﻿Review of the millipede genus Malayorthomorpha Mršić, 1996 (Diplopoda, Polydesmida, Paradoxosomatidae), with descriptions of two new species from Thailand and a key to its species

﻿Abstract The millipede genus Malayorthomorpha Mršić, 1996, so far monospecific and previously known only from Park Belum, Perak State, northern Malaysia, is recorded from a mountain in Betong District, Yala Province, southern Thailand for the first time, being represented there by two new species: M.halabalasp. nov. and M.hulutbeedasp. nov. Both new species are found to occur syntopically and can be assumed as narrowly endemic to the Titiwangsa Mountain Range which begins in southern Thailand, crosses the Malaysian border, and extends into east and west coast regions of the Malay Peninsula. In addition, the generic diagnosis is slightly updated, and a key to all three species is provided.


Introduction
The millipede genus Malayorthomorpha Mršić, 1996 was established for a single, and type species, Malayorthomorpha siveci Mršić, 1996, based on two males from northern Malaysia (Mršić 1996). This genus was immediately assigned, and still belongs, to the tribe Orthomorphini Brölemann, 1916, all 25 genera of which are basically confined to the Oriental Region (Nguyen and Sierwald 2013;Srisonchai et al. 2018a, b, c, d). The tribe is characterized by the gonopod that shows an elongate (not shortened) femorite and both solenophore (= tibiotarsus) and solenomere of medium size, the former's both mesal and lateral lobes (a lamina medialis and a lamina lateralis, respectively) sheathing, supporting and protecting a flagelliform solenomere (Jeekel 1968). The latest key to the genera of the tribe belongs to Golovatch (1997), but presently it is considerably out of date.
Thailand is located in the central part of mainland Southeast Asia within two significant biodiversity hotspots, Indo-Burma and Sundaland (Myers et al. 2000), both outstanding in supporting an especially rich and diverse diplopod fauna (Likhitrakarn et al. 2011Pimvichai et al. 2018Pimvichai et al. , 2020. To date, the Thai millipede list comprises 256 species in 52 genera, 17 families and nine orders, largely reported based on explorations during 2007-2022 throughout Thailand (e.g., Pimvichai et al. 2018Pimvichai et al. , 2020Srisonchai et al. 2018aSrisonchai et al. , b, c, d, 2021Likhitrakarn et al. 2020Likhitrakarn et al. , 2021aLikhitrakarn et al. , b, 2022Rosenmejer et al. 2021;Bhansali and Wesener 2022).
However, there are still some areas that have never been explored and prospected sufficiently well for millipedes, such as three southern border provinces within the Malay Peninsula: Pattani, Narathiwat and Yala. Some Diplopoda have only been documented from the Yala and Narathiwat provinces, while the Pattani Province has remained devoid of any millipede records (Enghoff 2005). Only ten species have been reported from the Yala Province (Table 1), nine of which share a single locality, the Bang Lang National Park (5°30'7"N, 101°26'21"E). Four species are only known from one locality, and three from two localities with a range of less than 50 km 2 . These seven species are presumably endemic to the country or restricted to a small area in the Malay Peninsula. However, only relatively small areas have been prospected, with just five sampling locations in the three provinces that have provided reports of millipede species.
Luckily, we have recently been privileged to survey an evergreen forest in the Betong District, Yala Province near the Thai-Malaysia border during the rainy season. Based on morphological examinations of the new specimens, we are able to herewith describe and illustrate two new species of the genus Malayorthomorpha which is formally reported from Thailand for the first time.

Materials and methods
New material was collected in a montane forest at a rather high elevation near the Thai-Malaysian border. The specimens collected were euthanized by a two-step method following the AVMA Guidelines for the Euthanasia of Animals (AVMA 2013). Material was then preserved in 75% ethanol for morphological observations and brought to 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 the stereo microscope equipped with a digital SLR camera. For scanning electron microscopy (SEM), the gonopods were coated with 8 nm gold layer using a CCU-010 high vacuum sputter and a carbon coater (Safematic), then imaged with a TESCAN VEGA3 scanning electron microscope operated at 5 keV of acceleration voltage and returned to alcohol after SEM examination. The holotypes and paratype are housed in the Museum of Zoology, Chulalongkorn University (CUMZ), Bangkok, Thailand.
The terminology concerning gonopodal and somatic structures mostly follows Mršić (1996), Golovatch and Enghoff (1993), Golovatch (1997), and Srisonchai et al. (2018bSrisonchai et al. ( , 2018c. Abbreviations of certain gonopodal structures are as follows: g groove, a distinct groove line running parallel to the solenomere, clearly seen in mesal view ll lamina lateralis, a flat lobe in the distal part of the gonopod lm lamina medialis, a large part located distally on the gonopod, tapered apically and unciform sl solenomere, usually a long and flagelliform structure originating at the base of the solenophore sph solenophore (= tibiotarsus), the apical part of the telopodite, consisting of a lamina lateralis and a lamina medialis The Animal Care and Use Protocol Review No. 1723018 was applied.
Coordinates and elevations were recorded by Garmin GPSMAP 60 CSx and Garmin eTrex 30 using the WGS84 datum and subsequently double-checked with Google Earth ver. 7.3.4 Amended diagnosis. Body medium-sized to large (ca. 24-41 mm long, ca. 1.2-2.7 mm wide), with 20 segments. Paraterga from poorly to rather well developed, without lateral incisions. Transverse metatergal sulcus distinct. Leg relatively long and slender, without modifications. ♂ tarsal brushes absent. Sternal lobe between ♂ coxae 4 present, other sternites unmodified.

Taxonomy
Gonopods rather simple to relatively complex; coxites elongate, subcylindrical, sparsely setose distoventrally, without tubercles; prefemoral (= setose) part of telopodite moderate to relatively large, 1/3-1/2 as long as acropodite; femorite moderately long and stout, slightly curved, devoid of a distinct distolateral sulcus demarcating a postfemoral part; a well-developed lamina medialis and a hypertrophied lamina lateralis of solenophore; the latter subterminally with a long, distally pointed and curved lobe broadened at base and protecting the tip of a curved solenomere. Apex of solenophore subquadrate. Solenomere flagelliform, starting about level to demarcation cingulum between femorite and solenophore, seminal groove running entirely or mostly mesally along an excavate femorite.
Affinities. As noted earlier (Golovatch 1997(Golovatch , 1998, the gonopodal conformation of Malayorthomorpha seems to especially similar to that of Cleptomorpha Golovatch, 1997, a monospecific genus of Orthomorphini from Sumatra, Indonesia. Yet both genera compared differ clearly in the gonopod femorite showing an indistinct, oblique, mesal fold, a relatively slender solenophore and an apically terminating solenomere in Cleptomorpha compared to the gonopod femorite that is clearly excavated mesally, has a considerably stouter solenophore, and the solenomere termnating mesally about the solenophore midway in Malayorthomorpha (Golovatch 1997). Remark. This species was described from Park Belum, 5°30'7"N, 101°26'21"E, ca. 320-350 m a.s.l., Hulu (Sungani), Perak, Malaysia (Mršić 1996). Only two male specimens have been obtained, and both have been discovered in a small area. This species is considered endemic to northern Malaysia. Diagnosis. This new species seems to be particularly similar to M. siveci Mršić, 1996, with which it shares most of the gonopodal characters. It differs from M. siveci by the wider body, 2.7-3.2 mm (vs smaller, 1.2 mm), the colour pattern which is uniformly red brown with lighter red brown paraterga (Fig. 3A-F) (vs a light brown body with the collum and caudal edges of metazonae margined darker brown; Fig. 1A), as well as the pleurosternal carinae present until segment 11 (vs until segment 5), the sternal lobe between ♂ coxae 4 with a pair of small cones laterally near base (Fig. 3E, H, I) (vs absent, Fig. 1D), and the tip of the gonopod with a denticulate margin (Figs 4A, B, 5C, D) (vs smooth and rounded; Fig. 1E-G).
Colouration of live animal rusty red ( Fig. 2A), edges of paraterga light red brown; antennae dark brownish, legs and venter contrasting light yellow ( Fig. 2A); colouration in alcohol, after one week of preservation, red brown ( Fig. 3A-F); edges of paraterga light red brown, head and antennae brown, legs, venter and a few basal antennomeres contrasting light yellow (Fig. 3A-G).
Calluses on paraterga rather narrow, delimited by a sulcus fully on dorsal side and in about posterior 2/3 on ventral side; on poreless rings more narrow than on porebearing ones in dorsal view (Fig. 3B, D, E). Ozopores evident, lateral, lying in an ovoid groove at about 1/3 in front of posterior edge of metaterga.
Etymology. To emphasize Hala-Bala Wildlife Sanctuary, the type locality. Noun in apposition.
Remarks. A comparison of these two species shows only a few differences, but they are sufficient to distinguish both. The type locality of M. siveci, Park Belum, is located quite far away (ca. 50 km) from this new place. In addition, because the elevations between the two localities are greater than 1000 meters above sea-level, it seems improbable that the species is one and the same. Consequently, we conclude that the two are obviously distinct species.
The specimens were collected in a primary sub-elfin montane forest with no significant disturbance due to human activity, in a high mountainous area of southernmost Thailand (Fig. 2B, C). The area is dominated by a single plant species, Dacrydium elatum. The canopy of Dacrydium elatum is low (ca. 10-15 m above ground), flat and continuously covering the area. The understorey is dense and covered with dwarf branches of small hardwood trees and teeming with bryophytes, lichens, orchids and ferns. The forest floor is with abundant orchids, ferns, liverworts, and thick slowly degraded bio-litters. The female specimen was easy to spot on the substrate and observed crawling on the leaf litter surface (Fig. 1A). Diagnosis. This new species is distinguished from its two congeners in sternal process between male coxae 4 linguiform with a rounded tip, and lamina lateralis of gonopodal solenophore triangular, apically bifid and protruded laterally.
Description. Length of holotype 31.5 mm, width of midbody pro-and metazonae 2.7 and 3.0 mm, respectively.
Colouration of alcohol material after one week of preservation dark red brown ( Fig. 6A-F); paraterga paler, head and antennae light brown to brown (Fig. 6A, B), legs and venter contrasting light yellow to brown (Fig. 6), antennae and legs increasingly darker brown distally (Fig. 6B, E, G).
All characters as in M. halabala sp. nov., except as follows.
Paraterga 2 broad, anterior edge angular and rounded, lateral edge with a small notch at about 1/4 in front of caudal corner (Fig. 6A). Calluses on paraterga rather narrow, delimited by a sulcus fully on dorsal side and in posterior half on ventral side; on poreless rings narrower than on pore-bearing ones in dorsal view (Fig. 6B, D, E).
Transverse sulcus distinct (Fig. 6A, C, F), complete on metaterga 5-17, narrow, line-shaped, rather deep, not reaching the bases of paraterga, smooth at bottom, incomplete and nearly wanting on ring 18. Stricture between pro-and metazona wide, deep, beaded at bottom down to base of paraterga starting with segment 5 (Fig. 6A-F). Pleurosternal carinae complete crests with a sharp caudal tooth on rings 2-4, increasingly reduced and retaining a sharp caudal tooth on rings 5 and 6 thereafter, retaining a small caudal tooth on ring 7, missing further on (Fig. 6B, D, E).
Sterna moderately setose, shining, cross-impressions shallow, without modifications; an entire, large, linguiform, sternal lobe between ♂ coxae 4, with a pair of small denticles laterally near base (Figs 6H, I, 7E). An inconspicuous and low ridge present in front of gonopod aperture. Legs long and slender, midbody ones ca. 1.6-1.9 times as long as body height, without modifications, ♂ tarsal brushes absent.
Etymology. To emphasize "hulutbeeda" which means "flat-back millipede" in Malay dialect, a noun in apposition. A Malay dialect language is mainly used in three provinces of southern Thailand where the holotype was obtained. Remark. This species was found living together with M. halabala sp. nov. Moreover, according to our observations, they may even occur syntopically, sharing the same habitat: leaf litter surface, branches of trees and tree trunks.

Discussion and conclusion
In accordance with the previous observations of related genera such as Orthomorpha Bollman, 1893, Desmoxytes Chamberlin, 1923and Tylopus Jeekel, 1968, the coexistence of congeners is quite common to come across in Paradoxosomatidae generally and Orthomorphini in particular. So the syntopy of Malayorthomorpha halabala sp. nov. and M. hulutbeeda sp. nov. is not unusual. For example, Desmoxytes planata (Pocock, 1895) was discovered beside D. octoconigera Panha, 2018, D. golovatchi Srisonchai, Enghoff &Panha, 2018 andD. purpurosea Enghoff, Sutcharit  & Panha, 2007 in several places (Srisonchai et al. 2018a). Nagaxytes erecta Panha, 2018 andN. gracilis Srisonchai, Enghoff &Panha, 2018 were found jointly at Daowadueng Cave and Wat Sunantha Wanaram in Kanchanaburi Province, Thailand. Both latter species also show a very similar pattern of gonopodal structure (Srisonchai et al. 2018b). In the genus Tylopus, numerous species have been reported co-occurring in larger mountainous regions, such as Doi Inthanon (10 species) and Doi Suthep (10 species) in Thailand. However, some of them, at least in adult stages, appear to reflect separate phenofaunas that are restricted to relatively limited time periods and therefore do not overlap with others (Likhitrakarn et al. 2010(Likhitrakarn et al. , 2014(Likhitrakarn et al. , 2016. . Malayorthomorpha hulutbeeda sp. nov., ♂ holotype, right gonopod A, B mesal and lateral views, respectively C-F submesal, subsuperior, oral and subcaudal views, respectively. Scale bars: 0.2 mm Therefore, it is far from surprising that both new species have been found to coexist at the same place. Although they share the same habitat, they may have distinct microhabitats, although this remains speculation at this stage. Malayorthomorpha species are presently endemic to southern Thailand and northern Peninsular Malaysia, both of which are located within the Titiwangsa Mountain Range, which is known as Peninsular Malaysia's backbone and longest mountain ridge. It begins in the north of southern Thailand, crosses the Malaysian border, enters the Negeri Sembilan valley, and terminates in the south near Jelebu, Negeri Sembilan (Chan et al. 2019). Mount Korbun is the highest peak in the Titiwangsa Range, reaching 2183 m above sea-level, and the second highest mountain in Peninsular Malaysia. In terms of biodiversity, the Mount Korbun area alone supports at least 18 amphibian, 134 bird, 42 mammal, and 18 reptile species, in addition to around 460 kinds of flowering plants and approximately 100 species of ferns and fern allies (Chan et al. 2019;Musthafa and Abdullah 2019). Due to the high biodiversity this mountain range supports, there are still many undiscovered species of flora and fauna. Thus, there are numerous unexplored millipede habitats in southern Thailand, particularly in the Pattani and Narathiwat provinces. Without doubt, new and exciting species will be discovered, and new localities reported, in this and surrounding regions in the future.