Hidden Diversity of Cave Millipedes From Mainland Southeast Asia Revealed by Species Delimitation and Phylogenetic Analysis, With a Description of a New Genus (Spirostreptida: Cambalopsidae)

Southeast Asia’s karst landscapes host a remarkable yet understudied diversity of cave‐dwelling fauna, where morphological conservatism often masks true species boundaries. Using an integrative taxonomic framework, we investigated the systematics of the millipede family Cambalopsidae Cook, 1895 inhabiting caves across Thailand, Laos, Myanmar, and Cambodia, combining morphological evidence with mitochondrial cytochrome c oxidase subunit I (COI) sequences. Analyses of 213 specimens revealed the polyphyletic relationships of the two largest genera, Glyphiulus Gervais, 1847 and Plusioglyphiulus Silvestri, 1923, necessitating major taxonomic revision. Consequently, we erect Somsakiulus Seesamut & Likhitrakarn, gen. nov. to accommodate a distinct clade formerly classified within Plusioglyphiulus , and resurrect Cambalomorpha Pocock, 1895 stat. resurr. from synonymy within Glyphiulus . Species delimitation analyses using four single‐locus methods (ASAP, GMYC, bPTP, and mPTP) consistently uncovered extensive cryptic diversity, with molecular data identifying substantially more putative species (molecular operational taxonomic units [MOTUs]) than morphology, most notably within the Trachyjulus unciger species complex (46–54 MOTUs vs. 25 morphospecies). This work underscores the limitations of relying on traditional gonopod‐based taxonomy alone, elevates the diagnostic importance of peripheral characters, such as the male first legs, and collum and mid‐body carinotaxy, and provides a robust systematic framework essential for future evolutionary studies and conservation planning within these vulnerable cave ecosystems.