Holotype ♂: Thailand, Nan Province, Pon, Thung Chang District, Tham Nam Lod, 19°25'13"N, 101°04'15"E, ca 1420 m a.s.l., 30.05.2018, S. Jantarit leg.; CUMZ-CAM171. Paratypes: 3 ♀: same locality as holotype; CUMZ-CAM171; 1 ♀: same locality as holotype; ZMUM; 1 ♀: same locality as holotype; NHM-PSU.

The species is so named to emphasize the long medial processes of the anterior and posterior gonopods; adjective.

This new species seems to be particularly similar to G. guangnanensis Jiang, Guo, Chen & Xie, 2018, from Yunnan Province, China (Jiang et al. 2018), with which it shares the following diagnostic characters: very strong metatergal crests and unique carinotaxic formulae, coupled with certain anterior and posterior gonopod structural details. Glyphiulus longus sp. nov. differs from G. guangnanensis by the smaller body size, 22–26 mm long (vs. larger, 38–54 mm long) and the carinotaxic formula of the collum: 1+2a+3c+4–5+6c+7a+pc+ma (vs. 1a+2c+3–4+5c+6a+pc+ma) (Fig. 1A, B), coupled with ♂ legs 1 showing very strongly reduced telopodites (Fig. 2C, D) (vs. nearly normal telopodites), the apicomesal processes (d) on the anterior gonopods being very long and slender (Fig. 2G, H) (vs. shorter and digitiform), and the presence of a very long, medially densely plumose and stout flagellum process (f) on the posterior gonopods (Fig. 2I, J) (vs. slim and smooth flagellum process).

Length of holotype ♂, 23.5 mm; that of paratypes, 22.8–26.5 mm (♀); midbody segments round in cross-section (Fig. 1F), their width (horizontal diameter) and height (vertical diameter) being similar; width of holotype ♂, 1.0 mm, of ♀ paratypes, 0.9–1.2 mm.

Coloration in alcohol (Fig. 1), after three years of preservation, uniformly red-brownish or dark castaneous brown to grey-brown, dorsal crests and porosteles usually dark brownish (Fig. 1A, B, D, E, H). Antennae and venter yellowish to pallid (Fig. 1A, C, E–G, I). Eyes blackish to brownish (Fig. 1A, C).

Body with 56p+2a+T rings/segments (♂ holotype); ♀ paratypes with 50–57p+2(1)a+T rings. Eye patches transversely ovoid, each composed of 2–5 rather flat ocelli in a single longitudinal row (Fig. 1A, C). Antennae short and clavate (Figs 1A, C, 2A), extending past ring 3 laterally, antennomeres 5 and 6 each with a small distoventral group or corolla of bacilliform sensilla (Fig. 2A). Gnathochilarium with a clearly separated promentum (Fig. 2B).

In width, head = ring 2 < collum = midbody rings (close to 13^th to 15^th) < ring 4 = 5 < 7 = 3 < 8 = 10; body abruptly tapering towards telson on a few posteriormost rings (Fig. 1B). Postcollar constriction very evident (Fig. 1B).

Collum nearly smooth, carinotaxic formula 1+2a+3c+4–5+6c+7a+pc+ma (Fig. 1A–C), with 6+6 longitudinal crests starting from anterior edge, but both median crests interrupted in about caudal 2/3–3/4, being replaced there by similar 1+1+1 crests.

Following metaterga very strongly crested (Fig. 1A, B, D–H), especially from ring 5 on, whence porosteles commence (Fig. 1A, B), smaller tubercles in their stead on legless rings in front of telson due to loss of ozopores (Fig. 1G, H). Porosteles large, conical, round, directed caudolaterad, rather higher than wide (Fig. 1A, B, D–H). Midway metatergal crests on ring 5 distinctly divided into two at about 1/3 metatergal height, each half evident and well rounded, nearly undivided small tubercles in their stead in legless rings in front of telson (Fig. 1G, H). Carinotaxic formulae 2/2+I/i+3/3+I/i+2/2, all dorsal crests subdivided transversely and sharper, especially so lateral crests (Fig. 1B, D–H).

Tegument rather smooth, shining throughout (Fig. 1A, B, D–H). Fine longitudinal striations in front of stricture between pro- and metazonae, remaining surface of prozonae very delicately shagreened (Fig. 1D, E). Metatergal setae absent. Rings 2 and 3 each with long pleural flaps. Epiproct (Fig. 1G–I) simple, with a long dorsal crest in middle and small paramedian tubercles, regularly rounded caudally, faintly convex medially. Paraprocts regularly convex, each with premarginal sulci medially and a row of sparse setae at medial margin (Fig. 1I). Hypoproct transversely bean-shaped, slightly concave caudally, with 1+1 strongly separated setae near caudal margin (Fig. 1I).

Ventral flaps behind gonopod aperture on ♂ ring 7 barely distinguishable as low swellings forming no marked transverse ridge.

Legs short, on midbody rings about 2/3 length of body height (Figs 1A, C, E–G, 2F). Claw at base with a very small accessory spine about 1/6 the length of main claw (Fig. 2F).

♂ legs 1 highly characteristic (Fig. 2C, D) in being very strongly reduced, 5-segmented telopodites and a pair of large, subdigitiform, medially contiguous, but apically diverging coxal processes with groups of long and strong setae at base.

♂ legs 2 nearly normal (Fig. 2E), claw long and slender; only anteriorly, prefemur somewhat reduced, and femur abbreviated on oral face; penes broad, rounded, each with four or five strong setae distolaterally (Fig. 2E).

♂ legs 3 missing, lost.

Anterior gonopods (Fig. 2G, H) with a typical shield-like coxosternum, the latter modestly microsetose on caudal face (Fig. 2G), on each side with a very long, slender, chopstick-shaped, apicomesal process (d) and an evident subsecuriform process (s), tip narrowly rounded. Telopodite rather small, stout, movable, 1-segmented, lateral in position, with three or four strong apical setae and a field of microsetae at base (Fig. 2G, H), about as long as adjacent lateral corner of coxosternum, but lower than paramedian processes (d) of coxosternum.

Posterior gonopods (Fig. 2I, J) compact, broadly subquadrate, micropapillate medially on oral face; with a very long, medially densely plumose and stout flagellum process (f) (Fig. 2I, J); lamelliform lobe (l) higher than caudal piece of coxite, membranous, smooth, with an apical field of coniform microsetae laterally (Fig. 2J); each median and lateral part of coxite with dense, strong and curved setae (Fig. 2J).

This new species belongs to the javanicus-group which currently encompasses 32 described species (Golovatch et al. 2007b, 2011b, 2012a; Likhitrakarn et al. 2017; Jiang et al. 2017, 2018, 2020). The above new congener is the second one in this group to be reported from Thailand. Due to the absence of any potential morphological cave adaptations, i.e., of a longer body, of a lighter coloration, and of elongated femora and tarsi of walking legs (Liu et al. 2017), G. longus sp. nov. is only to be considered as troglophilic at most. The cave where the species was discovered is located at an elevation of ca 1420 m a.s.l. in Nan Province. The cave is rather narrow, hidden in a steep-sided doline in a riparian area. It consists of a streamway and a series of oxbow passages, with many links between these two parts to the cave. The areas where the cave was developed are surrounded by agricultural landscapes, i.e., cabbage and corn fields. The specimens were found on the cave floor with scattered bat guano and some debris brought in by the stream, ca 50 meters from the entrance. The temperature of the cave was 20.8–22.4 °C (mid-May), and the relative humidity was 85–90%. In the same cave and habitat, we also found Collembola (Coecobrya sp. and Lepidocyrtus sp.), Formicidae (Ponera sp.), Thelyphonida, mites and pseudoscorpions.

Holotype ♂: Thailand, Nan Province, Na Noi District, Tham Chetawan, 18°16'26"N, 100°34'43"E; 520 m a.s.l., 18.05.2018, S. Jantarit leg.; CUMZ-CAM169. Paratypes: 5 ♂, 5 ♀: same locality as holotype; CUMZ-CAM169; 1 ♂, 1 ♀: same locality as holotype; ZMUM; 1 ♂, 1 ♀: same locality as holotype; NHM-PSU; 1 ♂, 3 juv.: same Province, Na Muen District, Tham La-ong, 18°14'18"N, 100°34'55"E, 648 m a.s.l., 18.05.2018, S. Jantarit leg.; CUMZ-CAM170; 3 ♂, 2 ♀: Phrae Province, Rong Kwang District, Tham Pha Phrai Wan, 18°25'18"N, 100°28'10"E, 419 m a.s.l., 17.05.2019, S. Jantarit leg.; CUMZ-CAM166.

The species is so named to honour Mr. Rueangrit Promdam, a carcinologist and researcher at the Princess Maha Chakri Sirindhorn Natural History Museum of Prince of Songkla University (NHM-PSU), who is interested in cave fauna in the country and who has collected many millipedes, including this new species, from various Thai caves.

This new species seems to be particularly similar to G. subbedosae Likhitrakarn, Golovatch & Panha, 2017, from Laos (Likhitrakarn et al. 2017), with which it shares the following diagnostic characters: body size, colour pattern and unique carinotaxic formulae, coupled with certain anterior and posterior gonopod structural details. It differs from G. subbedosae primarily by a uniformly yellow collum (Fig. 3A–C) (vs. its anterior half darker) and the median and lateral parts of the posterior gonopod coxite with 10–12 strong setae (Fig. 4K) (vs. six strong setae), coupled with the anterior gonopod coxosternum being moderately microsetose in the anterior and medial parts on the caudal face (Fig. 4H) (vs. only in medial part).

Length of holotype, 21.8 mm; adult paratypes 14.5–18.4 (♂) or 14.2–24.3 mm long (♀), juveniles 6.5–12.3 mm long; midbody rings round in cross-section (Fig. 5L), their width (horizontal diameter) and height (vertical diameter) being similar; width in holotype, 0.9 mm; in paratypes, 0.7–1.0 (♂), 0.8–1.1 (♀) or 0.5–0.7 mm (juveniles).

Coloration in alcohol (Fig. 3), after three years of preservation, uniformly red-yellownish to grey-brown, dorsal crests and porosteles usually dark brownish (Fig. 3A, B, D, E, H). Head, collum, antennae and venter yellowish to pallid (Fig. 3A, C, E–G, I). Eyes blackish to brownish (Fig. 3A, C).

Body with 58p+1a+T rings (♂ holotype); paratypes with 35–58p+1–2a+T (♂), 39–51p+1–3a+T (♀) or 25–35+2–5a+T (juveniles). Eye patches transversely ovoid, each composed of 8–13 blackish, rather flat ocelli in three or four irregular longitudinal rows (Fig. 3A). Antennae short and clavate (Figs 3A, C, 4A), extending past ring 3 laterally, antennomeres 5 and 6 each with a small distoventral group or corolla of bacilliform sensilla (Fig. 4A). Gnathochilarium with a clearly separated promentum (Fig. 4B).

In width, head = ring 4 = 5 < 6 < 7 < 3 < midbody rings (close to 8^th to 10^th) < 2 < collum; body abruptly tapering towards telson on a few posteriormost rings (Fig. 3B). Postcollar constriction very evident (Fig. 3B).

Collum nearly smooth, carinotaxic formula 1–6+7a+pc+ma (Fig. 3A–C), with 7+7 longitudinal crests starting from anterior edge, but both median crests interrupted in about caudal 2/3–3/4, being replaced there by similar 1+1+1 crests.

Following metaterga similarly strongly crested (Fig. 3A, B, D–H), especially from ring 5 on, whence porosteles commence (Fig. 3A, B), smaller tubercles in their stead on legless rings in front of telson due to loss of ozopores (Fig. 3G, H). Porosteles large, conical, round, directed caudolaterad, wider than high. Midway metatergal crests on ring 5 distinctly divided into two at about 1/3 metatergal height, each half evident and well rounded, nearly undivided small tubercles in their stead in legless rings in front of telson (Fig. 3G, H). Carinotaxic formulae 3+I/i+3/3+I/i+3 on rings 2–4, as well as on the last one or two leg-bearing, and on legless rings (Fig. 3A, B, G, H); midbody rings showing mostly dorsal crests distinctly divided into two at about 1/3 metatergal height, each half rather evident and well rounded (carinotaxic formulae 3/3+I/i+4/3+I/i+3/3) and sharper, especially so lateral crests (Fig. 3D, E).

Tegument rather smooth, dull throughout (Fig. 3A, B, D, E, G, H). Fine longitudinal striations in front of stricture between pro- and metazonae, remaining surface of prozonae very delicately shagreened (Fig. 3D, E). Metatergal setae absent. Rings 2 and 3 each with long pleural flaps (Fig. 3A). Epiproct (Fig. 3G, I) simple, regularly rounded caudally, faintly convex medially. Paraprocts regularly convex, each with premarginal sulci medially and a row of sparse setae at medial margin (Fig. 3I). Hypoproct transversely bean-shaped, slightly concave caudally, with 1+1 strongly separated setae near caudal margin (Fig. 3I).

Ventral flaps behind gonopod aperture on ♂ ring 7 distinguishable as low swellings forming a bare transverse ridge.

Legs rather short, on midbody rings about half the length of ring height (Figs 3A, C, E–G, 4G). Claw at base with an evident accessory spine about 1/3–1/4 the length of main claw (Fig. 4G).

♂ legs 1 highly characteristic (Fig. 4C, D) in being very strongly reduced, represented only by a sternum devoid of any median or paramedian structures, but carrying 1+1 strongly separated prongs, both evidently curved posteriad and bearing several strong setae, and rudimentary, 1-segmented leg vestiges at base on caudal face (Fig. 4C, D).

♂ legs 2 nearly normal (Figs 4E), prefemur somewhat reduced only anteriorly; penes broad, oblong-subtrapeziform, each with 3–5 strong setae distolaterally (Fig. 4E).

♂ legs 3 modified in having coxa especially slender and elongate (Fig. 4F).

Anterior gonopods (Fig. 4H, J) with a typical shield-like coxosternum, the latter moderately microsetose in anterior and medial parts on caudal face (Fig. 4H), with a high, digitiform, apicomesal process (d). Telopodite rather small, movable, 1-segmented, lateral in position, with two or three strong apical setae and a field of microsetae at base (Fig. 4H), moderately higher than adjacent lateral corner of coxosternum.

Posterior gonopods (Fig. 4J, K) compact, broadly subquadrate, micropapillate medially on oral face; coxite medio-apically with a long, plumose, apical flagellum (f) with evident spikes paramedially (Fig. 4J, K); lamelliform lobe (l) high, subquadrate, membranous, wrinkled frontolaterally, with an apical field of coniform microsetae laterally (Fig. 4K); each mediolateral part of coxite with 10–12 strong and curved setae (Fig. 4K).

The granulatus-group currently encompasses 36 described species, including our new species (Golovatch et al. 2007a, 2011a; Likhitrakarn et al. 2017; Liu and Wynne 2019). The new species is the fifth in this group to be reported from Thailand.

Three populations have been collected inside caves, with the longest distance of about 25 air-km between the collecting localities, and all show similar morphological characters as described above. Glyphiulus promdami sp. nov. fails to show any morphological adaptations to cave life and is considered here as a troglophilic species. It appears to have a rather narrow distribution, but has been found in a wide range of cave environments from the twilight (Tham Chetawan) to the dark and deep zones of the caves (all three caves). The temperature in the caves where the species was collected ranged between 24.2 and 29.8 °C, while the relative humidity was 70–94%. All populations were found to be quite large and associated with bat guano.