudal rami (n = 5, range 500–530 µm). Cephalothorax with narrow, saddle-shaped, well discernible integumentary window; several sensilla scattered on dorso-lateral surface. Prosome and urosome (segments 1–4) with free posterior margins smooth dorsally; rows of minute spinules dorsally on prosomites 2–4, urosomites 1–2. Genital double-somite completely fused (Figs 2B, 4A), about 0.5× as long as wide, with row of strong dorso-lateral and ventral spinules along distal margin, mid-ventral bell-shaped copulatory pore, duct and receptacles. Urosomite 3 with row of spinules ventrally inserted near posterior edge, row interrupted mid-ventrally. Urosomite 4 with continuous row of spinules ventrally near posterior margin. Anal somite (Figs 2B, C, 4A, B) with one pair of sensilla dorsally above base of anal operculum; seven to ten spinules (nine in holotype) ventrally near insertion of each caudal ramus; anal operculum concave and well-developed, with six strong spinules on free posterior margin.

Caudal ramus (Figs 2A–D, 4A, B) conical, about 2.0× longer than wide, inner margin unornamented; longitudinal keel located along dorso-inner margin, ending in acute tip. Ramus with seven setae (setae I–VII), all smooth, except setae IV and V. Anterolateral accessory seta (I) small, inserted near seta II. Anterolateral seta (II) inserted at two-quarters of ramus, accompanied by two spinules, about 1.5× longer than ramus length. Posterolateral seta (III) inserted at three-quarters of ramus, accompanied by two to three (two in holotype) spinules, about 1.3× longer than ramus length. Outer apical seta (IV) unipinnate, without a breaking plane, about 1.7× longer than ramus length. Inner apical seta (V) longest, bipinnate, without a breaking plane, more than 5.0× longer than ramus length. Inner accessory seta (VI) shortest, about as long as ramus length. Dorsal seta (VII) articulated, inserted on distal end of dorsal keel at three-quarters length of ramus, about 1.6× longer than ramus length.

Antennule (Fig. 3A) eight-segmented, with setal formulae as follow: 1(I), 8(II), 5(III), 1+a(IV) with conjoined bases, 1(V), 2(VI), 2(VII), 7 and 1+a(VIII) with conjoined bases (Arabic and Roman numerals refer to number of setae and segment number, respectively). Aesthetasc cylindrical; both aesthetascs fused at their bases with a seta forming an acrotheck. All setae slender, smooth.

Antenna (Fig. 3B) with small, unarmed coxa. Allobasis elongated, with two spinules on abexopodal margin. Exp one-segmented, with two inner and two apical unipinnate setae. Enp one-segmented, with two outer spines, one apical spine and five apical setae (two normal and three geniculated setae); inner and outer margins ornamented with two rows of spinules.

Mandible (Fig. 3C) with two large teeth, seven small teeth distally and one lateral seta on gnathobase, with a small knob on the disto-lateral margin. Palp one-segmented, with four apical setae.

Maxillule (Fig. 3D) with five apical spines and one anterior seta on praecoxal arthrite. Coxal endite with one spine and one seta apically. Basis with one spine, two setae apically. Exp and Enp reduced, represented by four lateral setae on basis.

Maxilla (Fig. 3E) with two endites on syncoxa, each endite with three apical setae. Basis elongated, drawn out into a claw, with one proximal accessory seta. Enp reduced and represented by two setae.

Maxilliped (Fig. 3F) with unarmed coxa, ornamented with row of spinules on median surface. Basis with two groups of spinules at inner and outer margins. Enp one-segmented, transformed into claw-like segment, accompanied by one small seta inserted proximally.

P1–P4 with three-segmented Exp and two-segmented Enp. The armature formula is as follows (Arabic and Roman numerals indicate number of setae and spines, respectively; not including spinules):

P1 (Fig. 5A) basis with short, strong outer spiniform spine and long, slender, smooth inner seta. Enp shorter than Exp; Enp-1 reaching to middle of Exp-2, with one inner unipinnate-tipped seta, ornamented with strong outer spinules and inner setules. Enp-2 with one smooth distal inner seta, one distal inner geniculated seta and one distal outer unipinnate seta; ornamented with strong outer spinules. Exp-1–2 with one distal outer spiniform spine; both segments ornamented with strong outer spinules. Exp-3 with one distal outer spiniform spine, two apical setae (inner seta geniculated, outer one unipinnate) and one distal inner geniculated seta; ornamented with few distal outer spinules.

P2 (Fig. 5B) basis with short, strong outer spine. Enp as long as Exp-1; Enp-1 shorter than wide, unarmed. Enp-2 with two elements apically; outer spine slightly longer than segment; inner seta bipinnate, long, extending beyond Exp. Exp-1 with one enlarged distal outer spine; ornamented with outer spinules. Exp-2 with one distal outer spine and one smooth distal inner seta. Exp-3 with two distal outer spines, two apical setae (inner seta bipinnate, outer one unipinnate) and one smooth inner seta; ornamented with disto-outer spinules.

P3 (Fig. 5C) basis with long, smooth, slender outer seta. Enp as long as Exp-1; Enp-1 shorter than wide, unarmed. Enp-2 with three elements apically; outer spine and innermost seta shorter than segment, subequal in length; middle seta bipinnate, reaching beyond Exp. Exp-1–2 with one distal outer spine, ornamented with spinules along disto-outer margin; Exp-2 with one additional smooth distal inner seta. Exp-3 with two distal outer spines, two apical setae (inner seta bipinnate, outer one unipinnate) and two inner setae (distal seta unipinnate, about 3.0× longer than proximal seta); ornamented with spinules along disto-outer margin.

P4 (Fig. 5D) basis and Exp as in P3, but Exp-3 shorter. Enp as long as Exp-1; Enp-1 small, shorter than wide, unarmed. Enp-2 with two bipinnate setae apically, both longer than segment; inner seta shorter than outer seta.

P5 (Fig. 5E) without ornamentation on surface. Basal seta smooth, slender. Baseoendopod separated from Exp, well-developed, exceeding Exp, with six spiniform setae; third inner seta longest apically; remaining setae decreasing in length to outer and inner margins of Enp. Exp sub-oval, with five setae, second inner seta longest; two innermost setae spiniform; three outer remaining setae smooth and decreasing in length to margin of Exp.

P6 (Fig. 4A) reduced to a single bipinnate seta inserted on the small plate, anterior to the seminal receptacle on the first half of genital double-somite.

Adult females with single egg sac containing 12–15 eggs (holotype: 12 eggs).

(allotype). Body length 510 µm (Fig. 2E–G) from rostrum to distal rim of caudal ramus, 470–590 µm (n = 5), smaller than female. Prosome (including antenna and mouthparts), anal somite and caudal ramus similar to those of female. Genital somite (Figs 2F, 6B) without row of posterior spinules; urosomites 3–5 with continuous posterior spinules along ventral to dorso-lateral sides.

Antennule (Figs 6–7A) ten-segmented, geniculated between segments 7 and 8. Segment 4 small, beneath segment 3. Setal formulae: 1(I), 7(II), 5(III), 2(IV), 3 and 1+a(V), 2(VI), 1(VII), 0(VIII), 0(IX), 7 and 1+a(X); aesthetasc on segments 5 and 10 fused to the base of seta, forming an acrotheck. All setae smooth.

P1, P4 (Figs 6B, 7D) and P2–3 Exp similar to those in female. P2 (Fig. 7B) Enp slightly longer than Exp-1; Enp-1 shorter than wide, unarmed. Enp-2 with long bipinnate seta apically, with two to three spinules on outer margin. P3 (Fig. 7C) Enp three-segmented; Enp-1 shorter than wide, unarmed. Enp-2 with thin inner apophysis with harpoon-like tip, long, exceeding beyond Exp. Enp-3 with two apical setae; outer seta bipinnate, longer than Exp; inner seta smooth, thin, shorter than segment. P4 (Fig. 7D) Enp two-segmented; Enp-1 small, shorter than wide, unarmed; Enp-2 shorter than Exp-1, about 2.5× longer than wide, with two bipinnate setae apically (outer seta longer than inner one; inner seta slightly longer than segment).

P5 (Figs 6C, 7E) separated from somite, baseoendopod of left and right sides fused medially. Basal seta long, slender and smooth. Baseoendopod separated from Exp, reaching one-half of Exp, with two spiniform setae; inner seta over 4.0× longer than outer seta. Exp with four setae; second inner seta longest, followed by second outer seta, innermost seta and outermost seta, respectively; two inner setae bipinnate, two outer setae smooth.

(a) The free distal margin of the anal operculum varies from six to ten spinules in females and six to eight spinules in males, a characteristic which is, perhaps, useless for differentiating amongst species, as mentioned by Gaviria and Defaye (2012). (b) The posterior margin of urosomite 3 in female has a continuous row of ventral spinules (Fig. 4A'; one of the other five examined specimens collected from the type locality). (c) The P2–P3 Enp-2 has a different number of spinules along the outer margin, with two spinules on P2 and three spinules on P3 in females (not shown in Figure; one of the other five female from the type locality) and with two spinules on P2 in males (Fig. 7B'; one of the other five examined specimens collected from the type locality). (d) The seta size of the P4 Enp-2 in both sexes, the outer seta about 2.0× longer than the inner seta in female and the inner seta longer than or equal in length to outer seta in male (Figs 5D', 7D'; two of the other five examined specimens collected from Pra Hor cave). (e) The female P5 in the southern population shows mostly shorter Exp and End setae (Fig. 5E'; two other females from Pra Hor cave).

Attheyella (C.) thailandica sp. nov. is usually found in pools of water at cave entrances, where water seeps through soil and plants before flowing down into the cave (see Fig. 1B, D, G). In this research, some samples were found in a pool at the twilight zone of a cave (Fig. 1C) and in a pool in the dark zone of a cave, which was directly fed by the dripping water from stalactites (Fig. 1F). The favourite habitats of A. (C.) thailandica sp. nov. in the present study are likely to be phytotelmata or wet soils, as is already known from most species of Attheyella (subgenus Canthosella) (Reid 2001). At times, A. (C.) thailandica sp. nov. has been found together with Bryocyclop muscicola (Menzel, 1926); both are recognised as stygophile species and are widely distributed in Thailand (Watiroyram 2018; present study).