Integrated taxonomy of two new species of the millipede genus Plusioglyphiulus Silvestri, 1923 from Cambodia (Diplopoda, Spirostreptida).

Two new sympatric cavernicolous species of the millipede genus Plusioglyphiulus Silvestri, 1923, are described from Battambang Province, northwestern Cambodia: Plusioglyphiulus parviserratus Likhitrakarn & Seesamut, sp. nov. and Plusioglyphiulus battambangensis Likhitrakarn & Seesamut, sp. nov. Both new taxa are diagnosed based on an integrated taxonomic approach combining morphological features and mitochondrial cytochrome c oxidase subunit I (COI) gene sequences from a previous study. Plusioglyphiulus parviserratus sp. nov. is distinguished from all congeners by having the male leg 1 telopodite distinctly 2-segmented with a rudimentary, knob-like second telopoditomere, the anterior gonopod coxosternal processes long, slender, and regularly curved anteriorly, resembling a bird's head, and a unique micro-serrate upper margin of the broad flagellum process. In contrast, P. battambangensis sp. nov. is characterized by a distinct carinotaxic formula of the collum, a significantly smaller second telopoditomere on the male leg 1, a slender, elongated, paramedian coxal process of the posterior gonopod, and a narrow, smooth-margined flagellum process. The two new species are separated from each other by an interspecific COI p-distance of 10.3% and show divergences of 9.6-9.9% from their closest known relatives, all lying well within the stable range observed for species boundaries in the Cambalopsidae and therefore strongly supporting their independent specific status. Beyond traditional descriptions, this study evaluates the resolution efficiency and practical limitations of multiple imaging modalities, including the standard transmission light photography, traditional line drawings, scanning electron microscopy, and micro-computed tomography scanning. While 3DµCT reconstructions provide powerful, non-destructive visualizations of external somatic architectures, we demonstrate that current in-situ tomographic data remain insufficient for resolving the highly condensed, tightly appressed gonopodial microscopic characters, thereby emphasizing the continued necessity of physical dissection and SEM in cambalopsid taxonomy. These findings increase the number of Plusioglyphiulus species currently known to occur in Cambodia to six, highlighting the significant, yet severely understudied, cavernicolous diplopod diversity in the country's extensive karst systems. An updated identification key to all six Cambodian Plusioglyphiulus species is provided. These discoveries underscore the critical need for continued taxonomic exploration and the implementation of conservation strategies for the imperiled limestone karst ecosystems in the region.