The family Glomeridesmidae was partly re-characterized recently (Iniesta et al. 2012; Enghoff et al. 2015).

Glomeridesmus porcellus Gervais & Goudot, 1844 (Colombia)

Javadesmus: Glomeridesmus javanicus Attems, 1907 (Indonesia: Java)

Zephroniodesmus: Glomeridesmus sumatranus Pocock, 1894 (Indonesia: Sumatra).

30, including the species described below (Jeekel 2003; Iniesta et al. 2012; Mauriès 2020).

Central America and northern half of South America and the Caribbean (26); India (1); Indonesia (2); Thailand (1 described below). Unidentified specimens from various localities in SE Asia and Oceania (Shelley 2011).

The position of the new species in the genus Glomeridesmus is tentative at best. As two genus names (Zephroniodesmus, Javadesmus) synonymized with Glomeridesmus (Loomis 1936, Carl 1942) are available for SE Asian Glomeridesmidae and were based on female specimens, both require redescriptions based on topotypic male material. Therefore, we hesitate to introduce a third genus name, which might later turn out to be synonymous. The species from Thailand described here differs significantly from the known Indian and American species at least in the telopod morphology, and potentially in other previously undocumented morphological characters.

Holotype: 1 M, ZCSWU Myr D000011 (THAI11), Thailand, Krabi Province, N. of Krabi Town, western aspect of Tiger Cave temple (Wat Tham Suea), overgrown rocks next to rubber plantation, 08°07'23.8"N, 098°55'18.9"E. leg. 27.VII.2017, Wesener, Wongthamwanich, Nawanetiwong, Moritz.

Paratypes: 1 immature M, ZFMK MYR10301, same data as holotype; 1 F, ZFMK MYR10302; 5 juveniles, ZCSWU Myr D000012–16; 4 juveniles, ZFMK MYR10303–10306, same data as holotype.

Siamensis, noun in apposition, after the type locality in Thailand.

Small (5–7 mm) dark grey glomeridesmid with white legs (Fig. 1A, B). Antennae short (Figs 1A, B, 2A–C). Coxal pouches starting at leg 9, all walking legs with apical spines and paronychium (Figs 3A–F, 4A–C). Last pleurite posterior margin well-rounded in female. Male sensory leg, podomere 1 with a conspicuous excavation on its mesal margin, not known from other Glomeridesmus (Fig. 3G, H). Male telopods unique: Inner horns slender, widely separated, a character only shared with G. indus Carl, 1942. Telopod, podomere 2 rectangular, with a unique, strongly elongated and slender process protruding between movable (podomere 4) and immovable finger (process of podomere 3) (Fig. 5A, B). Podomere 3 with a slender immovable projection and podomere 4 forming a long movable finger. Immovable finger long and visibly laterally in G. siamensis sp. nov. (Figs 1C, 5A, B), in contrast to South American species such as G. spelaeus Iniesta & Wesener, 2012 and G. arcostriatus Mauriès, 2020, where it is hidden in lateral view. Lamellae linguales of gnathochilarium fused with one another, but with a transverse suture towards mentum, lamellae with two central setae at margin and two lateral setae more posteriorly in G. siamensis sp. nov. (Fig. 6A), while all four setae are marginal in G. spelaeus (compare Iniesta et al. 2012).

Based on male holotype (ZCSWU Myr D000011) and female paratype (ZFMK MYR10302).

Measurements: Largest adult female (20+AS tergites, 35+1 leg pairs): length: ~6 mm; width (midbody): 1.0 mm. Male (19+AS tergites, 33+1+T leg pairs, holotype male): length: ~5 mm; width (midbody): 0.9 mm; immature (?) male (19+AS tergites, 32+1+T,): length: ~4 mm, width: 0.8 mm.

Colour in ethanol: Tergites and dorsal side of head grayish-brown (Fig. 1), legs, antennae and ventral side whitish (Fig. 1A, B).

Head: General shape typical for the family (Figs 1D, E, 2C). Epicranium glabrous; frons, clypeus and labrum with several isolated setae (Fig. 2C). Antennae widely separated by a distance slightly longer than twice width of first antennomere (Fig. 1A, E). Antennae and organ of Tömösváry both surrounded by individual cuticular rims, both on an elevated plateau (Fig. 2D). Genae (area below the antennae) almost non-existent, with incisura lateralis. Undivided basal joint of mandible large and visible in dorsal view.

Antennae consisting of seven joints, each covered with numerous setae. Size of joints 1 = 2 = 3 = 4 = 5 > 6 (Fig. 2A). Apical disc carrying four long apical cones; sensilla basiconica apparently absent (Fig. 2B).

Tömösváry organs as large as antennal base, of well-rounded oval shape, interior covered by a sclerotized plate; surrounded by cuticular rim and several very short setae (Fig. 2C, D).

Gnathochilarium with very broad gula (hypostoma). Cardines very small, separated from basal part of gnathochilarium. Proximal fourth of mentum towards gula distinctly elevated, transverse suture visible between mentum and lamellae linguales. Lamellae linguales fused to one another, not separated by a longitudinal suture, apically with four long setae, central pair at apical margin, lateral two setae located more posteriorly. Whole surface of gnathochilarium covered with few, isolated setae. Stipites laterally without sclerotized ledge. Stipites apically with 2 extra-long setae (Fig. 6A). Stipites lateral palpi slender, carrying 4 slender sensory cones; inner palpi very wide, carrying 12–15 long, tube-shaped sensory cones interspersed between long cuticular fibres (Fig. 6B). Central pads (modified central palpi of lamellae linguales) large, mesally pointing towards and touching one another, in ventral view covered with numerous long, tube-shaped sensory cones and long cuticular fibers (Fig. 6B). Basally of central pads on endochilarium with two parallel rows of 9 sensory cones along median furrow (Fig. 6C).

Mandible: Basal joint massive, undivided (Figs 1D, E, 2C). Apical joint (mandibular gnathal lobe) damaged during preparation, but with long slender external tooth and a long condylus. Inner tooth 4-combed. Seven rows of pectinate lamellae, large intermediate area covered by scale-like spines. Additional intermediate area located at a lower level next to molar plate with more elongated spine-plates. Molar plate plane, without indentations or grooves, margins entirely surrounded by thick membranous fringes (Fig. 6D).

Trunk: Collum (tergite 1) wider than head, shape similar to following tergites (Fig. 1A).

Tergites 2–19 (20) very thin, difficult to distinguish from one another. Width from tergite 1 to 8 gradually increasing, then decreasing to last tergite (Figs 1A, B, 7A). Each tergite bearing 6–8 longitudinal striae. Distances between striae increasing towards posterior margin. Surface of tergites with sparse isolated setae (Fig. 7A, B). Tergite 11 onwards posterior-lateral edge pronounced into a sharp-edged tip, becoming a longer and spine-like process at tergite 17. Limbus (posterior margin of tergite) smooth, without any structures. Endotergum (underside of posterior margin of tergite) smooth area without any specific structures (Fig. 7C). Posterior margin of tergite 19 and 20 with several short teeth (Fig. 7D)

Pleurites of rectangular shape, posterior margin overlapping first third of subsequent pleurite (Fig. 7B), wider than long. Pleurites similar to one another, but last pleurite on posterior margin with a well-rounded central indentation, providing space for the movements of the last leg.

Legs: In all except first coxae fused with stigmatic plates (typical for Glomeridesmida, Wesener et al. 2014) (Figs 3A–H, 4A–C). Starting at coxo-stigmatic plate 9 (male holotype) or 11 (female paratype), every second coxa up to pair 21 (male) or 25 (female) apically with an eversible coxal pouch posteriorly of prefemur (Fig. 4B).

Leg-pair 1 with a well-rounded elliptical sternite visible in oral view. Femur 1.5 × longer than wide, tarsus slender and elongated, >7 times longer than wide (Fig. 3A). Apical half of tarsus without any spines or setae, except 1 or 2 apical spines, claw and paronychium (Fig. 3A, B).

Leg 2 (male) femur 1.7, tarsus >10 × longer than wide (Fig. 3C). Tarsus in apical half without any spines or setae except for single apical spine, claw and paronychium (Fig. 3D).

Legs 3–32 similar to first legs, e.g. midbody leg femur 1.8, tarsus >7 times longer than wide. Tarsus with 1 or 2 apical spines, claw and paronychium (Fig. 3D).

Penultimate leg with coxosternite narrow, with stigma opening and sternal part located below (instead of lateral to) of coxal part. Femur slightly longer than prefemur, 1.3 × longer than wide, tarsus shorter, >7 × longer than wide, apically with claw, apical spine and paronychium (Fig. 3E, F).

Ultimate leg pair (sensory leg) modified, consisting of a free large sternite, fused coxa-stigmatic plates plus 3 podomeres (Fig. 3G). First podomere with a conspicuous mesal bend. Last two podomeres extending postero-laterally (confused with telopod by pre-1941 authors). Podomere 3 apically with a large, long spine, longer than the podomere (Fig. 3H). Both legs widely separated from one another, connected by large sternite. Coxa-stigmatic plates large, almost as long as all 3 podomeres combined (Fig. 3G).

Anal shield: glabrous, with a well-rounded edge. Subanal plate located behind last pair of legs, large and hyaline.

Sexual characters: Female: second coxae on posterior side with prominent ovipositors protruding back to leg pair 5. Coxa protruding mesally as a short lobe. Ovipositors basally supported by an undivided plate (sternite?) (Fig. 4C). Ovipositor with a basal part consisting of 21 or 22 segments of (eversible?) half-rings, each carrying isolated setae in a regular distance to each other. Half rings anteriorly and posteriorly interrupted by two different, much slender tubes, also consisting of half-rings, running along the whole length of the ovipositor. Apical part of ovipositor with opening surrounded by four larger plates, each arising out of a tube of rings. All four plates covered by longer setae (Fig. 4D).

Male: second coxa with gonopore located mesally (Fig. 3C). Gonopore flanked by two sclerotized, plates resembling an open bivalve shell. Apical part of both plates with 2 or 3 longer setae (Fig. 5C).

Male telopod consisting of syncoxite with inner horns and 4 podomeres (Figs 1C, 5A, D). Syncoxite covering basal podomeres laterally and in anterior view, medially rising into a smooth glabrous process. Process apically with two long inner horns; horns basally fused, completely separating more apically, diverging and running parallel to one another. Each horn apically slightly widening and spoon-shaped. Whole surface of horns sparsely covered by isolated, minute setae (Fig. 5D).

Telopod podomere 1 largest and most massive.

Podomere 2 wide, rectangular, apically with a large membranous area and mesally with a long and slender process. Membranous area well rounded, consisting of several inverse membranous folds. Slender process strongly elongated, as long as podomere 2, protruding between movable and immovable finger, inner surface excavated (Fig. 5A, B).

Podomere 3 long and slender, only 1/3 of width of podomere 2, inserting laterally on podomere 2. Posterior-lateral aspect of podomere 3 in apical part with slender, finger-shaped and well-rounded process called immobile finger, which makes up ca. half the length of podomere 3, inner surface of immobile finger excavated. Length of podomere 3, without immobile finger, 2 times longer than wide, with few isolated setae at margins in apical half (Fig. 5A, B). Immobile finger protruding up to 3/4 of length of podomere 4, called mobile finger (Fig. 5A, B).

Mobile finger (podomere 4), articulated to podomere 3 at base of immovable finger (Fig. 5A, B), slender and glabrous, 5 × longer than wide, posteriorly with an excavated area; at apex with a single long spine reaching 2/3 of length of immobile finger

The immature male has the same number of tergites, but one walking leg pair less than the mature male. The immature male is almost 1/3 shorter than the mature male. Coxal pouches starting at leg pair 9 as in mature male. Last pleurite with small well-rounded indentation at posterior margin. Telopod only slightly different from mature male: process of podomere 2 shorter, podomere 3 and 4 slenderer, immobile finger almost protruding up to apical end of movable finger.

All specimens were collected by hand in the morning during rainy season in Krabi province, the second reported area for Glomeridesmida in Thailand (the first in Shelley 2011). They were found in a karst landscape covered with evergreen forest (Fig. 8A, B) in soils at a depth of 10–20 mm near horizontal roots on a partially overgrown rock. Despite numerous attempts, no Glomeridesmus was discovered outside the less than 30×30 centimeter of habitat, showing that their distribution might be dependent on some unknown microhabitat requirements. The habitat was a flat area on a western aspect (Fig. 8C). Average air temperature and relative humidity during observation was approximately 29 °C and 85%, respectively. The specimens were fast moving (at least as fast as Chordeumatida), capable of bending their flexible body 180° to quickly change directions. Furthermore, the male holotype carried an ‘amphoromorph’ fungus on one of its legs (Fig. 3B).

2 imm. F; NHMD; Chiang Mai Province, Doi Suthep N. P.; Konthathan (Montha Than waterfall?); 600–700 m; leg. NHMD, 30.ix.1981. 1 imm. F.; NHMD; Chiang Mai Province, Doi Suthep N. P.; Montha Than; 600–700 m; leg. NHMD, 26.ix.1981. 1 juv.; NHMD; Doi Suthep, 1150 m; Lok.3a; 29/9.1958; Birgit Degerbøl leg. Zool. Mus. Kbh. 1/7 59. Doi Suthep, 11–1200 m. 1 juv. (16 pairs of legs); NHMD; Doi Suthep, No. Kasens DSal; 14/9.1959, Birgit Degerbøl leg., Zool. Mus. Kbh. 1/7 59. 1 mature F (with nemertini parasite); NHMD; Doi Suthep, No. sigteprøve (= sifting sample) 3a?; 2/8.1958, Birgit Degerbøl leg., Zool. Mus. Kbh. 1/7 59. 1 juv. (8 pairs of legs); NHMD; Doi Suthep, 1150 m; No. Lok.3a; 29/9.1958; Birgit Degerbøl leg., Zool. Mus. Kbh. 1/7 59.