Hylomus borealis, Nguyen & Vu & Nguyen, 2025

Nguyen, Anh D., Vu, Tam T. T. & Nguyen, Thu-Anh T., 2025, Mountainous millipedes in Vietnam. III. Two new dragon millipedes from limestone mountains in northern Vietnam (Polydesmida, Paradoxosomatidae, Hylomus), with an identification key to Vietnamese Hylomus species, ZooKeys 1223, pp. 247-262 : 247-262

Hylomus borealis sp. nov.

Figs 5 , 6 , 7

Material examined.

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; 850 m a. s. l.; 8 May 2021; Anh D. Nguyen leg.; bushes; IEBR-Myr 908 H . Paratypes. Vietnam • 2 females; same data as for holotype; IEBR-Myr 908 P • 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 .

Diagnosis.

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).

Etymology.

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

Description.

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. 5 A, 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. 5 B ): 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 5 C , 6 B ), the anterior row hardly visible, the posterior row more distinct (Fig. 6 B ). 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. 6 C ): 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. 6 D ).

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. 7 G ).

DNA barcoding.

(Appendix 1) A fragment of the COI gene is accessioned at NCBI GenBank with the following accession number PQ 676352. The new species exhibits a COI gene similarity of 85.29 % identity with Hylomus proximus Nguyen, Golovatch & Anichkin, 2005 ( MG 669371 ) and 83.11 % identity with Desmoxytes takensis Srisonchai, Enghoff, Likhitrakarn & Panha, 2016 ( OR 765894).

Figures

Figure 5. Hylomus borealis sp. nov., holotype (IEBR-Myr 908 H) A anterior-most body part, lateral view B frons and collum, anterior view (under UV light) C body rings 8–12, dorsal view D body rings 8–10, lateral view. Scale bars: 1 mm.

Figure 6. Hylomus borealis sp. nov., holotype (IEBR-Myr 908 H) A body rings 8–10, ventral view B body rings 8–10, dorsal view (under UV light) C telson, dorsal view (under UV light) D sternum 5, subposterior view. Scale bars: 1 mm.

Figure 7. Hylomus borealis sp. nov., holotype (IEBR-Myr 908 H), SEM A – D left gonopod, lateral view (A), ventral view (B), dorsal view (C), mesal view (D) E – G distal part of left gonopod, ventral view (E), dorsal view (F) G tip of left gonopod, ventral view. Abbreviations: co = coxite; pref = prefemorite; fe = femorite; ca = canula; lm = lamina medialis; ll = lamina lateralis; sl = solenomere. Scale bars: 200 µm (A – D); 100 µm (E), 50 µm (F), 30 µm (G).