Diversity, distribution patterns, and fauno-genesis of the millipedes (Diplopoda) of mainland China

Abstract Based on all available information, 339 species from 71 genera, 26 families, and eleven orders of Diplopoda have hitherto been recorded from mainland China, the fauna thus being very rich, albeit far from completely known, comprising various zoogeographic elements and populating very different environments. Diplopods mainly occur in various woodlands, in caves, and high in the mountains. Most species (> 90 %, usually highly localised, including 160 cavernicoles), 18 genera, and one family are strictly endemic to continental China. Mapping not only the horizontal, but also the vertical distributions of Diplopoda in China shows the bulk of the fauna to be expectedly restricted to forested lowland and mountain biomes or their remnants. Yet some Chordeumatida, Callipodida, Polydesmida, Julida, and even Spirobolida seem to occur only in the subalpine to alpine environments and thus may provisionally be considered as truly high-montane. The long-acknowledged notions of China being a great biogeographic zone transitional between the Palaearctic and Oriental regions generally find good support in millipede distributions, in particular at the higher taxonomic levels (generic, familial, and ordinal). While the Palaearctic/Holarctic components expectedly dominate the fauna of the northern parts of the country, the Oriental ones prevail in its south and along the Pacific coast. Both realms are increasingly mixed and intermingled towards China’s centre. However, in addition to the above traditional views, based on distribution patterns alone, southern China seems to harbour a rather small, but highly peculiar faunal nucleus or origin centre of its own, whence Himalaya, Myanmar, Thailand, Indochina and/or Taiwan could have become populated by younger lineages. The millipede fauna of continental China is thus a tangled mixture of zoogeographic elements of various origins and ages, both relict and more advanced. The few anthropochores must have been the latest faunal “layer” to populate China.


Introduction
Millipedes (Diplopoda) form a highly diverse, yet strongly understudied arthropod class with > 11,000 described species (Minelli 2015). Apparently, only ca. 20 % of the global species diversity of millipedes are currently known, with the actual number of species being estimated between 50,000 and 80,000 species (Minelli and Golovatch 2013). Being mainly represented by mesophilous forest-dwelling detritivores, millipedes have long been recognised as playing important ecological roles, mostly in temperate and tropical land ecosystems where their diversity is especially pronounced (Golovatch and Kime 2009).
The class encompasses 16 extant orders, 140+ families, and ca. 2,000 genera (Minelli and Golovatch 2013), while the distributions of higher taxa fully agree with the major biogeographic divisions of Earth into the Holarctic (Palaearctic + Nearctic), Afrotropical, Oriental, Neotropical and Australian regions which are accepted since Alfred Russel Wallace and Joseph Dalton Hooker. Antarctica is completely devoid of diplopods, whereas the Oriental Region appears to be the sole one to harbour all 16 orders. Being very ancient (Silurian, early Palaeozoic) and diverse taxonomically, widespread (present on all continents except Antarctica), virtually fully terrestrial (even fossils show spiracles), poorly vagile (with highly limited dispersal capacities) and highly limited in compensatory ecological faculties (strongly restricted by a single limiting ecological factor even if the others are favourable), Diplopoda have long been considered as an exemplary group for biogeographic studies and reconstructions (e.g. Shelley and Golovatch 2011).
China has long been considered as a huge territory lying between and linking the Palaearctic and Oriental realms, with very considerable areas of southern China representing not only a marked transitional zone (e.g. Wulf 1944;Zherikhin 2003;Holt et al. 2012), but also the largest karst belt of the world particularly rich in cavernicoles, including millipedes (Golovatch 2015a). Continental China as conventionally understood here includes Hainan and Hong Kong but excludes Taiwan. The territory in question covers ca. 9,326 million sq. km, spanning ca. 5,500 km from north to south and ca. 5,200 km from west to east. China's topography is very complex. The outline descends step by step from west to east: mountains, high plateaus and hilly land prevail and take up nearly 70 % of the total area, with deserts also located in the west, but mostly plains, deltas and hills in the east. The climates are likewise varied, ranging from sharply continental in the north, through temperate in the middle, to monsoon subtropical and tropical in the south, with a warm humid influence along the eastern sea coasts (https://en.wikipedia.org/wiki/Geographic_information_systems_in_China).
China with its highly varied climates and relief (ca. 70 % national land area being mountains or plateaus) is exceptionally rich in ecological conditions and it supports as many as 18 natural latitudinal belts or biomes (Ni et al. 2000). They range from Polar desert and Alpine tundra in Tibet, through grasslands (savanna, steppe) or desert in the northern parts, to various woodlands (scrub, boreal forest, temperate forest, tropi-cal forest etc.) (Fig. 1). Nature zonation is generally well-expressed, forested biomes prevailing in total area and forming a succession of boreal forest in the north, through temperate (conifer, deciduous and evergreen), to tropical rainforest in the far south. Altitudinal zonation follows the same general pattern which varies depending on location and grows increasingly complex from seven vegetation or eco-geographic belts in the Tianshan Mountains in the northwest or Tibetan Plateau in the southwest to 14 in Yunnan in the south (review by Zhang et al. 2004). Even though the millipede fauna of China enjoys a very long history of taxonomic study, dating back to 1833 (Wang and Mauriès 1996), it still remains far from wellknown. Based on all available information, 339 species from 71 genera, 26 families, and eleven orders of Diplopoda have hitherto been recorded from mainland China (Table 1), but there can be no doubt that our review will soon be out of date.
The present paper is an attempt not only to summarise the Chinese species list (as of the end of 2019), but also to provide an analysis of the distribution patterns revealed, both altitudinal and horizontal, and to hypothesise the main sources, routes and stages of fauno-genesis. A very similar approach has recently been applied to treating the millipedes of the Himalaya (Golovatch and Martens 2018).

Materials and methods
Only described species and published records are considered in our paper, while dubious taxa and those not identified to the species level have been omitted from both checklist and bibliography.
Several broken transects have been chosen to grossly reflect the macro relief of mainland China that accompanies the usual mapped distributions (Figs 2-15). The maps and their corresponding transects at the bottom show both horizontal and vertical distributions of all or most species in a number of largely speciose genera from different families and of various origins across China. The species on the maps and along transects are arranged from west to east and/or north to south. The generic level has been chosen as the most suitable to be accepted in historical biogeography (Kryzhanovsky 2002). The above novel approach to a graphic presentation of faunistic data allows us to combine the horizontal and vertical distributions of millipedes in the easiest and most vivid way on the same map. Mapping largely concerns endemic species and only the territory of mainland China.
The colour maps were generated using Google Earth Pro version 7.3.2.5495 and Adobe Photoshop CS6. The final images were processed with Adobe Photoshop CS6.
A very similar pattern is demonstrated by the subendemic genus Paracortina (Paracortinidae, Callipodida), with 12 species, of which ten (Fig. 3) are confined to the mountains of southwestern China (Liu and Tian 2015c), mostly high-montane (3300 m a.s.l., Table 1). Only a few are cavernicoles.
Nepalmatoiulus (Julidae, Julida) is another very large genus which presently comprises 55 species that span from the central Himalaya in the west, through Bhutan, Myanmar, Indochina, Thailand and West Malaysia, to the Ryukyus, Japan and Taiwan in the east (Enghoff 1987b). Seven species range across the southern parts of China (Fig.  4), including two high-montane ones (2750-3650 m a.s.l., Table 1). Although Beron (2008) reported closer unidentified Diplopoda from up to 5300 m a.s.l. from Nepal, the world's highest record for a known species belongs to N. ivanloebli Enghoff, 1987, also from Nepal: 4800 m a.s.l. (Enghoff 1987b, Shelley andGolovatch 2011). The same general pattern is observed in the similarly speciose (ca. 50 spp.), but more boreal genus Anaulaciulus (Julidae), the distribution of which covers northern Pakistan and India, the Himalaya, northern Myanmar, the Far East of Russia, all Japan and Korea, Taiwan, as well as central and eastern China. The highest record belongs to A. bilineatus Korsós, 2001 from Nepal: 3600-4300 m a.s.l. (Korsós 2001). Unlike Nepalmatoiulus, no Anaulaciulus spp. are known to occur in southern China, both these genera being allo-to parapatric. Among the Julidae in China, only very few are cavernicoles.
Particularly clear Palaearctic origins are observed in the large genus Skleroprotopus (Mongoliulidae, Julida), most species of which inhabit the Russian Far East, Korea, Japan and China (Table 1), the small Siberian genus Angarozonium (Polyzoniidae, Polyzoniida) only marginally encountered in northern China (Table 1), the rather small Siberio-Nearctic genus Orinisobates (Nemasomatidae, Julida) represented in China by a single species endemic to the southern Tianshan Mountains (Table 1) (Mikhaljova 2017). The same concerns Polydesmus (Polydesmidae, Polydesmida), a very large genus with ca. 80 species, most of which occur in Europe, the Mediterranean area, Anatolia and the western Caucasus, but a few are known from Japan, and one each in northern Vietnam and Hong Kong (Table 1) (Golovatch 1991a, Nguyen 2009). The large genus Nepalella (Megalotylidae, Chordeumatida), with its 27 species spanning from Nepal (10 species) in the west, through Myanmar (two species) and Thailand (two species), to Vietnam (one species) in the south, and southwestern China (12 species, including several presumed troglobionts) in the north (Liu, Wesener et al. 2017d), shows the same general pattern (Fig. 5). Most congeners are mid-montane, but one, N. marmorata, has been recorded from ca. 4350 m a.s.l. (Table 1).
Basically the same picture is revealed in the distribution of the huge Central to East Asian genus Epanerchodus (Polydesmidae, Polydesmida) which presently encompasses 118 species or subspecies, both epi-and endogean, including 25 across almost entire continental China (Liu and Golovatch 2018b) (Table 1, Fig. 6). Their vertical distributions range from nearly sea-level to high-montane (3090 m a.s.l.), but a few congeners from the Himalaya occur even up to 4250 m a.s.l. (Golovatch and Martens 2018).
The genus Pacidesmus (Polydesmidae, Polydesmida) shows a highly peculiar distribution (Fig. 7), with all of its eleven Chinese species being low-to mid-montane   and restricted to karst caves in the south (Liu and Golovatch in press), whereas the type species, P. shelleyi Golovatch, 1991, comes from the summit (2200-2500 m a.s.l.) of Mount Doi Inthanon, northern Thailand (Golovatch 1991a). Similarly, the small genus Glenniea (Polydesmidae) contains five lowland to mid-montane epigean species from the Himalaya of India and Bhutan (Golovatch and Martens 2018), as well as another three species (including two cavernicoles) from southern China (Golovatch and Geoffroy 2014) (Table 1, Fig. 8).
The great Holarctic family Xystodesmidae (Polydesmida) presently encompasses 66 genera and ca. 410 species, most of which occur in the Nearctic. Only few genera and species are known from Central and northern South America (to Ecuador in the south), the Antilles, the Mediterranean region and East Asia (Shelley and Smith 2018). The largest East Asian genus Riukiaria currently contains 35 species or subspecies from southern Japan, southern Korea, Taiwan and China (Korsós et al. 2011, Golovatch 2014d, 2015b, Nguyen 2016. We disagree with Nguyen (2016), who split Riukiaria into two genera and created a new genus, Parariukiaria Nguyen, 2016, to accommodate a new species from northern Vietnam and three previously described ones from China. To our mind, Riukiaria and Parariukiaria show all transitional stages in the reduction of a gonoprefemoral process and, albeit without formal synonymy advanced here, both may well be regarded as representing a single large genus, in which several peripheral, southernmost congeners demonstrate a more or less strongly suppressed process on the gonopodal prefemur, from relatively small to totally missing. All nine Riukiaria species in China are epigean and span across the central and southern parts of the country, occurring in lowland to high-montane habitats (170-4440 m a.s.l., Table 1, Fig. 9).
As noted above, in China the great family Paradoxosomatidae, which is amongst the largest in the class (200+ genera, 1,000+ species), dominates most of the tropical faunas across the world, but is absent from the Nearctic, contains remarkably few troglobionts (Golovatch 2015a) and comprises genera of various origins. Some seem to be rooted in the Palaearctic (including several endemic or subendemic ones), the others are likely to be Oriental. Among the former elements, the following two rather species-rich genera can be taken as examples.
The genus Hedinomorpha is subendemic to China, with most of its 17 species known from the country being high-montane (up to 4490 m a.s.l., Table 1, Fig. 10), and only one more restricted to Tajikistan, Central Asia (Golovatch 2019b). The genus Sigipinius is strictly endemic to mainland China and contains nine high-montane species (2810-4195 m a.s.l., Table 1, Fig. 11). Such paradoxosomatid genera as Cawjeekelia, Kronopolites, Mandarinopus and Orthomorphella likewise seem best to be attributed to Palaearctic elements in the fauna of China.   Figure 7. Distribution of the family Polydesmidae, genus Pacidesmus in mainland China. Red lines show the transect Qujing -Wenshan -Dafang -Fengshan -Huanjiang -Guilin, along which the elevations are crudely indicated below. 1 P. uncatus 2 P. trilobatus 3 P. martensi 4 P. sinensis 5 P. whitteni 6 P. bifidus 7 P. superdraco 8 P. tiani 9 P. bedosae 10 P. armatus 11 P. trifidus.  In contrast, Paradoxosomatidae also contain a good number of presumed Oriental components, mostly tropical to subtropical. Thus, the genus Hylomus presently comprises 36 species from Myanmar, Thailand, Laos, Vietnam and China (Srisonchai et al. 2018, Liu and Wynne 2019, Golovatch 2019b). Many of them are presumed troglobionts. The distributions of all 20 Hylomus spp. recorded from China cover much of the southern and eastern parts of the country and are only confined to lowland to mid-montane habitats (ca. 140-910 m a.s.l., Table 1, Fig. 12). At the moment, with its 73 species (Golovatch 2019b) that range from southern China in the north, through most of Indochina, to Myanmar in the south, Tylopus remains the largest genus of Paradoxosomatidae globally. However, the altitudinal distributions vary from lowland to high-montane (350-4025 m a.s.l., Table 1), cavernicoles are few, while the Chinese congeners mark the northern range limit of the genus and are confined to the southwestern parts of the country (Fig. 13). Because Tylopus and Hedinomorpha seem to be particularly similar morphologically and co-occur, albeit probably never strictly sympatric, in southwestern China (at least Yunnan, Figs 10, 13), these areas seem to mark the southern range limit of Hedinomorpha.
The relatively large genera Anoplodesmus, Antheromorpha, Enghoffosoma, Nedyopus and Sellanucheza also seem best to refer to as Oriental components in the fauna of China, because it is southern China that marks their northern range limits. The same concerns the small genera Hirtodrepanum, Inversispina, Piccola, Polylobosoma and Tetracentrosternus, all of which show one or a few congeners either in the Himalaya and/or Myanmar,        . 1 G. basalis 2 G. liangshanensis 3 G. beroni 4 G. paragranulatus 5 G. semigranulatus 6 G. subobliteratus 7 G. subgranulatus 8 G.  obliteratus 9 G. latus 10 G. intermedius 11 G. zorzini 12 G. guangnanensis 13 G. foetidus 14 G. sinensis  15 G. pergranulatus 16 G. quadrohamatus 17 G. paracostulifer 18 G. latellai 19 G. obliteratoides 20 G.  or northern Vietnam, or Taiwan. The mono-or oligotypic Belousoviella, Gonobelus, Sinomorpha, Wulingina, and Yuennanina are all strictly endemic to China, mostly to its southwestern parts, but their Oriental stem is clear-cut due to their closest affinities.
The immediately above paradoxosomatid genera endemic or subendemic to southern China which all seem to be of Oriental stock, together with some other polydesmidans like Carlotretus and Martensodesmus (both Opisotretidae), Glenniea and Pacidesmus (both Polydesmidae , Figs 7, 8), as well as several others (e.g. Cryptodesmidae, Haplodesmidae, Pyrgodesmidae), regardless of whether they are Oriental or Palaearctic in origin, seem to be sufficiently numerous and manifest to warrant the recognition of a separate, albeit secondary, subordinate, southern Chinese diversity and faunogenetic centre which must have seriously contributed to at least the faunas of the adjacent parts of the Himalaya, Myanmar, Thailand, Indochina and Taiwan. The influence of that southern Chinese centre in the Himalaya has recently been emphasized (Golovatch and Martens 2018).  The Oriental realm as one of the main sources for the formation of the millipede fauna of China can also be exemplified by the basically tropical to subtropical orders Sphaerotheriida, Spirobolida and Spirostreptida, as well as the families Cryptodesmidae, Haplodesmidae, Opisotretidae, Pyrgodesmidae (all Polydesmida) and Sinocallipodidae (Callipodida), some of which often vary a lot in altitudinal distributions just like numerous Holarctic/Palaearctic groups. The often presumed rule "tropical elements for low elevations only" does not always work.
The genus Glyphiulus, the largest in the family Cambalopsidae (Spirostreptida), presently comprises 60+ species in East and Southeast Asia (to Borneo in the east), 42 of which are encountered at 105-4150 m a.s.l. across China (Fig. 14). Most of them are cavernicoles (Liu and Wynne 2019). A similarly large and even more widespread genus, Eutrichodesmus (Haplodesmidae), presently encompasses 50 species (Liu et al. 2017b, Liu and Wynne 2019) which range from southern Japan and Taiwan in the north, through entire Southeast Asia, to Vanuatu, Melanesia in the south. The distributions of all 24 species that populate continental China seem to be more typical, much better agreeing with the above rule: 65-1495 m a.s.l. (Table 1, Fig. 15). At least half of them are also cavernicoles.

Discussion
The diversity estimates presented in Table 1, i.e. 339 species, 71 genera, 26 families, and eleven orders, are much or significantly higher than those reported from the main adjacent areas. The similarly huge territories of Siberia and the Russian Far East that lie north of China support only ca. 130 species, 46 genera, 18 families and five orders of Diplopoda, while the fauna is reasonably well known (Mikhaljova 2017). This is hardly surprising because the prevailing permafrost and sharply continental climates of Asian Russia are largely too harsh to sustain a rich millipede fauna. The even harsher, mostly arid Mongolia is extremely poor in millipedes, with some nine species, five genera and families, and three orders involved (Mikhaljova 2012, Nefediev et al. 2015. In contrast, the great Himalayan Range spanning for >2,300 km from northwest to southeast and mostly lying south of China supports >275 species, 53 genera, 23 families and 13 orders of diplopods (Golovatch and Martens 2018). Similarly, the fauna of India presently amounts to > 270 species, at least 90 genera, 25 fam ilies, and eleven orders (Golovatch and Wesener 2016), vs. 92 species from 34 genera, 13 families, and eight orders recorded from Myanmar (Likhitrakarn et al. 2017) or ca. 230 species in Thailand (Likhitrakarn et al. 2019). A direct correlation between area and latitude is clear: the larger the area and the closer it lies to the equator, the richer the biota, including the diplopod faunas. However, the more southerly, the greater the diversity, and the more incomplete and fragmentary is our knowledge.
Certainly the Chinese millipede fauna still remains strongly understudied, given the country's great size and habitat diversity, including the globe's greatest karst areas. It may well amount to 1,000 species (Golovatch 2015a), chiefly due to the still particularly poorly studied micropolydesmidans, as well as cavernicoles. Southern China's karsts are unique in often harbouring up to 5-6 diplopod species per cave (Golovatch 2015a). At least some of the remaining orders such as Glomeridesmida, Siphonocryptida, Siphonophorida, Siphoniulida, and Stemmiulida that occur in the Oriental Region (Table 2), including areas immediately adjacent to mainland China, may also be expected to populate the country. For example, Jiang et al. (2019) have recently described a fossil Siphonophorida from Cretaceous amber (ca. 99 Mya) in northern Myanmar, and an extant species is long known to occur in northern Pakistan (Golovatch 1991b). In addition, the same Burmese amber contains still undescribed Stemmiulida (Stoev et al. 2019) and two described species of Siphoniulida (Liu et al. 2017c). Likewise, as noted above, an extant species of Siphonocryptida and Glomeridesmida each is known from Taiwan and northern Thailand, respectively (Korsós et al. 2008, Shelley 2011. While the Palaearctic/Holarctic components expectedly dominate the fauna of the northern parts of the country, the Oriental ones prevail in its south and along the Pacific coast. Both realms are increasingly mixed and intermingled towards China's centre. However, in addition to the above traditional views, based on millipede distribution patterns alone, southern China seems to harbour a subordinate, but highly peculiar faunal nucleus, or origin centre of its own, whence the adjacent Himalaya, Indochina and/or Taiwan could have become populated by younger lineages. The presence of a family (the monobasic Guizhousomatidae) and numerous genera endemic or subendemic to southern China, both apparently relict and relatively advanced, seems to be evidence of this. Within the order Callipodida alone, the family Sinocallipodidae seems to be the basalmost and representing a suborder of its own, the Paracortinidae is a more advanced subendemic, same as the mostly Central Asian Caspiopetalidae Geoffroy 2004, Stoev andEnghoff 2011). More importantly, a fossil family representing a separate suborder has recently been discovered in the Cretaceous Burmese amber, ca. 99 Mya (Stoev et al. 2019).
The millipede fauna of mainland China is thus a tangled mixture of zoogeographic elements of various origins and ages, apparently both relict and more advanced. The few anthropochores/introductions must have been the latest faunal "layer" to populate China.