A revision of dragon millipedes IV: the new genus Spinaxytes, with the description of nine new species (Diplopoda, Polydesmida, Paradoxosomatidae)

Abstract Nine new species constituting the ‘spiny’ group of dragon millipedes are assigned to the new genus Spinaxytes Srisonchai, Enghoff & Panha, gen. n. Seven new species are described from Thailand: S.biloba Srisonchai, Enghoff & Panha, sp. n. and S.palmata Srisonchai, Enghoff & Panha, sp. n. from Surat Thani Province, S.hasta Srisonchai, Enghoff & Panha, sp. n. from Chumphon Province, S.krabiensis Srisonchai, Enghoff & Panha, sp. n. (type species) and S.sutchariti Srisonchai, Enghoff & Panha, sp. n. from Krabi Province, S.uncus Srisonchai, Enghoff & Panha, sp. n., and S.macaca Srisonchai, Enghoff & Panha, sp. n. from Phang Nga Province; as well as one from Malaysia, S.tortioverpa Srisonchai, Enghoff & Panha, sp. n., and one from Myanmar, S.efefi Srisonchai, Enghoff & Panha, sp. n. The new genus is endemic to South Myanmar, South Thailand, and Malaysia, and all new species are restricted to limestone habitats. All were exclusively found living on humid rock walls and/or inside small caves. Complete illustrations of external morphological characters, an identification key, and a distribution map are provided.


Introduction
This is the fourth paper in a series of articles about revision of the dragon millipedes. Srisonchai et al. (2018a) provided general information on dragon millipedes, split Desmoxytes Chamberlin, 1923, sensu Golovatch andEnghoff (1994) into five genera based on morphological and genetic data, and revised the genus Desmoxytes in its new, restricted sense. Subsequently, Srisonchai et al. (2018a, b) described two new genera of dragon millipedes containing several new species and several species transferred from Desmoxytes. In the present study, we describe nine new species constituting the group that we (Srisonchai et al. 2018a) provisionally named the 'spiny' group, of which no species has hitherto been named, and assign them to Spinaxytes gen. n.
The new genus is narrowly distributed in the Malay Peninsula (Malaysia, Myanmar, and Thailand).

Specimen collection and preservation
Specimens were hand-collected from many localities throughout South Thailand, in some parts of Malaysia and in southern Myanmar. We also observed the habitats of all specimens. Specimens were mostly stored in 70% ethanol for morphological study and partly in 95% ethanol for molecular analysis. Latitude, longitude, and elevation were recorded by using a Garmin GPSMAP 60 CSx, and all coordinates and elevations were checked with Google Earth.
The main collectors in this work were staff and students of the Animal Systematics Research Unit, Department of Biology, Faculty of Science, Chulalongkorn University which we here refer to as 'ASRU members'.

Illustrations
All living specimen photos were taken with a Nikon D700 equipped with a AF-S VR Micro-Nikkor 105 mm lens during fieldwork. Newly collected specimens preserved in ethanol were imaged with an Olympus DP72 camera on an Olympus SZX16 stereomicroscope, using image stacking Cell-D auto-montage software. Scanning electron micrographs were generated with a JEOL -JSM-5410 LV. All samples studied with SEM were carefully dissected under a microscope, mounted on aluminium stubs, and coated with gold. After imaging with SEM, all objects were removed and kept in dry condition. Drawings were outlined under a stereo microscope (Leica Wild M10) with a drawing tube and finished using dot-line technique (stipple). Plates were composed in Adobe Photoshop CS6.
Gonopod terms for the genus Spinaxytes gen. n., and their abbreviations acropodite Apical part of the gonopod; including femur, postfemoral part, solenophore and solenomere ca Cannula: a tube, lever-like, curved and slender; originating from coxa, tip inserted into concavity in prefemoral part cx Coxa: basal part of the gonopod, rather long, connecting to seventh body ring, attached to apertural rim dorsally; with a distoanterior group of setae fe Femur: longest part of the gonopod, straight; with lateral and mesal sulci distally; accommodates seminal groove (terminology for femorite vs femur, see Jeekel (2003 p. 48) ll Lamina lateralis: a small lobe on distal part of gonopod; seen in lateral view lm Lamina medialis: the longest part distally on the gonopod, very long, curved ls Lateral sulcus: a distinct sulcus distally on femur, seen obviously in lateral view ms Mesal sulcus: a distinct sulcus distally on femur, seen obviously in mesal view pfe Prefemoral part (=prefemur): basal portion of the telopodite, densely setose pof Postfemoral part (=postfemur): short part of telopodite, supporting solenophore and solenomere, demarcated from femur by lateral and mesal sulci sg Seminal groove: a conspicuous groove, similar to a tunnel, seen as a transparent line, visible on femur in mesal view sl Solenomere: a long and curved, flagellum-like appendage, originating from base of solenophore sph Solenophore (=tibiotarsus): apical part of telopodite, consisting of lamina lateralis and lamina medialis telopodite Main part of the gonopod, pivoting on coxa; including prefemoral part, femur, postfemoral part, solenophore and solenomere

Deposition of holotypes, paratypes, and other new specimens
All holotypes and most paratypes are kept at CUMZ. Some paratypes are deposited at NHMUK, NHMW, ZMUC, and ZMUM.
Etymology. The name is a noun in apposition; from the Latin 'spina', referring to the spine-like paraterga of all constituent species; '-xytes' ensures harmony with Desmoxytes (and its synonym 'Pteroxytes').
Included species.

Remarks.
The new genus is easily distinguished from other genera of dragon millipedes by having spiniform paraterga, lamina lateralis smaller and shorter than lamina medialis, lamina medialis long and curved. Some species of the genus Hylomus Cook & Loomis, 1924, share spine-like paraterga; however, the gonopod details are totally different.
General description of the genus Spinaxytes. The description applies to adult males and females, except for the gonopods and when "male" is specified (Figs 1, 2, 4). The general description of the gonopods is based mainly on Spinaxytes krabiensis gen. et sp. n. (Figs 3, 5).
Colour. Most species in life with dark brown colour. Colour in alcohol: all specimens partly faded to pale brown after 5 years' preservation in alcohol; specimens kept in darkness faded more slowly.

Key to species of Spinaxytes gen. n. (based mainly on males)
Diagnosis. Differs from other species by having: metaterga 5-19 with 2+2 cones in anterior row and 3+3 cones in posterior row; sternal lobe between male coxae 4 subtrapeziform; male femora 8 and 9 with apophyses distally; lamina lateralis with an additional process-like protruding lobe; lamina medialis basally with an additional protruding process-like lobe.
Distribution and habitat ( Figure 6C). Known only from the type locality which is a small isolated limestone mountain between Khao Sok and Khlong Phanom National Parks. The new species blended perfectly with the humid rock walls, and most specimens were found inside rock holes/crevices. S. biloba sp. n. co-occurs with Desmoxytes cervina (Pocock, 1895) (Srisonchai et al. 2018a) in the same habitat. Several attempts (2017-2018) have been made to find further specimens near the type locality, but none were found. As the new species has only been found at the type locality only, we regard S. biloba sp. n. as endemic to Thailand. Remarks. Among all Spinaxytes species, S. biloba sp. n. is obviously the smallest (length 15-18 mm, width of midbody metazona 1.0-1.5 mm), and the live pale brown colouration is lighter than that of other species.
Etymology. The name is an artificially constructed homophone (efefi = FFI) honouring FFI (Fauna and Flora International, Myanmar), an organization for biodiversity conservation; in recognition of their hard work to protect wildlife including invertebrates.
Legs (Figs 4E,10J). Male femora without modification. Gonopods (Figs 5B,11). Coxa subequal in length to femur. Prefemoral part ca. 2/3 as long as femur. Femur not enlarged distally, ventrally swollen in middle part. Postfemoral part broad; mesally with a long triangular process (directed mesoanteriad) and a long triangular ridge, between process and ridge with a wide furrow. Mesal sulcus and lateral sulcus wide. Solenophore bigger and longer than postfemoral part; basally very broad: lamina lateralis long and slender, curved, tip round: lamina medialis long and slender; with two ridges in middle portion; slightly attenuated near tip; tip in situ resting very close to solenomere, terminating in small spines. Solenomere flat, curving down; tip terminating in three sharp spines, directed mesoventrad. Distribution and habitat ( Figure 9C). Known only from the type locality. The specimens were found exclusively on rock walls or in caves. We have tried to find this species in other places near the type locality, but no further specimens have been collected. Given the finding only at the type locality, the new species is considered to be endemic to southern Myanmar.

Remarks.
No variation was found. Body ring 19 of S. efefi sp. n. seems to be shorter than in other species, and the tip of paraterga on collum is obtuse whereas in other species (except S. biloba sp. n.) it is sharp. Etymology. The name is a Latin noun in apposition meaning spear, referring to the shape of the sternal lobe between male coxae 4 which is somewhat similar to a spear.
Distribution and habitat ( Figure 12D). The specimens were found on rocks or walls with plants, and some were found in a small cave. S. hasta sp. n. is distributed only in Chumphon Province, and we regard the new species as endemic for the Thai fauna. At Wat Nam Cha the new species coexists with Desmoxytes cervina.
Remarks. There are some variations: the lateral setiferous tubercles of the epiproct are conspicuous in some specimens, inconspicuous in others; the hypoproct is subtrapeziform in some individuals, subsemicircular in others; the caudal margin of the hypoproct is rounded in some specimens, angular in others.  Etymology. The new species is named after the province where the type locality lies.

Spinaxytes krabiensis
Diagnosis. Male femora 6 and 7 humped distally. Similar in this respect to S. macaca sp. n., but differs by having: paraterga orange, longer; male femora 6 smaller; tip of lamina lateralis round, not protuding as digitiform; tip of lamina medialis terminating in two lobes.
Metaterga ( Figure 16A, C, E). With two transverse rows of setiferous tubercles and setiferous cones/spines; metaterga 2-7 with 2+2 tubercles in anterior row and 2+2 spines in posterior row; 8-19 with 2+2 tubercles in anterior row and 2+2 cones in posterior row; lateral cones/spines of posterior row bigger and longer than mesal ones, gradually reduced in size and length on the following rings.
Distribution and habitat ( Figure 15E). S. krabiensis sp. n. inhabits Krabi Province. Considering its narrow distribution, we regard this species as endemic for the Thai fauna. It is syntopic with Desmoxytes delfae (Jeekel, 1964), Desmoxytes cervina and Gigaxytes gigas (Golovatch & Enghoff, 1994), which were collected from the same location (Than Bok Khorani and Wat Tham Sue (Tiger Cave)), but the new species was encountered living on rock walls or in small caves while the others were usually found on leaf litter or on tree branches.

Spinaxytes macaca
Etymology. The species is named after the monkey, long-tailed macaque (Macaca fascicularis) living at the type locality (Monkey Cave). Diagnosis. Male femora 6 and 7 humped distally. Similar in this respect to S. krabiensis sp. n., but differs from it by having: paraterga brownish white, shorter; male femora 6 larger; tip of lamina lateralis (ll) protruding as a small lobe, digitiform; tip of lamina medialis (lm) bent, terminating in one lobe.
Gonopods (Figs 5E,20). Coxa shorter than femur. Prefemoral part ca. half as long as femur. Femur quite enlarged distally. Postfemoral part short and narrow. Mesal sulcus and lateral sulcus wide. Solenophore longer than postfemoral part: lamina lateralis small, compact; apically protruding as a small lobe, directed mesoventrad: lamina medialis long; basally enlarged and slightly attenuated near the tip; tip bent, sharp and curving up. Solenomere curved and twisted, metazona in transverse section, tip directed posteriad.
Distribution and habitat ( Figure 18C). All specimens were collected in small caves near the big Monkey Cave, crawling on rock walls. It is difficult to see the new species without using a flashlight/torch as the black body colour blends in with dark rocks. This species can be found in syntopy with Desmoxytes cervina. For the time being, S. macaca sp. n. is known only from the type locality and we regard it as endemic to Thailand.
Metaterga ( Figure 22A, C, E). With two transverse rows of setiferous cones and setiferous spines; metaterga 2-19 with 2+2 cones in anterior row and 2+2 spines in posterior row; lateral cones/spines of posterior row bigger and longer than mesal ones, gradually reduced in size and length on the following rings.
Distribution and habitat ( Figure 21D). S. palmata sp. n. is known only from the type locality. We regard this species as endemic for the Thai fauna. The new species can be found in syntopy with Desmoxytes corythosaurus Srisonchai, Enghoff & Panha, 2018, crawling on humid rock walls.
Etymology. The name honours associate professor Dr. Chirasak Sutcharit, malacologist of ASRU (CUMZ), collector of this new species and numerous other dragon millipedes.
Diagnosis. Male femora without modification, sternal lobe between male coxae 4 incompletely bilobed. Similar in these respects to S. palmata sp. n., but differs by having: a large and round lamina lateralis; tip of lamina medialis terminating in two spines; distal part of solenomere circular in tranverse section.
Metaterga. With two transverse rows of setiferous tubercles and setiferous spines; metaterga 2-19 with 2+2 tubercles in anterior row and 2+2 spines in posterior row; lateral spines of posterior row bigger and longer than mesal ones, subequal in size and length on all body rings.
Legs ( Figure 4Q). Male femora without modification. Gonopods (Figs 5G,25). Coxa subequal in length to femur. Prefemoral part ca. 2/3 as long as femur. Femur not enlarged distally. Postfemoral part narrow. Mesal sulcus and lateral sulcus wide. Solenophore bigger and longer than postfemoral part: lamina lateralis oval, large, long, tip round: lamina medialis long and slender; base enlarged, slightly attenuated near the tip; tip curving down, with two sharp spines (one smaller, one bigger). Solenomere curving up, circular in transverse section, tip directed anteriad. Distribution and habitat ( Figure 24D). S. sutchariti sp. n. is known only from the type locality, and we regard it as endemic to Thailand. The new species can be found in the same area as Gigaxytes gigas, but we assume that they live in different microhabitats: G. gigas was collected from the ground in leaf litter, whereas the new species was found on humid rock walls. Remarks. We found variation in the hypoproct: in some specimens subtrapeziform, in others subtriangular; caudal margin in some individuals round, in others angular. Parasitic mite larvae, probably of the genus Leptus Latreille, 1796, were found attached to the anterior body part of some female specimens. Larvae of ?Leptus have previously been found on species of Desmoxytes (D. cervina) and Nagaxytes (N. acantherpestes (Golovatch & Enghoff, 1994)) (Srisonchai et al. 2018a(Srisonchai et al. , 2018b, see also Southcott 1992).  rung, limestone mountain, 4°24'58"N, 101°11'16"E, ca. 92 m a.s.l., 27 Sep. 2007, B. W. Ng and ASRU members leg. (CUMZ-pxDGT00219). Paratypes. 1 ♂, 1 ♂ gonopods missing, 1 ♀, same data as for holotype (CUMZ-pxDGT00220).

Spinaxytes tortioverpa
Etymology. The name is a noun in apposition, from the Latin tortio meaning torsion and verpa for penis, refers to the distal part of gonopod (postfemoral part, solenophore and solenomere) which makes a 90 degrees torsion relative to the femoral part.
Diagnosis. Male femora without modification; collum with 4+4 tubercles in anterior row, 1+1 tubercles in intermediate row and 2+2 tubercles in posterior row. Similar in these respects to S. hasta sp. n. and S. palmata sp. n., but differs from them by having: a completely bilobed sternal lobe between male coxae 4; postfemoral part, solenophore and solenomere angled 90 degrees with femoral part; lamina lateralis with two lobes -the lateral one spine-like, the mesal one shorter and ridge-like; solenomere very long, longer than lamina medialis.
Colour. Specimens in life with body black/brownish black. Colour in alcohol: after 10 years changed to brown; paraterga brownish white; antennae brown (except whitish distal part of antennomeres 7 and 8); head, collum, metaterga and prozona blackish brown; surface below paraterga brown/blackish brown; sterna, epiproct and legs pale brown; a few basal podomeres whitish brown.
Gonopods (Figs 5H,26). Coxa subequal in length to femur. Prefemoral part about 2/3 as long as femur. Femur quite enlarged distally. Postfemoral part large, broad and wide; angled 90 degrees with femur. Mesal sulcus and lateral sulcus wide. Solenophore smaller than postfemoral part: lamina lateralis apparently with two lobes demarcated from each other; lateral lobe very long, process-like, its tip in situ directed ventrad; mesal lobe short and wide, supporting solenomere: lamina medialis long, base not enlarged, tip directed mesad. Solenomere obviously longer than lamina medialis, circular in transverse section, curving down, tip directed laterad.
Distribution and habitat. Known only from the type locality which is currently a tourist attraction (cave). We regard this species as endemic to Malaysia.
Metaterga ( Figure 28A, C, E). With two transverse rows of setiferous cones and setiferous spines; metaterga 2-19 with 2+2 cones in anterior row and 2+2 spines in posterior row; lateral spines of posterior row very long, bigger and longer than mesal ones.
Gonopods (Figs 5I,29). Coxa subequal in length to femur. Prefemoral part almost half as long as femur. Femur obviously enlarged distally. Postfemoral part small, narrow. Mesal sulcus wide; lateral sulcus narrow. Solenophore bigger and longer than postfemoral part: lamina lateralis very small, compact, tip round: lamina medialis long; basally enlarged and slightly attenuated near the tip; apically sharp, long, hook-like; tip curving down, in situ resting close to solenomere. Solenomere curved and twisted, compressed in transverse section, tip directed posteriad.
Distribution and habitat ( Figure 27F). Known only from Phang Nga Province; we regard S. uncus sp. n. as endemic for the Thai fauna. Most specimens were found on rock walls near the cave, some were seen crawling on leaf litter on the rock. The new species has been encountered in syntopy with Desmoxytes cervina at Phung Chang Cave, Tham Nam Pud and Wat Kerewong (Tham Koab).
Remarks. Some variation in the hypoproct was observed in this species: in some specimens subtrapeziform, in others subsemicircular; caudal margin in some individuals conspicuous, in others inconspicuous. In addition, specimens from Wat Kerewong (Tham Koab) have smaller and shorter paraterga than other specimens.

Discussion
The new genus Spinaxytes, defined by Srisonchai et al. (2018a) as the "spiny" group of dragon millipedes, at that time without described members, adds to the challenge of understanding the patterns of paratergal and gonopod evolution in the dragon millipedes. The nine species, all new, described here are recorded from Malaysia, Myanmar, and Thailand. They are united in the new genus by sharing the diagnostic characters of subspiniform paraterga; lamina lateralis distinctly demar- Based on a comparatively large number of specimens and species of the new genus, our study confirmed that gonopod characters can be used confidently to discriminate the species, just as we found in Desmoxytes and Nagaxytes (Srisonchai et al. 2018a(Srisonchai et al. , 2018b. Using the gonopods in combination with other morphological characters, such as modification of male femora, sternal lobe between male coxae 4, and number of tubercles/cones/spines on collum and on metaterga, further facilitates reliable taxonomic identification. It is particularly interesting that a process on the postfemoral part of the gonopod is found in S. efefi sp. n. Only two species of dragon millipede, Hylomus specialis (Nguyen et al. 2005) and H. spectabilis (Attems, 1937), have hitherto been known to have this process (z-spine) at the base of the solenophore. However, the overall gonopod characters of S. efefi sp. n. are markedly different from the gonopod of the two mentioned Hylomus species, warranting its inclusion in the new genus. Spinaxytes gen. n., is quite possibly a monophyletic group, considering both gonopodal and nongonopodal characters. A phylogenetic study using molecular as well as morphological characters seems warranted in order to better understand the taxonomic position and the true relationship of the genus with other dragon millipede genera.
Almost all specimens collected by us were found on humid rock walls in small caves. Therefore, we strongly suspect that all species in this genus prefer to live on rock walls. The black or dark brown body colour makes them difficult to see against darkcoloured rocks. Quite often some specimens of Desmoxytes and Gigaxytes species are encountered in the same habitat as species of the new genus, but it seems likely that those species live on leaf litter, on the ground or on tree branches instead of rock walls. Considering currently known distributions of species of Spinaxytes gen. n., and their restriction to small limestone areas (Figure 30), we regard all described species here as locally endemic. Of the nine species of Spinaxytes gen. n., only one (S. hasta sp. n.) has been shown to have a somewhat wider range, but it still inhabits less than approximately 50 km 2 along the coast of Thailand.
The discovery rate of new dragon millipede species has been increasing in recent years (Liu et al. 2014(Liu et al. , 2016Likhitrakarn et al. 2015;Golovatch et al. 2016;Srisonchai et al. 2016Srisonchai et al. , 2018aSrisonchai et al. , 2018bSrisonchai et al. , 2018c. Including the nine new species described here, the diversity of dragon millipedes (Desmoxytes + Hylomus + Nagaxytes + Gigaxytes + Spinaxytes gen. n.) has now reached 59 species. Dragon millipedes are thus a significant element in the biodiversity of Southeast Asia, especially Thailand and the Malay Peninsula. We believe that the number of endemic dragon millipede species will certainly increase further when collecting efforts in very remote or otherwise difficult-to-access places are made.
History Museum of Denmark. We gratefully acknowledge the Plant Genetic Conservation Project under the Initiative of Her Royal Highness Maha Chakri Sirindhorn and Center of Excellence on Biodiversity for permission to field work in several restricted/ remote areas. This work would not have been possible at all without the impressive collections made by ASRU members, especially Dr. Chirasak Sutcharit for his kind help in collecting numerous specimens. Thanks are due to B. W. Ng. (Malaysia) and Fauna & Flora International (Myanmar) for helping us to collect specimens in Malaysia and Myanmar. We are the most grateful to the subject editor R Mesibov, and journal referees, SI Golovatch and N Akkari, for all valuable comments and advices that greatly improved the manuscript. The first author also thanks to Ms Thita Krutchen for her kind teaching the drawing skills.