The Oriental genus Nepalella is reviewed, rediagnosed and shown to comprise 28 species, including N. siamensis sp. nov. from southeastern Thailand. All Nepalella species are keyed, and their distributions mapped, being highly localized and mainly allopatric. Unlike most congeners, which are largely confined to subtropical environments (including montane to high-montane conditions, up to 3800 m a.s.l.) or karst caves (eight species, all in southern China alone), the new species is the southernmost in the distribution area of the entire genus, also being among the very few (four) that are restricted to lowland, purely tropical habitats.

Distribution, Indochina, key, taxonomy

Nepalella Shear, 1979 is one of the relatively few Indo-Malayan genera of the millipede order Chordeunatida and only the second in the small family Megalotylidae (Enghoff et al. 2015). Unlike the oligotypic, more boreal, East Asian Megalotyla Golovatch, in Golovatch and Mikhaljova 1978, represented by only two species from the Russian Far East or North Korea, Nepalella is far more southerly in distribution, being also regarded as one of the most species-rich diplopod genera in the entire Oriental Realm (Golovatch et al. 2006b).

Nepalella is presently known to comprise 27 described species ranging from Nepal (10 species) in the west, southern China (12 species) in the north, through Myanmar and northern Thailand in the south (2 species each), to northern Vietnam (1 species) in the east (Liu et al. 2017b; Fig. 1). Most species of Nepalella are only known from a single locality, being highly localized in distribution (Table 1, Fig. 1). This concerns not only the rather numerous cavernicoles (eight species, largely presumed troglobionts confined to karst caves in southern China), but also epigean congeners, among which most are montane (>800 m a.s.l.) to high-montane (2200–3800 m a.s.l.) and allopatric (Table 1), with only two pairs that have been found to occur syntopically (Shear 2002; Liu et al. 2017b). Some Nepalella species are among the largest Chordeumatida globally and they mainly appear to be restricted to subtropical rather than purely tropical environments, all lying between 23.5° and 34°N (Fig. 1), whereas lowland, typically tropical encounters are only very few.

Therefore, the discovery of another lowland, tropical species of Nepalella, this time in southeastern Thailand, is noteworthy, especially as it represents both the southernmost and the most lowland congener reported to date. The new species was collected in a dipterocarp forest in the Ta Phraya National Park, Sa Kaeo Province, Thailand (Fig. 1). The opportunity is also taken to update the previous key to Nepalella spp. (Golovatch et al. 2006b) and to revisit its taxonomy and distribution.

Figure 1. 

Distributions of Nepalella species (28 species), arranged from northwest to southeast 1 N. phulcokia Mauriès, 1988 2 N. gairiensis Mauriès, 1988 3 N. thodunga Shear, 1979 4 N. deharvengi Mauriès, 1988 5 N. ringmoensis Mauriès, 1988 6 N. tragsindola Mauriès, 1988 7 N. khumbua Shear, 1979 8 N. jaljalae Mauriès, 1988 9 N. taplejunga Shear, 1987 10 N. gunsa Shear, 1987 11 N. marmorata Golovatch, Geoffroy & Mauriès, 2006 12 N. grandoides Golovatch, Geoffroy & Mauriès, 2006 13 N. lobata Liu, in Liu et al. 2017 14 N. jinfoshan Liu, in Liu et al. 2017 15 N. wangi Liu, in Liu et al. 2017 16 N. grandis Golovatch, Geoffroy & Mauriès, 2006 17 N. troglodytes Liu, in Liu et al. 2017 18 N. caeca Shear, 1999 19 N. kavanaughi Shear, 2002 20 N. pianma Shear, 2002 21 N. pallida Mauriès, 1988 22 N. birmanica Mauriès, 1988 23 N. magna Shear, 2002 24 N. griswoldi Shear, 2002 25 N. vietnamica Golovatch, 1983 26 N. taiensis Mauriès, 1988 27 N. inthanonae Mauriès, 1988 28 N. siamensis sp. nov.

Material was euthanized using a two-step method following Guidelines for the Euthanasia of Animals (AVMA 2013). Specimens were then preserved in 75% ethanol for morphological observations which were carried out in the laboratory. The specimens were examined, measured and photographed under a Nikon SMZ 745T trinocular stereo microscope, equipped with a Canon EOS 5DS R digital SLR camera. Digital images obtained were processed and edited with Adobe Photoshop CS5. Line drawings were based on photographs and examined under a stereo microscope equipped with a digital SLR camera. Scanning electron micrographs (SEM) of gonopods coated with a 8 nm gold layer using a CCU-010 high vacuum sputter and a carbon coater (Safematic) were imaged with a TESCAN VEGA3 scanning electron microscope operated at 5 keV of acceleration voltage and returned to alcohol after SEM examination. The images were enhanced and arranged in plates with Adobe Photoshop CS6 software. Collecting sites were located by GPS WGS84 datum using a Garmin GPSMAP 60 CSx, and all coordinates and elevations were checked with Google Earth. The holotype of Nepalella siamensis sp. nov. is housed in the Museum of Zoology, Chulalongkorn University (CUMZ), Bangkok, Thailand. The Animal Care and Use Protocol Review No. 1723018 was applied.

In the synonymy sections, D stands for the original description and/or subsequent descriptive notes, K for the appearance in a key, L for the appearance in a species list, and M for a mention.

Terminology concerning gonopodal and somatic structures, including the following abbreviations used in the text, mostly follows Spelda (2001), Golovatch et al. (2006a) and Liu et al. (2017b).

Abbreviations of certain gonopodal structures in the figures are explained both in the text and figure captions.

CIX macrochaetal index; distance between the exterior and median macrochaeta divided by the distance between the interior and median macrocheata;

MA macrochaetal angle; formed between the arm from the median and exterior macrochaetae and that between the median and interior macrochaetae;

MIX median index; distance between the interior macrochaeta and axial (longitudinal) suture divided by the distance between the interior and median macrochaeta;

PIX paraterga index; distance between the edges of both pataterga and the edges of the prozonite divided by double the length of a paratergum.

At the moment, 28 species of Nepalella have been described, mostly (22, ca 79%) from Nepal or China. In Nepal, many species have been encountered at very high elevations of 2200–3800 m a.s.l., although the occurrence in montane habitats (>800 m a.s.l.) is typical of most congeners elsewhere. Allopatry prevails, but sympatry or even syntopy of two congeners has occasionally been recorded as well. As the distributions of all species, both epigean and cave-dwelling, tend to be highly localized, narrow endemism is most characteristic. Cavernicoly seems to be restricted to the karsts of the southern half of China alone, whereas more to the south, even in the abundant karsts of Thailand or Myanmar, all Nepalella encounters appear to be only epigean and increasingly sporadic (Table 1). Moreover, there seem to be no troglobionts among the Chordeumatida presently known to occur in Thailand or Myanmar, although at least the cave millipede faunas of Thailand and Indochina are quite well studied (e.g., Golovatch 2015; Likhitrakarn et al. 2015, 2016, 2017, 2018, 2020a, 2020b, 2021). The most common group, likewise both highly diverse and abundant, that clearly dominates the subterranean millipede faunas of Southeast Asia together with southern China is long known to be the family Cambalopsidae (Spirostreptida) (Golovatch 2015; Likhitrakarn et al. 2018, 2020a, b, 2021).

Basically, these characteristics and patterns strongly resemble those of many groups of Diplopoda such as the orders Polydesmida (4 families, 8 genera), Chordeumatida (3 families, 3 genera), Callipodida (3 families, 3 genera), Spirostreptida (2 families, 3 genera), Glomerida (1 family, 1 genus), and Julida (1 family, 1 genus) encountered in caves of southern China (Golovatch and Liu 2020). Thus, it is there that caves appear to be exceptionally rich in millipedes, often with 5–6 diplopod species, mostly very local endemics and presumed troglobionts (Golovatch 2015), occurring per cave. The animals are largely characterized by pronounced troglomorphic features such as reduced and mostly unpigmented eyes, unpigmented bodies, thinner and more delicate teguments, clearly elongated appendages (antennae, legs, claws, tergal outgrowths etc.), and often also the so-called “cave gigantism” (Liu et al. 2017a).

A few Nepalella species are among the largest Chordeumatida globally (e.g., N. grandis, which is up to 42 mm long) and nearly all appear to be restricted to subtropical rather than purely tropical environments lying between 23.5° and 34°N (Fig. 1). In contrast, lowland, typically tropical occurrences are only very few: N. vietnamica from Vietnam, and both N. taiensis and N. inthanonae from Thailand (Table 1). The new species, N. siamensis sp. nov., definitely joins the trio, at the same time representing the most lowland and the southernmost record of a Nepalella.

Liu et al. (2017b) recovered the phylogeny of five species of Nepalella, based both on morphological and molecular evidence. Barcoding results revealed that interspecific p-distances amounted to 8.5–15.9%, vs 0–6.8% for intraspecific p-distances. The genus was split into two groups associated with such morphological characters as the presence or absence of a median lobe on the sternum of the anterior gonopods. Because of a limited amount of Nepalella material used in that pioneering study, future investigations are required to confirm both hypotheses. There is little doubt that further novelties concerning the species diversity and distribution of Nepalella are ahead.

This project was funded through grants received from TRF Strategic Basic Research BDG 6080011 (2017–2019) to CS and NL, and Center of Excellence on Biodiversity (BDC-PG4-163008) to SP. We thank the members of the Animal Systematics Research Unit for their invaluable assistance in the field. One of us (SIG) was partly supported by the Presidium of the Russian Academy of Sciences, Program No. 41 “Biodiversity of Natural Systems and Biological Resources of Russia”. Special thanks go to William A. Shear (Virginia, U.S.A.) and an anonymous reviewer, as well as to Dragan Ž. Antić (Belgrade, Serbia), the editor, whose critiques and help have allowed us to considerably improve the paper.