A new genus of the millipede family Pachybolidae from Southeast Asia is described: Macrurobolus gen. nov., with Spirobolus macrurus Pocock, 1893 as type species. This latter species is DNA barcoded (COI) and redescribed based on male morphological characters, which hitherto were unknown. The new genus differs from other pachybolid genera by having (1) the preanal ring process long and protruding beyond the anal valves and (2) the anterior gonopod telopodite distally abruptly narrowed, forming an extremely long, slender, elevated process curved caudad. Given that Macrurobolus gen. nov. is a monotypic genus, it is aphyletic and thus requires further taxonomic revision.

Aphyly, Myanmar, taxonomy, Thailand

Spirobolus macrurus Pocock, 1893 is, with its length of up to 110 mm and diameter of up to 10 mm, the largest pachybolid millipede in SE Asia, but despite its large size, the species is still poorly known. Its original description was based on a single female specimen from Kawkareet, Tenasserim, Myanmar, and did not include the genital parts. Yet, Pocock (1893) separated S. macrurus from other Spirobolus species by its much longer and thinner preanal ring process. Much later, Hoffman (1962: 773) transferred the species to the genus Tonkinbolus Verhoeff, 1938 and remarked “said to be closely related to moulmeinensis, differing only in the longer and more slender epiproct”. However, based on gonopod characters and strongly supported by DNA sequence data, Pimvichai et al. (2018) assigned Tonkinbolus scaber Verhoeff, 1938 (type species of Tonkinbolus) to the genus Litostrophus Chamberlin, 1921. Thus, Tonkinbolus became a subjective junior synonym of Litostrophus. At the same time, Pimvichai et al. (2018) moved all other Tonkinbolus species, including T. macrurus, to the genus Atopochetus Attems, 1953 because they share the unique anterior gonopod telopodite of this genus. Yet, since T. macrurus was until then only characterised on the basis of a single female specimen, its transfer to Atopochetus was qualified as “incertae sedis” (Pimvichai et al. 2018).

In the present paper we redescribe and barcode Spirobolus macrurus based on an old male specimen discovered in the collections of the Natural History Museum of Denmark, Copenhagen, and new live material, including an adult male specimen, collected during recent fieldwork in Thailand. As a result we also create the new genus Macrurobolous gen. nov. to accommodate Spirobolus macrurus, so that this species will be referred to as Macrurobolus macrurus comb. nov.

Live specimens were hand collected and preserved in 70% ethanol for morphological study or placed in a freezer at –20 °C for DNA analysis. Specimens were also examined from the following collections:

CUMZ Museum of Zoology, Chulalongkorn University, Bangkok, Thailand;

NHMD Natural History Museum of Denmark, University of Copenhagen, Denmark.

This research was conducted under the approval of the Animal Care and Use regulations (numbers U1-07304-2560 and IACUC-MSU-037/2019) of the Thai government.

The uncorrected p-distance between the sequences ranged from 0.03 to 0.25 (Tables 2, 3). The mean interspecific sequence divergence within Atopochetus was 0.13 (range: 0.08–0.16). The mean sequence divergence between Atopochetus and M. macrurus comb. nov. was 0.15 (range: 0.14–0.17). The mean interspecific sequence divergence within Litostrophus was 0.10 (range: 0.09–0.11). The mean sequence divergence between Litostrophus and M. macrurus comb. nov. was 0.13 (range: 0.11–0.14).

PartitionFinder indicated that the best substitution model for BI analysis was GTR+ G. The ML, BI, and NJ trees were congruent with respect to some of the well-supported branches (by visual inspection of the branching pattern). Yet, in several instances BI provided good support for branches that were not well-supported by both ML and NJ (e.g., the Litostrophus + Benoitolus clade or the Coxobolellus + Pseudospirobolellus clade).

In the phylogenetic trees (Fig. 1) the clade of Pachybolidae + Benoitolus is poorly supported by ML (BV = 63) and NJ (BV = 27), but well supported by BI (PP = 0.97), while Trigoniulinae is well supported by the three methods (BV = 96 and 92; PP = 1.00). Although the monophyly of Pachybolidae is clearly challenged by the inclusion of Benoitolus, which involves a long branch, removing Benoitolus from the analysis yields a Pachybolidae clade with the same pattern of support as the Pachybolidae + Benoitolus clade (Suppl. material 1).

Figure 1. 

Phylogenetic relationships of pachybolid and several other spirobolidan millipede species based on maximum likelihood analysis (ML) of a 660 bp COI gene fragment. Numbers at nodes indicate branch support based on bootstrapping (ML) / posterior probabilities (BI) / bootstrapping (NJ). Scale bar: 0.3 substitutions/site. # indicates branches with < 50% ML and NJ bootstrap support or < 0.95 posterior probability, - indicates non-supported branches. The coloured areas mark the Pseudospirobolellidae (minus Benoitolus) (purple), Trigoniulinae (red), and non-trigoniuline Pachybolidae (plus Benoitolus) (yellow).

Irrespective of the in- or exclusion of Benoitolus, Macrurobolus macrurus comb. nov. is nested within a clade comprising Litostrophus and Atopochetus. Yet, this clade is poorly supported by ML, well supported (but just so) by NJ, and convincingly well supported by BI. The position of M. macrurus comb. nov. within this clade, however, is poorly supported by the three methods.

Taxonomy

Class DIPLOPODA de Blainville in Gervais, 1844

Order SPIROBOLIDA Bollman, 1893

Suborder TRIGONIULIDEA Attems, 1909

Family PACHYBOLIDAE Cook, 1897

Genus Macrurobolus gen. nov.

http://zoobank.org/A428FDFE-D777-4B7B-8D29-F603088A0AC2

Figures 1, 2, 3, 4, 5

Diagnosis

A genus of Pachybolidae characterised by the following combination of characters: preanal ring with long process protruding beyond anal valves; the anterior gonopod telopodite distally abruptly narrowed, forming an extremely long, slender, elevated process curved caudad.

Etymology

The generic name is a combination of the name of the type species and “-bolus”, the ending of many pachybolid genus names.

Type species

Macrurobolus macrurus (Pocock, 1893) comb. nov.

Spirobolus macrurus Pocock 1893: 396.

Tonkinbolus macrurus: Hoffman 1962: 773.

Atopochetus macrurus: Pimvichai et al. 2018: 174.

Macrurobolus macrurus

(Pocock, 1893), comb. nov

The original description was based exclusively on a female from “Kawkareet” (Tenasserim), Myanmar (see Distribution section for information on this locality). Pocock (1893) described the female external morphology and mentioned that this species differed from Spirobolus caudulanus [= Atopochetus caudulanus (Karsch, 1881)] and Spirobolus moulmeinensis [= Atopochetus moulmeinensis (Pocock, 1893)] by having a “much longer and thinner tail”.

Material studied

Thailand, 1 ♂, 3 ♀♀; Tak Province, Mae Sot District, Wat Tham Inthanin; 16°45'59"N, 98°40'21"E; 660 m a.s.l.; 27 July 2016; P. Pimvichai, T. Backeljau and P. Prasankok leg. (CUMZ). • Myanmar, 1 ♂; Meetan; Fea; “ex typ.”; NHMD 621698.

Description of Thai specimens

Adult male with 51 podous rings, no apodous rings. Length ca 11 cm, diameter ca 9.0 mm. Adult females with 48–51 podous rings, no apodous rings. Length ca 10–11 cm, diameter ca 10.0–10.4 mm.

Head capsule smooth, area below antennal sockets with wrinkles (Fig. 2A). Occipital furrow extending down between, but not beyond eyes; clypeal furrow reaching level of antennal sockets. Area below antennal sockets and eyes impressed, forming part of antennal furrow. Incisura lateralis open. 2+2 labral teeth, a row of labral setae, 1+1 supralabral setae (mentioned as “the labral region furnished with 4 punctures” by Pocock 1893: 401). Diameter of eyes ca half of interocular space; 9 vertical rows of ommatidia, 8 horizontal rows, 53–55 ommatidia per eye. Antennae short, not reaching beyond collum when stretched back, accommodated in a shallow furrow composed of a horizontal segment in the head capsule and a vertical segment in the mandibular cardo and stipes. Antennomere lengths 2 > 3 = 5 > 4 > 6 > 1 > 7; antennomere 1 glabrous, 2 and 3 with some ventral setae, 4, 5 and 6 densely setose; 4 apical sensilla. Mandibles: stipes (Mst) broad at base, apically gradually narrowed. Gnathochilarium (Fig. 2B): each stipes (Gst) with 3 apical setae; each lamella lingualis with 2 setae, one behind the other. Basal part of mentum (Me) transversely wrinkled; basal part of stipites longitudinally wrinkled.

Figure 2. 

External morphology of a male Macrurobolus macrurus comb. nov. from Wat Tham Inthanin, Thailand, CUMZ-D00147-1 A head, frontal view B gnathochilarium, ventral view C posterior end, lateral view D posterior end, latero-ventral view E midbody leg, latero-ventral view. Av = anal valves; Gst = gnathochilarial stipes; Me = mentum; Mst = mandibular stipes; Sub = subanal scale.

Collum smooth, with a marginal furrow along lateral part of anterior margin; lateral lobes narrowly rounded, extending as far ventrad as the ventral margin of body ring 2.

Body rings 2–5 ventrally concave, hence with distinct ventrolateral “corners”. Body rings very smooth, parallel-sided in dorsal view. Prozona smooth. ‘Tergo-pleural’ suture visible on pro- and mesozona; mesozona ventrally with fine oblique striae, dorsally punctate; metazona ventrally with fine longitudinal striae, otherwise smooth. “Pleural” parts of rings with fine oblique striae. Sterna transversely striate. Ozopores from ring 6, situated in mesozona, ca 1/2 pore diameter in front of metazona (mentioned as “the repugnatorial pores situated in front of the transverse sulcus” by Pocock 1893: 401).

Telson smooth; preanal ring with slightly concave dorsal profile, with thick and long process protruding beyond anal valves (Fig. 2C). Anal valves (Av) impressed submarginally (Fig. 2D); margins hence distinctly protruding, liplike. Subanal scale (Sub) broadly triangular.

Legs (Fig. 2E): length of midbody legs 72–77% of body diameter in males, 54–56% of body diameter in females. Prefemur basally constricted, tarsus longer than other podomeres. First and second legs with 2 or 3 prefemoral, 2 or 3 femoral, 2 or 3 postfemoral, and 2–4 tibial setae, and 4 or 5 ventral and 1 dorsal apical setae on tarsi, numbers of setae reaching constancy from pair 3: each leg podomere from coxa to tibia with 1 seta; tarsi with 2 ventral apical and 1 dorsal apical seta, the apical ventral seta larger than the more basal one. Claw very slender, more than half as long as tarsus.

Colour. Living animal reddish brown except for grey pro- and mesozona (Fig. 4).

Male sexual characters. Tarsus from third to before the last 4 body rings with large ventral soft pad occupying entire ventral surface. Body ring 7 entirely fused ventrally, no trace of a suture. Tip of anterior gonopods visible when the animal is stretched out (not when it is rolled up).

Anterior gonopods (Fig. 3A, B, D, E) with triangular mesal sternal process, not reaching so far as the tip of coxae, apical margin bilobed, with basal longitudinal triangular ridge in posterior view. Coxa oval, apically gradually narrowed, rounded, projecting slightly beyond sternal process. Telopodite apically far overreaching coxa, distally abruptly narrowed, forming an extremely long, slender, elevated process curved caudad.

Figure 3. 

Male (A–F, H–L) and female (G) genital parts of Macrurobolus macrurus comb. nov. (specimens from Wat Tham Inthanin, Thailand, CUMZ-D00147-1) A anterior gonopod, anterior view B anterior gonopod, posterior view C right posterior gonopod, posterior-mesal view D anterior gonopod, anterior view E anterior gonopod, posterior view F right posterior gonopod, posterior-mesal view G left female vulva, posterior mesal view H–L SEM H left posterior gonopod, posterior-mesal view I tip of posterior gonopod, mesal view J apical part of posterior gonopod, mesal view K spiny lamellae near tip of posterior gonopod, mesal view L meso-distad process of posterior gonopod, posterior-mesal view. at = anterior gonopod telopodite; av = anterior valve; cx = coxa; pt = posterior gonopod telopodite; pv = posterior valve; st = sternum.

Posterior gonopods (Fig. 3C, F, H–I) strongly curved mesad, laterally with a massive ridge; with efferent canal (Enghoff 2011) running along mesal margin terminating in slender, pointed meso-distad process, covered with fine hairlike spinules (Fig. 3L); tip of posterior gonopod concave, apically ending in a rounded lobe (Fig. 3I, showed serrated margin, dorsally covered with short spines); with spiny lamellae mesally near tip.

Figure 4. 

Live female Macrurobolus macrurus comb. nov. from Wat Tham Inthanin, Thailand (CUMZ-D00147-3).

Female vulvae (Fig. 3G). Valves prominent, of equal size; basally with open space between free margins.

DNA barcode

The GenBank accession number of the COI barcode of the Thai specimen is MZ905519 (voucher code CUMZ-D00147).

Ecology

. Found under leaf litter.

Notes on the male from Meetan, Myanmar

This specimen is labelled as “ex typ” in the NHMD collection and was, like the female type specimen, collected by Fea. It agrees with the Thai male in all characters, including all details of gonopod shape, with the following exceptions: Colour after > 100 years in alcohol is faded, but there is still a clear contrast between greyish pro- and mesozone and reddish-brown metazona. Size: length ca 8 cm, diameter 6.7 mm, 50 podous rings, no apodous rings in front of telson. Head capsule smooth. 11 vertical rows of ommatidia, of which 3 are very incomplete, 7 horizontal rows, 47 ommatidia per eye. Antennomeres 2–4 with some ventral setae, 5 and 6 densely setose. Gnathochilarium not dissected.

Distribution

Tak Province, Thailand; Kawkareet (Tenasserim) and Meetan, Myanmar (Fig. 5). The names Kawkareet and Meetan do not appear on maps available to us. However, Brandis (2002: 1312) mentioned “Meetan (= Mitan Chaung (= river) 15°59'00"N 98°24'00"E at the south-west slope of the Dawna mountain”, whereas Randall and Page (2012: 344) located Meetan at “16.555556°N, 98.24°E (coordinates estimated)”. Annandale (1911: 118) stated that Kawkareet refers to Kawkareik and remarked in a footnote that “This locality [i.e. Kawkareik] is often referred to in zoological literature as Kawkareet or Kawkarit, or even Kokarit”. Finally, Likhitrakarn et al. (2017) located Kawkareet (= Kawkareik) at 16°33'20"N, 98°14'24"E and Meetan (= Mi Tan) at 16°00'12"N, 98°23'25"E.

Figure 5. 

Distribution of Macrurobolus macrurus comb. nov.

The male specimen of Spirobolus macrurus from Meetan in NHMD, although labelled “ex typ.”, should not a priori be regarded as a type (ICZN Art. 72.4.7.) because Pocock (1893: 396) explicitly mentioned that the species description was based on “A single ♀ from Kawkareet (Tenasserim)”. However, its non-sexual characters agree with Pocock’s (1893) description. Hence, we do not hesitate to refer it to Macrurobolus macrurus comb. nov.

The new male specimen from Thailand and the old specimen from Myanmar share the long preanal ring process with the female type specimen, which is a remarkable character for a pachybolid, since most pachybolid genera (except Aulacobolus Pocock, 1903 and Trachelomegalus Silvestri, 1896) have a short preanal ring process. So, in this respect, Macrurobolus gen. nov. is clearly differentiated from most other pachybolid genera, including Atopochetus and Litostrophus, the two genera with which Macrurobolus gen. nov. appears the be most closely related in our phylogenetic tree (Fig. 1). Similarly, the anterior gonopod telopodites of Macrurobolus (telopodite distally abruptly narrowed, forming an extremely long, slender, elevated process curved caudad) clearly differ from those of Litostrophus (telopodite simple, without process, narrowly rounded) or Atopochetus (telopodite with a triangular process directed laterad originating on posterior surface at ~1/2 or 2/3–4/5 of its height). Hence, given that Macrurobolus shares neither the defining morphological synapomorphies of Atopochetus, nor those of Litostrophus, we think that the creation as a separate monotypic genus is warranted.

The interpretation of Macrurobolus as a separate genus is somehow in line with the COI tree (Fig. 1), which places the new genus in a clade comprising Atopochetus and Litostrophus, but which supports neither joining M. macrurus comb. nov. with Atopochetus (which itself forms a consistently well-supported clade), nor joining it with Litostrophus (which itself forms also a well-supported clade) (Fig. 1). Moreover, the mean interspecific COI sequence divergence between M. macrurus and other pachybolid and pseudospirobolellid species is 18% (range: 11–23%) (Tables 2, 3), a value that rather points to an intergeneric divergence (Table 3).

In conclusion, this study suggests that Pimvichai et al. (2018) appropriately labelled the transfer of Tonkinbolus macrurus to the genus Atopochetus as “incertae sedis”. Indeed, the species can be accommodated in neither Atopochetus nor Litostrophus, i.e., the two genera with which it appears to be most closely associated. Hence, it would be ill-advised to maintain Macrurobolus macrurus comb. nov. in the genus Atopochetus, for this would undermine both the definition and the support of the monophyly of this taxon. Therefore, the creation of the monotypic genus Macrurobolus gen. nov. seems the best solution to provide a generic name for Spirobolus macrurus Pocock, 1893. Still, the monotypy of Macrurobolus gen. nov. renders it aphyletic sensu Ebach and Williams (2010), and hence in need of further study (Williams and Ebach 2020: 134).

This research was funded by the Thailand Science Research and Innovation (TSRI) together with Mahasarakham University as a TRF Research Career Development Grant (2019–2022; RSA6280051) (to P. Pimvichai). Additional funding came from the Royal Belgian Institute of Natural Sciences (RBINS). We thank, Pongpun Prasankok for assistance in collecting material. We are indebted to Julien Cillis (RBINS) for help with SEM photographs, to Yves Barette (RBINS) for help with gonopod photographs and to Thita Krutchuen for the excellent drawings. Last but not least, we thank Thomas Wesener (Bonn) and Sergei Golovatch (Moscow) for their helpful reviews of this paper.