Two new species of the millipede genus Plusioglyphiulus Silvestri, 1923 from Cambodia (Diplopoda, Spirostreptida)

Abstract Two new species of Plusioglyphiulus are described from southern Cambodia. Plusioglyphiulus biserratussp. nov. is clearly distinguished from all congeners by the shape of the telopodites of the posterior gonopods which are distinctly serrate laterally and by the anterior gonopods showing only a pair of single, smooth and curved coxosternal processes. Plusioglyphiulus khmer sp. nov. is distinguished by having most crests on the collum being complete and male legs 1 showing long, prominent, one-segmented telopodites, coupled with the oblong-subtrapeziform, membranous, posterior gonopods with a small bifid process at about a third of the telopodite length. Notes on the variation of Plusioglyphiulus boutini Mauriès, 1970 are also given, including a colour photograph of fresh, live material. A key to all four species of Plusioglyphiulus currently known to occur in Cambodia is also presented.


Introduction
The strictly Southeast Asian genus Plusioglyphiulus Silvestri, 1923 is one of the most diverse, common and often highly abundant groups of millipedes that dominate cave faunas (Golovatch 2015). At present, this genus comprises 28 described species ranging from southern Myanmar, northern Thailand, and Laos in the west to Borneo in the east and southeast (Golovatch et al. 2009Likhitrakarn et al. 2018). The genus is very ancient, as one, still undescribed species is known from Burmese amber approximately 99 Mya in age (Wesener in litt.).
Only two species of Plusioglyphiulus have hitherto been documented from Cambodia: P. dubius (Attems, 1938) and P. boutini Mauriès, 1970. Both are presumably endemic to the country. Like in Thailand, where there are 14 recorded species, mostly cavernicolous, large karst limestone areas blanket Cambodia's western and southern parts, but they are not prospected yet for their cave fauna.
Most Plusioglyphiulus species have been recorded and described from a single locality or cave in Southeast Asia, where karst habitats are recognized as hotspot for species diversity and endemism (Clements et al. 2006). Each cave tends to be populated by a single species, with the only exception being P. bedosae Golovatch et al., 2009 andP. pallidior Golovatch et al., 2009, which are sympatric in the same cave on Borneo. Golovatch et al. (2009) suggested that the difference of more than twice the body size between these two species may be the reason for their niche segregation. On the other hand, there are two species, P. erawan Golovatch et al., 2011 from Thailand and P. digitiformis Likhitrakarn et al., 2018 from Myanmar, each of which has been reported from several adjacent caves.
In 2019, during a field survey in Kampot and Kep provinces, southern Cambodia, we found another two new species of Plusioglyphiulus, as well as fresh material of an earlier described congener. The present paper is devoted to descriptions and illustrations of these new species and also includes a key to all four Cambodian Plusioglyphiulus species known to date.

Material and methods
Specimens were collected in Cambodia under the Animal Care and Use Protocol Review No. 1723018. The collecting sites were located by GPS (WGS84 datum) using a Garmin GPSMAP 60 CSx, and all coordinates and elevations were checked with Google Earth. Live animals were photographed. The specimens collected were euthanized by a twostep method following AVMA Guidelines for the Euthanasia of Animals (AVMA 2013). Specimens were then preserved in 95% ethanol for morphological and molecular studies.
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 stereomicroscope equipped with a digital SLR camera. The terminology used and the carinotaxic formulae in the descriptions follow those in Golovatch et al. (2007aGolovatch et al. ( , b, 2012, while body segment counts are after Enghoff et al. (1993) and Golovatch et al. (2007a).
The holotype, as well as most of the paratypes are housed in the Museum of Zoology, Chulalongkorn University (CUMZ), Bangkok, Thailand; a few paratypes have also been donated to the collections of the Zoological Museum, State University of Moscow, Russia (ZMUM), the Natural History Museum of Denmark, University of Copenhagen, Denmark (NHMD), and the Zoological Reference Collection of the Lee Kong Chian Natural History Museum, National University of Singapore (ZRC), as indicated in the text.
Remarks. The new specimens fully agree with the original description (Mauriès 1970), which was sufficiently detailed and beautifully illustrated. Instead, we only present a few notes on variation and a new illustration (Fig. 1A) to show coloration based on live material. This species was originally described from near Kampong Trach, 10.554N, 104.471E, Kampot Province, Cambodia (Mauriès 1970 Name. To emphasize the telopodites of the posterior gonopods being clearly serrate apicolaterally; adjective. Diagnosis. This new species is distinguished from all congeners by its anterior gonopod structure: in having only a pair of single coxosternal processes (cxp) (Fig. 3H, I) it is especially similar to that observed in P. hoffmani Golovatch, Geoffroy, Mauriès & VandenSpiegel, 2009, but both these species differ in cxp being smooth and distally curved in P. biserratus sp. nov. vs serrate and suberect in P. hoffmani. The posterior gonopods of P. biserratus sp. nov. are unique in showing laterally fringed/serrate telopodites (te), both elongate and membranous (Fig. 3J, K), and ♂ legs 1 with very long, slender and one-segmented telopodites (Fig. 3D, E).
Coloration of live animals light brown ( Fig. 1B) with lighter anterior and posterior parts of body; antennae, venter and legs light yellowish; coloration in alcohol, after six months of preservation (Fig. 2), uniformly red-brownish or dark castaneous brown to grey-brown, dorsal crests and porosteles usually dark brownish. Antennae and venter yellow-brownish to brownish ( Fig. 2A-C, E-G, I). Eyes brown to blackish ( Fig. 2A, C).
Ventral flaps behind gonopod aperture on male segment 7 distinguishable as low swellings with rounded flaps bent abruptly caudad.
Legs short, nearly as long as body diameter (Figs 2F, 3L), claw at base with a strong, spiniform, accessory claw almost half as long as claw itself (Fig. 3L).
Male legs 1 with a usual strong and long central hook (actually a pair of tightly appressed hooks) regularly curved forward; a pair of strong, sac-shaped, one-segmented telopodites, the latter being nearly as long as central hook (Fig. 3D, E).
Anterior gonopods (Fig. 3H, I) simple, coxosternum halves being touching but not really fused medially; each coxite bearing only a single, digitiform, coxosternal process (cxp) with an unciform and laterad directed tip, and a few strong setae medially near base; telopodites (te) simple, lateral in position, movable, one-segmented, digitiform, rounded and bearing several apical setae at tip, shorter than cxp.

Plusioglyphiulus khmer
Name. To emphasize "khmer", referring to the main people of Cambodia; a noun in apposition.
Diagnosis. This new species differs from all congeners by all crests on the collum being undivided, mostly complete (Fig. 5B) and male legs 1 with very long and one-segmented telopodites (Fig. 5D, E), as well as by the presence of 2+2 paramedian, characteristically long, slender, coxosternal processes of the anterior gonopods, a shorter and only apically curved posterior (cxp1) pair, and a longer, mesally micropapillate and regularly curved anterior (cxp2) one (Fig. 5H, I); the posterior gonopods are oblong-subtrapeziform, membranous, each with a small apical spike (d) and a bifid process (k) at about 1/3 telopodite length on anterior face (Fig. 5J, K).
Coloration of live animals light brown to chocolate brown ( Fig. 1C) with blackish or dark brown anterior and posterior parts of body; antennae light brown to brown; venter and legs light yellowish to yellowish white; rather contrasting dark brownish, lateral, longitudinal stripes above ozopores on each side, both interrupted mid-dorsally by a light wide axial stripe; ommatidia brown to blackish (Fig. 4A, C); coloration in alcohol, after six months of preservation (Fig. 4), similar to live one, but body red-brownish or dark castaneous brown to grey-brown; vertex dark brown to yellowbrown; antennae dark brown to blackish; venter and legs light yellowish to yellowish red ( Fig. 4A-C, E-G, I).
Male legs 1 with a usual, strong, central hook (actually a double structure of tightly appressed hooks), regularly curved forward; a pair of 1-segmented, sac-shaped and very long telopodites, the latter almost as long as central hook (Fig. 5D, E).
Male legs 3 modified as usual, with particularly elongate and slender coxae and shortened telopodites (Fig. 5G).
Posterior gonopods (Fig. 5J, K) rather long (high), simple; coxites well-separated from sternum, fused only basally, oblong-subtrapeziform, membranous, with traces of telopodites in the form of a small, latero-parabasal field of microsetae on posterior face of each gonopod; each coxite with a terminal medial spike (d), a subterminal lobule at d base, and a small bifid process (k) at about 1/3 coxite length on frontal face; distal half lamellose, fringed and with a deep fovea subapically.
Remarks. This species was found together with a single male specimen of Orthomorpha coarctata (De Saussure, 1860).
Key to Plusioglyphiulus species currently known to occur in Cambodia 1 All crests on collum undivided and mostly complete (Fig. 5B)  Male leg 1 with 1-segmented, very long telopodites, the latter almost as long as central hook (Fig. 3D, E). Posterior gonopods with membranous, elongate, laterally clearly fringed/serrate telopodites (Fig. 3J, K)

Conclusions
According to the latest catalogue of the Diplopoda of Cambodia (Likhitrakarn et al. 2015), and considering two new Plusioglyphiulus described above, the millipede fauna of the country currently comprises only 21 species from 15 genera, 12 families, and eight orders. In addition, all new records came from only the southern parts of Cambodia. The collecting localities for the millipedes in Cambodia are still very few, especially when compared to the neighboring countries such as Thailand (more than 300 reported localities) (i.e. Likhitrakarn et al. 2011;Pimvichai et al. 2016;Srisonchai et al. 2018). Finally, as regards the present knowledge of the Cambalopsidae, we seem to have only touched the tip of the diversity iceberg of the family (Golovatch et al. 2007b). Cambalopsids are especially diverse and common in karst areas, where they are usually associated with bat guano in caves (Golovatch 2015). There is little doubt that many additional new species of Diplopoda, including Cambalopsidae, can be expected to be revealed by future explorations in Cambodia, especially in the limestone karsts of the country.