Review of the millipede genus Orthomorpha Bollman, 1893 (Diplopoda, Polydesmida, Paradoxosomatidae) in Vietnam, with several new records and descriptions of two new species

The genus Orthomorpha is shown to currently be represented in Vietnam by ten species or varieties, including new records of O. arboricola (Attems, 1937), O. coarctata (de Saussure, 1860), O. rotundicollis (Attems, 1937) and O. scabra Jeekel, 1964, and two new species, O. caramel sp. nov. and O. vietnamica sp. nov. A key to all eight Orthomorpha species and two varieties known to occur in Vietnam is provided. Although the morphological characters that have been traditionally used for Orthomorpha taxonomy are here considered superior to molecular ones, molecular-based phylogenetic relationships and taxon assignments within the tribe Orthomorphini are provisionally analyzed using fragments of the cytochrome c oxidase subunit I (COI) mitochondrial gene. The preferred phylograms, both rooted and unrooted, demonstrate the monophyly of the tribe Orthomorphini, but due to the special, uncertain or even controversial position of O. coarctata, which occurs closer to the genera Antheromorpha and Hylomus, the genus Orthomorpha in current usage appears to be polyphyletic. However, if O. coarctata is to be treated within the monotypic genus Asiomorpha, the monophyly of Orthomorpha becomes manifest. On the other hand, a cautious approach is followed to avoid descriptions of suspicious new taxa/species. Thus, solely because the average genetic distance between O. rodundicollis subrotundicollis var. nov. and O. rodundicollis, as well as that between O. scabra grandis var. nov. and O. scabra, are both found to be negligibly small, the statuses of the sympatric and closest yet morphologically different varieties are treated only as such, i.e., infrasubspecific categories. The apparent discord observed between morphological and molecular data is obviously due to only partial and single-gene topologies used, possibly also to hybridization already known to occur in some closely related and sympatric paradoxosomatid species or even genera.