Introduction

The millipede genus Plusioglyphiulus Silvestri, 1923, restricted to Southeast Asia, is one of the most conspicuous, species-rich, widespread, and frequently abundant groups dominating the cavernicolous fauna of the region (Golovatch 2015). To date, the genus comprises 30 described species ranging from southern Myanmar, Thailand, and Laos in the west to Borneo in the east and southeast (Golovatch et al. 2009, 2011; Likhitrakarn et al. 2018, 2020a). The highest diversity has been recorded from Thailand, which alone harbors 14 species (Golovatch et al. 2011; Likhitrakarn et al. 2023), followed by Malaysia and Indonesia (seven species combined from the Malay Peninsula and Borneo) (Golovatch et al. 2009), Cambodia (four species) (Likhitrakarn et al. 2020a), and Laos (three species) (Golovatch et al. 2009). In contrast, Myanmar and Vietnam are currently known to support only a single species each (Golovatch et al. 2009; Likhitrakarn et al. 2018; Nguyen et al. 2025). From an evolutionary perspective, the Cambalopsidae represents a remarkably ancient lineage. This is evidenced by the recently described Laeviglyphiulus patrickmuelleri Wesener & Rühr, 2025, an amber-preserved fossil from the Late Cretaceous of Myanmar (ca 99 Mya) which exhibits closer morphological affinities to the extant genus Hypocambala Silvestri, 1897, than to Plusioglyphiulus (Wesener and Moritz 2017; Wesener and Rühr 2025).

Plusioglyphiulus species are strongly related to karst landscapes, which are long recognized as biodiversity hotspots that are threatened in Southeast Asia (Clements et al. 2006). Most congeners show a significant degree of micro-endemism due to their limited dispersal capabilities and specific habitat requirements. Species tend to generally be confined to a single cave or a solitary limestone outcrop (Golovatch et al. 2009, 2011; Likhitrakarn et al. 2020a). Even though narrow endemism is common, there are some important exceptions where species occupy more than one cave next to each other. For example, P. erawan Golovatch, Geoffroy, Mauriès & VandenSpiegel, 2011 in northern Thailand (Golovatch et al. 2011; Likhitrakarn et al. 2023) and P. digitiformis Likhitrakarn, Golovatch, Srisonchai, Brehier, Lin, Sutcharit & Panha, 2018 in southern Myanmar (Likhitrakarn et al. 2018). In most circumstances, a single cave usually supports only one cambalopsid species. However, there is a rare case of syntopy in Borneo, where P. bedosae Golovatch, Geoffroy, Mauriès & VandenSpiegel, 2009 and P. pallidior Golovatch, Geoffroy, Mauriès & VandenSpiegel, 2009 coexist in one cave (Golovatch et al. 2009). The significant size difference between these two syntopic congeners strongly suggests niche segregation to minimize resource competition (Golovatch et al. 2009). Notably, despite frequently forming massive populations within caves, nearly all Plusioglyphiulus species lack troglomorphic features and are thus regarded as troglophiles (Golovatch et al. 2011; Likhitrakarn et al. 2018), with bat guano serving as the primary nutrient source in these subterranean ecosystems (Golovatch 2015).

Even though large karst limestone areas blanket Cambodia’s western and southern parts, they are not too thoroughly surveyed yet for their cave fauna (Clements et al. 2006; Likhitrakarn et al. 2015). As a result, the country remains dramatically understudied for its diplopod diversity, especially when compared to the neighboring Thailand which currently supports as many as 14 recorded Plusioglyphiulus species, mostly cavernicolous (Likhitrakarn et al. 2023). Until recently, only two species of this genus had hitherto been documented from Cambodia: P. dubius (Attems, 1938) and P. boutini Mauriès, 1970, both being presumably endemic to the country. It was not until 2020 that a field survey yielded another two new cavernicolous congeners: P. biserratus Likhitrakarn, Golovatch, Thach, Chhuoy, Ngor, Srisonchai, Sutcharit & Panha, 2020 and P. khmer Likhitrakarn, Golovatch, Thach, Chhuoy, Ngor, Srisonchai, Sutcharit & Panha, 2020, from the southern provinces of Kampot and Kep, respectively (Likhitrakarn et al. 2020a). Consequently, the present list of Cambodian Plusioglyphiulus comprises only four species. Such a paucity of records undoubtedly reflects the severe lack of collecting efforts and taxonomic explorations in Cambodia’s extensive karst systems, rather than the true biogeographical patterns of this genus.

The main objective of the present paper is to describe and illustrate further two new cavernicolous species of Plusioglyphiulus from Cambodia, both recently discovered from karst cave ecosystems in Battambang province, respectively, and recognized as distinct evolutionary lineages in the previous study (Seesamut et al. 2026: fig. 1). The present study also explores a multi-faceted imaging approach to document the delicate somatic and gonopodal structures of these millipedes. By integrated application and comparison using the standard transmission light photography, detailed line drawings, scanning electron microscopy (SEM), and micro-computed tomography (µCT) scanning, we further aim to evaluate the resolution efficiency and cost-effectiveness of each technique, thereby suggesting an optimal visualization pipeline for future taxonomic work on this millipede group. Furthermore, to enhance the value of this contribution for future research, we provide an updated identification key to all six species of Plusioglyphiulus currently known to occur in Cambodia. These discoveries not only fill in the existing biogeographical gaps for this genus but also strongly reiterate the urgent need to evaluate and drive conservation strategies for the imperiled karst ecosystems in the region.