Holotype. Vietnam • 1 male; Cao Bang Province, Pia Oac - Pia Den National Park; 22.5943°N, 105.8846°E; 1200 m a.s.l.; 9 May 2021; Anh D. Nguyen leg.; bushes; IEBR-Myr 904H. Paratypes. Vietnam • 10 males, 5 females; same data as for holotype; EBR-Myr 904P • 1 male, 1 female; same data as for holotype; IEBR-Myr 901.

The species can be discriminated from the congeners by the presence of long spiniform paraterga, midbody metaterga with only two rows of 1+1 small setiferous spines in the middle and 2+2 longer setiferous spines near posterior margin; male femora 6 with a large tubercle ventrally; sternite 5 with a large, sparsely setose, bifid, trapeziform lamina between male coxae 4; epiproct without conspicuous setiferous knobs near tip; gonopod lamina lateralis broadly rounded, partly folded to sheathe distal part of solenomere; gonopod lamina medialis with a small rounded lobe at middle.

The new species is similar to Hylomus namek Nguyen et al., 2019 but differs from this species by the following characters: the ventral side of male femora 6 with a big, robust tubercle in the middle (vs. femora 6 and 7 with large tubercles); longer spiniform paraterga; and gonopod lamina medialis with a small rounded lobe in the middle (vs. without processes).

The name refers to “piccolo”, a main character of the Japanese manga “Dragon balls” by Toriyama Akira (Japan). Noun in apposition.

Length c. 11.6–12.8 mm (male), 13.9–15.2 mm (female); width of midbody pro- and metazona (distance between two paratergal tips) 0.6–0.8 mm (male), 1.12–1.25 mm (female) and 3.1–3.3 mm (male), 3.4–3.6 mm (female), respectively. Holotype length c. 11.8 mm, width of midbody pro- and metazona 0.74 mm and 3.2 mm, respectively.

Coloration : Generally darkish-red to darkish-brown except paraterga, sterna, legs whitish-yellow; distal part of main branch pinkish.

Head (Fig. 1A–C): Clypeolabral region densely setose, vertex sparsely setose. Epicranial suture distinct, dividing frons into two equal parts; with setae along the suture. Antenna slender, extremely long, reaching back to body ring 7 if stretched along the body axis; antennomere 1 < 7 < 6 < 2 < 3 = 4 = 5 in length.

Collum (Fig. 1C): Slightly narrower than head; surface dull, coarsely microgranulate; with two rows of spines: 3+3 spines in anterior row and 2+2 spines in posterior row. Paraterga of collum well developed, spiniform; directed dorsad; highly elevated above dorsal surface; with two conspicuous teeth on anterior side.

Body rings : Ring 3 < 4 < 2 = 5–16 in width, thereafter gradually tapering towards telson. Prozona finely shagreened; metazona and pleura with microgranulations. Transverse sulcus present, but inconspicuous on metaterga 5–18. Axial line missing. Metaterga with two rows of 1+1 smaller setiferous spines in middle and 2+2 longer setiferous spines near posterior margin (Fig. 2B). Suture between pro- and metazona broad, very shallow. Pleurosternal carinae present as a complete keel on body rings 2–3, then missing on subsequent body rings.

Paraterga (Figs 1, 2): Very well developed; directed dorsad; long and spiniform with a large branch and 2–3 tiny teeth on anterior side (2 on poreless body rings and 3 on pore-bearing rings) and with 1 larger spine on posterior side. Paratergum on ring 19 directed caudad. Ozopore located between the second tooth and main branch of paraterga, visible in dorsal view.

Telson (Fig. 3A, B): Epiproct without conspicuous setiferous knobs near tip, but broadly truncated; tip with four spinnerets; lateral tubercles well developed (Fig. 3A). Hypoproct subtrapeziform, with two distolateral, completely separated, setiferous knobs (Fig. 3B).

Legs : Extremely long, slender and thin, c. 1.8–2.0 times as long as midbody height. Prefemora not swollen. Male femora 6 each ventrally with a large, robust tubercle in middle (Figs 1A, 3C).

Sterna : With distinct cross-impressions, no modification – except a large, sparsely setose, bifid, trapeziform lamina between male coxae 4 (Figs 1A, 3D).

Gonopod (Fig. 4): Suberect. Coxite (co) cylindrical, larger than femorite, sparsely setose distodorsally. Prefemorite (pref) densely setose, equal to femorite as well. Femorite (fe) slightly enlarged distad (from ventral view); without a demarcation with postfemoral region. Postfemoral region inconspicuous. Solenophore (sph) well developed; lamina lateralis (ll) broadly rounded, partly folded to sheathe distal part of solenomere; lamina medialis (lm) with a small rounded lobe at about midway. Seminal groove running entirely on mesal side, then entering a flagelliform solenomere (sl), solenomere partly sheathed by solenophore. Tip of solenophore rounded.

(Appendix 1) A fragment of the COI gene is accessioned at NCBI GenBank with the following accession number PQ676351. The new species exhibits a COI gene similarity of 86.22% and 86.57% (OR765910 and MG669370, respectively) identity with Hylomus cervarius (Attems, 1953).

Holotype. Vietnam • 1 male; Cao Bang Province, Pia Oac - Pia Den National Park, on the way to Hang Ong; 22.5540°N, 105.8622°E; 850m a.s.l.; 8 May 2021; Anh D. Nguyen leg.; bushes; IEBR-Myr 908H. Paratypes. Vietnam • 2 females; same data as for holotype; IEBR-Myr 908P • 2 females; same data as for holotype; IEBR-Myr 906 • 2 females; same data as for the holotype, but 8 Jun. 2020; IEBR-Myr 851 • 1 female; same data as for sample IEBR-Myr 851; IEBR-Myr 854.

The species can be discriminated from the congeners by the presence of spiniform paraterga; metaterga densely covered with microgranulations; midbody metaterga with two rows of setiferous spines: 2+2 in anterior row and 2+2 near posterior margin, the anterior row hardly seen, the posterior row more distinct; male femora 6 each with a large tubercle ventrally; sternite 5 with a large, sparsely setose, bifid, trapeziform lamina between male coxae 4; epiproct with several evident setiferous knobs near tip; gonopod solenophore partly folded to sheathe distal part of solenomere; tip of solenophore consisting of seven overlapping laminae.

The new species is similar to H. proximus in body size and shape, but the two species are distinguished by the number of metatergal posterior spines (2+2 vs 3+3), male femoral modifications (femur 6 vs femora 5 & 6), and gonopod conformation. The new species has a well-developed gonopod solenophore (sph); a broadly rounded lamina medialis, partly folded to sheathe distal part of solenomere; and gonopod tip consisting of seven overlapping laminae while H. proximus has a gonopod femorite that is subequal to the postfemoral region in length; both solenophore and solenomere long; and a serrated solenophore tip.

Hylomus borealis sp. nov. is also similar to H. jeekeli (Golovatch & Enghoff, 1994) from northern Thailand in terms of general body and gonopod shape. However, the new species can be distinguished from it by the combination of these characters: smaller in size with 10.4 mm in males and 12.3–13.4 mm in females (vs 15–16 mm in males and 18–20 mm in females); metaterga with 2+2 spines in posterior rows (vs 3+3 spines); modification in only femur 6 (vs femora 6 and 7); tip of solenophore consisting of seven overlapping laminae and not serrate (vs serrated solenophore).

An adjective epithet “borealis” refers to the northern-most province (Cao Bang) of Vietnam, the type locality.

Holotype length 10.4 mm, width of mid pro- and metazona 0.6 mm and 1.8 mm (distance between two paratergal tips), respectively. Female length 12.3–13.4 mm, width of mid pro- and metazona 0.9–1.0 mm and 1.7–1.8 mm, respectively.

Coloration : Generally dark to castaneous brown except paratergal bases, sterna, leg coxae and prefemora whitish-yellow.

Head (Fig. 5A, B): Clypeolabral region densely setose, vertex sparsely setose. Epicranial suture distinct, dividing frons into two equal parts; with setae along suture. Antenna slender, extremely long, reaching to body ring 5 if stretched along the body axis; antennomere 1 < 7 < 6 < 2 < 3 = 4 = 5 in length.

Collum (Fig. 5B): Subequal to head in width; surface dull, coarsely and densely microgranulate, with three rows of spines: 3+3 spines in anterior row, 1+1 spines in intermediate row, and 2+2 spines in posterior row; all spines equal in size. Paratergum well developed; directed dorsad; highly elevated above dorsal surface; with two conspicuous teeth on anterior side.

Body rings : Rings 3 < 4 < 2 = 5–16 in width, thereafter gradually tapering towards telson. Prozona finely shagreened; metazona and pleura with microgranulations. Transverse sulcus present, but inconspicuous on metaterga 5–18. Axial line missing. Metaterga with two rows of setiferous spines: 2+2 spines in anterior row and 2+2 spines near posterior margin (Figs 5C, 6B), the anterior row hardly visible, the posterior row more distinct (Fig. 6B). Suture between pro- and metazona broad, very shallow. Pleurosternal carinae present as a complete keel on body rings 2–3, then missing on subsequent body rings.

Paraterga (Fig. 5): Very well developed; directed laterodorsad; antler-shaped with a large branch and 2 small teeth on anterior side and 1 smaller spine on posterior side. Ozopore located between the first tooth and main branch of paraterga, visible in dorsal view.

Telson (Fig. 6C): Epiproct with several evident setiferous knobs near tip; tip with four spinnerets; lateral tubercles well developed. Hypoproct sub-trapeziform, with two distolateral, completely separated, setiferous knobs.

Legs : Extremely long, slender and thin, c. 1.5–1.6 times as long as midbody height. Prefemora not swollen. Male femora 6 each ventrally with a large, robust tubercle in middle.

Sterna : with distinct cross-impression, no modification – except a large, sparsely setose, trapeziform lamina carrying two distal, separated lobes between male coxae 4 (Fig. 6D).

Gonopod (Fig. 7): Suberect. Coxite (co) cylindrical, much shorter than femorite, sparsely setose distodorsally. Prefemorite (pref) densely setose, shorter than femorite as well. Femorite (fe) slightly enlarged distad, without a demarcation with postfemoral region. Postfemoral region slightly twisted mesad. Solenophore (sph) well developed; lamina medialis (lm) broadly rounded, partly folded to sheathe distal part of solenomere, lamina lateralis (ll) well developed. Seminal groove running entirely on mesal side, then entering a flagelliform solenomere (sl) which is partly sheathed by solenophore. Tip of gonopod consisting of seven overlapping laminae (Fig. 7G).

(Appendix 1) A fragment of the COI gene is accessioned at NCBI GenBank with the following accession number PQ676352. The new species exhibits a COI gene similarity of 85.29% identity with Hylomus proximus Nguyen, Golovatch & Anichkin, 2005 (MG669371) and 83.11% identity with Desmoxytes takensis Srisonchai, Enghoff, Likhitrakarn & Panha, 2016 (OR765894).