Holotype: male (IEBR-Myr 710H), Vietnam, Lao Cai Province, Hoang Lien National Park, natural forest, 22.32250°N, 103.77081°E, 2478 m a.s.l., 7 July 2018, coll. Nguyen Dac-Dai. Paratype: 1 female (IEBR-Myr 710P), together with holotype.

The species is easily recognized by having a black sub-moniliform body, small and crest-shaped paraterga, a long and slender gonofemorite, a simple solenophore with neither modifications nor additional processes, a poorly developed lamina medialis, a well-developed lamina lateralis with a tongue-shaped process, and the gonopod tip with a broad apical lobule.

Holotype body length about 22.1 mm, width of pro- and metazona 1.6 mm and 2.1 mm, respectively.

Colouration (Figs 1–2): body black except whitish yellow legs and sterna; several podomeres light brown.

Head (Fig. 1A, B) somewhat larger than collum, clypeolabral region densely setose, vertex sparsely setose. Epicranial suture distinct; frons with 2+2 setae along epicranial suture. Antenna long, slender, reaching segment 3 if stretched laterally; antennomere 2>3=4=5=6>1>7; tip with four sensory cones.

Collum (Fig. 1B) semicircular; surface smooth, shining with two rows of setae: 4+4 anterior and 1+1 intermediate; transverse sulcus present, but short, indistinct. Paratergum present, crest-shaped.

Body sub-moniliform (Figs 1C, D, 2C). Surface smooth and shining, without metatergal setae. Transverse metatergal sulci deep, line-shaped and present on all segments. Waist between pro- and metazonae relatively deep, striolate. Paraterga present, but small, crest-shaped, lying lower than metatergal surface. Pleurosternal carinae present as complete crests on segments 2–3, reduced to caudal teeth on segments 4–7, and missing on subsequent segments. Axial line missing.

Epiproct (Fig. 2B–D) broadly truncated, flattened dorsoventrally; tip with four spinnerets. Hypoproct subtriangular, with two distolateral, separated setiferous knobs (Fig. 2D).

Sterna sparsely setose, with distinct cross-impressions, without modifications except for a setiferous, broadly tongue-shaped lobule between coxae 4 (Fig. 3A, B).

Legs slender, long about 1.6–1.8 times as long as midbody height. Femora without modifications. Prefemora not swollen. Tarsal brushes absent.

Gonopods simple (Figs 3C–F, 4, 5). Coxite long, cylindrical, distoventral part sparsely setose. Prefemorite densely setose, set off from femorite by an oblique sulcus laterally. Femorite long, slender, without processes or modifications. The demarcation between postfemoral region and femorite present laterally. Lamina medialis of solenophore poorly developed while lamina lateralis well developed, with a tuberculiform process. Tip of gonopod broadly rounded lobule. Seminal groove running entirely on mesal side of femorite, then entering a flagelliform solenomere sheathed by solenophore.

Fragments of COI and 16S rRNA genes were uploaded to GenBank with the accession numbers: MW647898 and MW648327, respectively. The new species has a close COI gene similarity with Orthomorphoides setosus (KU234720) of 87.17%. It also shares 74.04% and 72.68% of its 16S rRNA gene sequence with Asiomorpha coarctata (KU721885) and Pogonosternum nigrovirgatum (KU745218), respectively.

Named after Sapa, the type locality. It is a noun in apposition.

The genus Orthomorphoides was extracted from the genus Orthomorpha by Likhitrakarn et al. (2011) for two species: Orthomorpha setosa (Attems, 1937) and Orthomorpha exarata (Attems, 1953). The genus, therefore, currently contains only two species, Orthomorphoides setosus from Lam Dong Province (Vietnam) and Orthomorphoides exaratus from Xieng Khouang (Laos).

This new species obviously belongs to the genus Orthomorphoides because of its generic characters, such as: long and slender femorite without visible modifications and processes, and the solenomere being sheathed by a solenophore except for the exposed tip. The new species does, however, clearly differ from the two known Orthomorphoides species in body shape and the degree of development of the paraterga. In addition, the solenophore of the new species carries a tuberculiform process laterally while neither O. setosus nor O. exaratus possess additional processes on the solenophore.

The two previously described Orthomorphoides species were found only in high mountains in southcentral Vietnam and Xieng Khouang (>1500 m a.s.l.) (Attems 1953). The discovery of this new species, found in Sapa, may further support the mountainous distribution of this genus, although this must be confirmed with additional field studies and possible discoveries.

Holotype. male (IEBR-Myr 712), Vietnam, Lao Cai Province, Hoang Lien National Park, natural forest, 22.32250°N, -103.77081°E, 2478 m a.s.l., 7 July 2018, coll. Nguyen Dac-Dai. Paratype. 1 male, 1 female (IEBR-Myr 714), Lao Cai Province, Hoang Lien National Park, natural forest, 22.32129°N, -103.77094°E, 2547 m a.s.l., 7 July 2018, coll. Nguyen Dac-Dai.

The species differs from its congeners by having wing paraterga, two rows of 2+2 and 2+2 knobs on the metaterga, and a well-developed, broad gonopod solenophore with an additional distal process.

Holotype body length about 16.0 mm, width of pro- and metazona 1.2 mm and 2.0 mm, respectively.

Coloration (Figs 6, 7): whole body blackish brown or black, except labrum, sterna and legs yellowish brown. Projected caudal corners of paraterga yellowish brown.

Head (Fig. 6C, D) slightly smaller than body segments, but somewhat larger than collum. Epicranial suture clearly distinct; frons with 2+2 setae along epicranial suture. Clypeolabral region moderately setose. Antenna extremely long, reaching segment 5 if stretched laterally; antennomere 3=4=5>2>6>1>7; tip with four sensory cones.

Collum (Fig. 6A) slightly smaller than segment 2, semicircular; surface not smooth, with 3 rows of setiferous knobs: 3+3 anterior, 2+2 intermediate and 1+1 posterior; transverse sulcus present, located near a row of 1+1 posterior knobs. Paratergum broadly triangular, well developed, with a lateral incision (Fig. 6A).

Body segment 3<4<2=5–17, thereafter gradually tapering toward telson (Figs 6A, B, 7A–C). Prozonae smooth, shagreened while metazonae densely covered with microgranulation. Transverse metatergal sulci shallow, broad, present on all segment. Metaterga 2–4 with two rows of setiferous knobs: 2+2 and 2+2 in front of and behind transverse sulcus, respectively; other metaterga with two rows of 2+2 and 3+3 setiferous knobs. Axial line distinct, thin. Waist between pro- and metazonae indistinct, shallow, and broad. Pleurosternal carinae absent.

Paraterga (Figs 6A, 7A–C)) wing-shaped, with pointed, projected caudal corners, lying horizonally, but reduced as broad-base spine with two lateral incisions on segments 7–18.

Epiproct long, broadly truncated, flattened dorsoventrally; tip with four spinnerets (Fig. 7D). Hypoproct trapeziform, with two distolateral, separated setiferous knobs (Fig. 7D).

Sterna sparsely setose, with distinct cross-impressions, without modifications except for an anterioventrally directed, large, strongly bi-lobuled process between coxae 4 (Fig. 8A, B).

Legs slender, long about 1.5–1.7 times as long as midbody height. Prefemora not swollen. Femora without modifications. Tarsal brushes absent.

Gonopods simple (Figs 8C, D, 9, 10). Coxite cylindrical, distoventral part sparsely setose. Prefemorite densely setose, set off from femorite by an oblique sulcus laterally. Femorite slightly curved mesad, somewhat enlarged distally, without modifications and processes. Demarcation between femorite and postfemoral region absent. Postfemoral region consisting of solenophore and solenomere. Femorite and solenophore subequal in length. Solenophore simple, broad, slightly spiral and somewhat curved down. Tip of gonopod broadly round, with an additional distoapical process. Seminal groove running entirely on mesal side of femorite, then entering the flagelliform solenomere sheathed by solenophore.

Fragments of COI and 16S rRNA were uploaded to GenBank with accession numbers: MW647899 and MW648328, respectively. The new species has a close COI identity with Hylomus cervarius (MG669370) and Hylomus enghoffi (MG669369) of 85.1% and 83.45%, respectively. It also shares 79.57% and 78.72% of its 16S rRNA gene sequence with Hylomus cervarius (MG564329) and Hylomus enghoffi (MG564330), respectively.

An epithet “solenophorus” is used to emphasize the well-developed, broad solenophore carrying a triangular distoapical process.

The genus Hylomus Cook & Loomis, 1924 used to be a synonym of the genus Desmoxytes Chamberlin, 1923 (Golovatch and Enghoff 1994), but it was recently re-evaluated by Srisonchai et al. (2018) and currently comprises 39 species (Sierwald and Spelda 2021). The new species differs distinctly from the majority of known Hylomus species (except for H. spectabilis) in having wing-shaped paraterga (vs. antler-like or spiniform paraterga). As already stated, Hylomus solenophorus sp. nov. is closely similar to H. spectabilis (Attems, 1937) from central Vietnam by having wing-shaped paraterga and the same gonopod conformation. On the other hand, the newly described species differs clearly from H. spectabilis by being smaller in size (length: 16 mm vs. 30 mm; width of metazona: 2.0 mm vs. 6.1 mm) and having darker body coloration (darkish brown or black vs. brown). Furthermore, leg femora of the new species possess no visible modifications, metaterga contain two rows of 2 + 2 and 3 + 3 setiferous knobs and the gonopod has no spine z, but possesses a triangular distoapical process, instead. By comparison, the 7^th femur of the H. spectabilis male has a ventral hump, metaterga are characterised by two transversal rows of 2 + 2 well-developed spines and 1 + 1 smaller spines and the gonopod is characterised by well-developed spine z, but no process k.