Species concepts are only little addressed in diplopod taxonomy. To think about species limits and species definitions is not at all trivial; in nearly every case, a morphological species concept is used with the background idea that these entities, defined by external characters, fit well to the Biological Species Concept. In practical alpha-taxonomy it circumscribes reproductively isolated groups of specimens. Diplopod taxonomists largely base their identifications on adult male samples. Differences in male genitalic structure usually provide the basic characters that allow us to safely determine millipede species. In most cases this raises no problems. In the Himalayas, however, we have to tackle with numerous populations in a wide array of forest habitats found at various altitudes and in remote and secluded valleys. Hardly surprisingly, the Himalayas do support quite a number of examples of species swarms among Diplopoda as well. Species delimitation may then cause problems like those described by Martens (1978) for the polymorphous biantid harvestman Biantes pernepalicus Martens, 1978. Such situations also resemble the few known cases of insular species swarms in millipedes of Macaronesia (e.g. Enghoff 1992).

The following examples can be given and easily added to the roster of similar observations that Martens (2015) made or compiled for Himalayan mammals, birds, arachnids, insects and several other animal groups. Such a distribution pattern can be termed fanned (see below) and is also found in the endemic Himalayan diplopod genera Beronodesmus and Beronodesmoides containing 11 and seven species, respectively (Golovatch 2016c, Golovatch et al. 2016). The main species-specific characters are in minor details of gonopodal structure, in particular, the shapes of the various outgrowths (Fig. 3). Vicariance speciation must have taken place in situ, with several of the congeners forming pairs or trios that can occur sympatrically or even syntopically and thus implying a series of secondary dispersal events.

Figure 3. 

Gonopodal structural variations between several species of Beronodesmus: B. martensi Golovatch et al., 2016 (1–3), B. serratus Golovatch et al., 2016 (4, 5), B. simplex Golovatch, 2016 (6, 7), B. distospinosus Golovatch, 2015 (8, 9), B. latispinosus Golovatch, 2015 (10, 11), B. sinuatospinus Golovatch, 2015 (12, 13) and B. gorkhalis Golovatch, 2015 (14). Scale bars: 1.0 mm (4–5, 14), 0.5 mm (1–3, 8–13) or 0.4 mm (6, 7). After Golovatch (2015a, 2016c) and Golovatch et al. (2016).

Ecology and dispersal abilities

The vast majority of Himalayan diplopod species are highly localized in distribution, both geographical and altitudinal. There are only few relatively widespread species like Zephronia manca, Siphonorhinus cingulatus (both recorded from Vietnam & Darjeeling District, India) or Trichopeltis watsoni (Bangladesh, Myanmar, Bhutan and India, Darjeeling Distr., West Bengal, Assam & near Kolkata). Despite extended collection acitivities over most parts of Nepal during several decades, most millipedes in the Himalayas remain known from only a single or very few localities. This particularly concerns the best-explored fauna of Nepal, Central Himalayas (ca 160 spp.), including species of the dominant family Paradoxosomatidae (82 spp., or >50%).

In contrast, most genera occur through a range of altitudes (Fig. 4), but are more or less restricted to forest habitats. The alpine (= nival) zone of the Central Himalayas which lies above closed forests appears to only marginally be populated by Diplopoda (Table 1, Figs 2, 4), the bulk of the fauna being confined to the tropical and subtropical forest belts. The highest records belong to Nepalozonium trimaculatum and Nepalmatoiulus ivanloebli, both found at 4800 m a.s.l. This pattern conforms to general knowledge that millipedes are basically a class of forest-dwelling terrestrial arthropods both trophecologically and historically largely associated with woodlands and ranging from nemoral (= broadleaved forest) and coniferous forest in temperate regions in the north to rainforest tropical areas in the south (Golovatch 1997a, 1997b). Such a background is accepted and it serves as the basis for faunogenetic reconstructions using phyto- and palaeogeographical evidence.

Figure 4. 

Vertical distribution of several genera of Diplopoda in the Himalayas (modified, after Golovatch and Martens 1996).

According to Martens (1993, 2015), broad vertical distribution belts appear to be exceptional in animals at least in the Central Himalayas, in contrast to fanned ones which are especially characteristic of species-rich groups, both vertebrates and invertebrates. Yet the vertical width of a distributional belt varies very considerably between taxa. Diplopods in their natural habitats seem to be particularly restricted to their forest habitats, with little capacity for enlarging their areas, both vertically and horizontally. There is hardly any other larger land arthropod group that shows a similarly strong specialization and relation to a habitat. In many cases, congeners occupy only limited vertical belts and such patterns are the result of multi-layered speciation processes that occurred in secluded valleys and mountain massifs. There are obvious altitudinal adaptations resulting from the interaction between Pliocene/Pleistocene climate oscillations and speciation processes. Whilst species of different clades occupy narrow vertical belts, close congeners, i.e. sister species, are mostly horizontally allopatric because of their allopatric vicariance speciation. Sympatry or syntopy are quite common among congeners (see above for Beronodesmus, Fig. 3), this alone implying a series of secondary dispersal events. On the other hand, all 13 species of Nepalmatoiulus known from Nepal and Bhutan are nearly exclusively allopatric in distribution (Enghoff 1987).

Narrow belts seem to be rare, when each individual species has been recorded from a single or very few localities, but even the whole species set combined remains restricted to a narrow altitudinal range. Much more common, rather usual are the situations when the vertical and horizontal distribution of a species is highly restricted, but that of the corresponding species-group or genus is very considerable (Table 1, Fig. 4).

Among the Diplopoda of the Himalayas, most if not all of the rather to highly species-rich genera show fanned vertical distribution patterns. Such are all genera at least in the orders Sphaerotheriida, Glomerida, Chordeumatida, Julida and Spirostreptida, as well as most in the order Polydesmida. Their origins seem to be very different, but profound in situ speciation is their general characteristic. No evident narrow belts seem to be distinguishable in the combined vertical distribution of millipede congeners in the Himalayas (Table 1, Figs 2, 4). Even within Nepal, a gradual east-west decline in diplopod diversity is clearly observed towards the country’s central regions, apparently following the climate aridity gradient and increased absence of humid forest.

As noted above, zoogeographically the Himalayas are traditionally viewed as a vast, yet clear-cut transitional zone between the Palaearctic and Oriental (= Indo-Malayan) realms. Martens (2015, p. 212) writes that “The renowned species diversity of the Himalayan fauna results from the area’s position between tropical SE Asia to the south and southeast, temperate High Asia to the north and dry Central Asia to the northwest, in addition to its proximity to endemic-rich SW China, which harbours many Tertiary relicts. Elements from all these areas contributed to and influenced the present faunal composition, creating one of the world’s 25 biodiversity hotspots, Indo-Burma and the adjacent South Central China.” He discriminated a predominantly immigrant fauna with five main sources: SW Chinese Himalayan from the northeastern Himalayan Arc; Indochinese Himalayan from the southeast; tropical Indian from the south; central Asian from the north via Tibet; and W Asian Himalayan from mountain ranges westwards to the Mediterranean. This fauna is mainly composed of species with good dispersal abilities such as bats, birds, butterflies, dragonflies and many other groups. Since millipedes are poor dispersers and only relatively few species are anthropochores, we refrain from enlisting any as belonging to this category, even though a few widespread Oriental species do reach the Himalayas from the east and/or southeast (e.g. the above Zephronia manca, Siphonorhinus cingulatus, Streptogonopus phipsoni, Gonoplectus malayus malayus or Trichopeltis watsoni). Human introductions cannot be excluded in such cases, these being especially apparent as regards the few unquestioned anthropochores like Trigoniulus corallinus, Orthomorpha coarctata or Oxidus gracilis.

In addition, a Sino-Himalayan fauna (or even a Sino-Himalayan region) is distinguished, which is characterized by remarkable diversity, partially of Indo-Malayan origins and partially Palaearctic in nature, but with marked peculiarities. Holt et al. (2012), based on the modern distributions and phylogenies of amphibians, birds and non-marine mammals, defined 20 distinct zoogeographic regions grouped into 11 realms, including a Sino-Japanese realm which appears to show closer phylogenetic affinities to the Palaearctic than to the Oriental realm. It includes Japan, Tibet and nearly all of China. Eventually, that paper represents one of the most consistent, but no less unsuccessful attempts at uncritically combining the landscape-typological (= zonal) and faunogenetic approaches to biogeography which, however, must be clearly separated at least as regards the biotas of older biomes (e.g. Chernov 1975, Golovatch 2015b). To reiterate Chernov (1975) briefly, from the “viewpoint” of a biome or more local landscape it only matters whether the constituent species properly function as its biotic elements, regardless of their origins. In contrast, from a faunogenetic point of view, the more ancient the biome or landscape, the more ancient its biota and the more complex its history. Thus both approaches must be clearly distinguished, especially as regards the relatively younger zonal biomes like tundra or taiga (= boreal forest) from the particularly ancient, rather regional than zonal, subtropical and tropical ones.

In terms of its faunal composition, the Sino-Himalayan region represents a mixed zone of elements derived from both the Palaearctic and Oriental realms, but it includes moreover a wealth of endemics with surprisingly small and often relict distributions (Martens 2015). It is within this category that many of the Himalayan Diplopoda seem best to place. Perhaps the most conspicuous example of such a pattern is represented by the definitely relict order Siphonocryptida which globally contains only two genera and seven species. Thus, the genus Siphonocryptus Pocock, 1894, comprises three species: one in Sumatra, Indonesia, the other two in continental Western Malaysia. In contrast, the distribution pattern of Hirudicryptus Enghoff & Golovatch, 1995 is trans-Palaearctic (Fig. 5). The type species occurs only on Madeira and the Canaries, where it is largely confined to the relict, subtropical, laurisilva biome. One species each is endemic to Taiwan, to Nepal and to the NW Caucasus (Golovatch et al. 2015, Zuev 2017). It may well be that the distribution pattern under consideration dates back at least to the Oligocene times of the so-called “Warm Earth” to have highly probable explanations rooted in palaeobotanical evidence. These imply a gradual shrinkage and disruption ever since of the previously dominating and continuous subtropical biome (Golovatch 1997a, 1997b, Zherikhin 2003). Being so vastly disjunct, the present-day distribution of Siphonocryptida is best accounted for by extinction events (Shelley and Golovatch 2011).

Figure 5. 

Distribution of the genus Hirudicyptus (Siphonocryptidae, Siphonocryptida). After Golovatch et al. (2015).

Genus- and species-level relations

Further possible examples of the Sino-Himalayan pattern seem to be represented by Glenniea (Polydesmidae, a largely Holarctic family), with five species from the Himalayas of Nepal and Bhutan, and three in S China (Golovatch 2015b), Nepalella (Megalotylidae, the genus being the westernmost in that temperate to tropical Asian family), with 23 species or subspecies from Nepal, S China and SE Asia (Minelli 2015, Table 1), Hirtodrepanum (Paradoxosomatidae, a subcosmopolitan family absent only from N America) with one species each in Nepal and S China (Golovatch 2014a), and Martensodesmus (Opisotretidae, the genus being the westernmost in that Indo-Australian family), with at least five species from the Himalayas of Nepal and Bhutan, two in S China, and one in S Vietnam (Golovatch et al. 2013). The latter example, however, may likewise illustrate the predominating zoogeographical connections of the Himalayan fauna with the Indo-Malayan one. Such are also the genera Cryptocorypha (Pyrgodesmidae), with 14 described species from S India, Myanmar, S China and Australasia, eastwards to Vanuatu, Melanesia (Golovatch and VandenSpiegel 2015, Golovatch and Wesener 2016), Zephronia (Sphaerotheriida), with 38 species or subspecies from Nepal, NE India, Myanmar, Thailand, Cambodia, Vietnam, Western (= mainland) Malaysia and Hong Kong (Wesener 2016), Siphonorhinus (Siphonophorida), with eight species from Indonesia, Vietnam, Cambodia, Laos, India and Madagascar (Minelli 2015) and Pocockina (Paradoxosomatidae), with three species from Nepal and Myanmar (Golovatch 2016b). The small genus Trichopeltis (Cryptodesmidae) includes nine described species, largely from Indochina, Sumatra, Indonesia, Myanmar and S China; only the much more widespread T. watsoni and Gonoplectus malayus malayus reach as far west as the Himalayas of India (Table 1). Similarly, amongst the Paradoxosomatidae the genus Trogodesmus contains four species in Myanmar and one more in Assam, India (Nguyen and Sierwald 2013); Touranella harbours two species from Nepal and further four from Vietnam (Golovatch 2016b); Kronopolites has one species in Jammu & Kashmir, India, one in Thailand, one in Laos, and another nine in China, including one in Taiwan (Likhitrakarn et al. 2015, Golovatch 2015a); Anoplodesmus is a large genus which, regardless of a few pantropical anthropochores, comprises approximately 35 species in S (India and Sri Lanka), E (Taiwan) and SE Asia (eastwards up to Sumatra, Indonesia), including nine congeners confined to the Himalayas (Golovatch 2016a, Table 1); Streptogonopus contains not only S. phipsoni, a species quite widespread in the Himalayas and certain adjacent countries (Table 1), but also two more in India, and one each in Eritrea, Thailand and Vietnam (Nguyen et al. 2016); Delarthrum is one of the most species-rich genera of Diplopoda (56 spp.), with most (55 spp.) of its diversity confined to the Himalayas of Pakistan, Nepal and India (Table 1), and only one outlier congener in S India (Golovatch and Wesener 2016, Golovatch 2016b). Much like Delarthrum, the genus Sholaphilus demonstrates faunal connections between the Himalayas (6 spp., Table 1) and S India (1 sp., Golovatch and Wesener 2016).

Suprageneric relations

At these taxonomic levels, the fully to largely tropical orders Sphaerotheriida, Siphonophorida and Spirostreptida, as well as most of Spirobolida, Platydesmida and Polydesmida (at least some Haplodesmidae, Cryptodesmidae, Paradoxosomatidae, Pyrgodesmidae and Trichopolydesmidae) seem to represent Indo-Malayan elements in the Himalayan millipede fauna. A siphonophoridan and a cryptodesmid species, both have been recorded as far north as N Pakistan (Golovatch 1991). However, at the species level the bulk of diversity is endemic and highly localized, both horizontally and vertically, clearly due to allopatric vicariance (cf. Golovatch and Martens 1996). There are several Himalayan endemic to subendemic diplopod genera, but not a single suprageneric taxon: Kophosphaera (Sphaerotheriida, seven species, Wesener 2016 & Table 1), Koponenius (Polydesmida, four species from Nepal, NW India and Myanmar (Golovatch and VandenSpiegel 2014, 2016), Kashmireuma (Chordeumatida), with three species from Nepal and N India (Table 1), Himalodesmus (Polydesmidae, Table 1), Beronodesmoides, Beronodesmus, Substrongylosoma (all Paradoxosomatidae, Table 1), Magidesmus (Trichopolydesmidae, Table 1), as well as the monotypic Assamodesmus, Bhutanodesmus, Topalodesmus, Topalosoma and Typhlopygmaeosoma (Table 1).

Faunal connections to the northwest and/or north are demonstrated by a few examples only. Even at the generic level, not all of them could unequivocally be treated as likely Palaearctic components in the Himalayan millipede fauna. Thus, the genus Kaschmiriosoma (Paradoxosomatidae) is composed of three species endemic to N Pakistan, and one to both N Pakistan and N India (Jeekel 2003). Such a pattern might seem to have been accounted for by an originally northwestwards dispersal. Even in the absence of a phylogenetic reconstruction, the gonopodal structure in Kaschmiriosoma, especially the particularly complex, strongly coiled and thus apomorphous solenophores as observed in the southernmost, Himalayan K. contortipes and K. nulla, may rather be evidence of a Palaearctic origin of the genus. Its deemed closest relatives within the tribe Sulciferini, also often showing particularly complex and strongly twisted gonopodal solenophores, are observed in the genera Gonobelus Attems, 1936, Inversispina Zhang, in Zhang et al. 1997 and a few others, all confined to southern China, occasionally including Taiwan (Jeekel 1980, Golovatch 2012, 2016b).

Ties to the north are much better pronounced, e.g., in the genera Tianella (Cleidogonidae), Epanerchodus (Polydesmidae), Bollmania (Caspiopetalidae) and Anaulaciulus (Julidae). Tianella has two described and a number on still undescribed species in Kyrgyzstan and Kazakhstan, Central Asia, as well as 11 named species in Nepal and a few undescribed ones from both Nepal and N India (Mauriès 1988, Read and Golovatch 1994, Table 1). Epanerchodus is a very large genus comprising 70+ species from Central and E Asia: Russian Far East, Korea, China, Taiwan and Japan, as well as several species from Nepal (Minelli 2015, Table 1). Bollmania is composed of eight described and a few undescribed species ranging from Turkmenistan, Iran, Uzbekistan, Tajikistan and Afghanistan in Central Asia to S China; one species is known from the Himalayas of Pakistan (Stoev et al. 2008, Table 1). Anaulaciulus is also a highly speciose Asian genus which contains nearly 50 species ranging from across the Himalayas, through China, to the Russian Far East, Korea, Japan and Taiwan (Korsós 2001, Table 1). It is partly sympatric with still another, similarly large, Asian genus Nepalmatoiulus (Julidae), which harbours 55 species also distributed across the Himalayas, but then extending more to the east and southeast (S China, Taiwan, S Ryukyus, Myanmar, Indochina and peninsular Malaysia) (Enghoff 1987, Korsós and Lazányi 2013, Table 1). In the Himalayas, many species from these genera are high-montane (Table 1), thus reinforcing their presumed Palaearctic origins. The occurrence of the sole known species of Nepalozonium (Polyzoniida) at 4700–4800 a.s.l., i.e. among the highest records in the entire class, coupled with the family Hirudisomatidae where it belongs being strictly Holarctic (Minelli 2015), is clearly evidence of its Palaearctic roots.

The pattern demonstrated by the very large genus Hyleoglomeris (Glomeridae) strongly resembles that of the family Siphonocryptidae (see above and Fig. 5), but in no way is it relictual. Indeed, its 100+ species range from the Balkans and Greek islands in the west, through Anatolia, the Caucasus, Central Asia and the Himalayas, to China, Korea, Japan, and Taiwan in the east, and through Indochina to the Philippines and Sulawesi, Indonesia in the southeast (Golovatch et al. 2006, Table 1). This picture actually reflects one of the fundamental patterns of historical biogeography as evidenced in the entire class Diplopoda (Shelley and Golovatch 2011). Generally, west-east trans-Himalayan faunal connections are traced in numerous millipede higher taxa: Glomerida, Julida, Chordeumatida, Callipodida, Siphonocryptida, Spirostreptida, Polydesmida etc. Southeast Asia is the only corner in the world where all 16 extant orders of Diplopoda are still to be found. In many cases, the Himalayas might have served as a paramount stepping stone and refugium in linking, much more in the past than at present, the faunas of SE Asia to those of Europe and W Asia. In the past, the Himalayas started rising and absorbing surrounding faunal elements often already present in the area. After having gained a certain height, the Himalayas functioned more as a trap, a “prison”, mountain ridges and deep valley systems hindering further faunal exchange, with the tremendous speciation process that came into action. Old migration routes both ways are thereby evident, although the influence of the Oriental fauna is by far greater. Since the uplift of the Himalayas started in the early Oligocene (about 27 Mya), the diplopod fauna of the region has gradually acquired its own, highly characteristic, very rich and diverse composition, multi-layered and very complex, with profound in situ radiations and vicariance events (cf. Golovatch and Martens 1996, Martens 2015). At least some of the oldest components are presently highly disjunct and clearly relict, as is the order Siphonocryptida (see above & Fig. 5). The most recent faunal layer is certainly represented by the few pantropical or subcosmopolitan introductions like Trigoniulius corallinus, Orthomorpha coarctata or Oxidus gracilis.