Brachelytry (elytral reduction) is common in the basal subfamilies of extant Cantharidae, but it rarely occurs in crown groups such as Cantharinae. In cantharine soldier beetles, shortened elytra and reduced hind wings are considered a morphological adaptation to windy, high-altitude mountainous habitats. In the present study, we document the first record of a brachelytrous species of Cantharinae in Taiwan. Based on a combination of morpho-anatomical evidence, COI-based phylogenetic analyses and species-delimitation methods, we recognise it as a new species of Habronychus (Monohabronychus) Okushima & M. Satô, 1999, which is described herein as H. (Mo.) hsui Hsiao sp. nov., representing the first occurrence of brachelytry within the Stenothemus genus-group. The description of the new species is supplemented with illustrations of the habitus and diagnostic characters, a distribution map and a spatial visualisation. Moreover, we explore the phylogenetic placement of Habronychus (Monohabronychus) within the Stenothemus genus-group. Our results unexpectedly suggest the non-monophyly of H. (Monohabronychus), H. (Macrohabronychus) Wittmer, 1981, and Stenothemus Bourgeois, 1907, suggesting that current generic boundaries may require future revision. However, considering the limitations of single-locus data, we refrain from proposing formal taxonomic changes pending more comprehensive, multilocus phylogenetic and morphological studies.
Asia, brachelytry, Cantharinae, new species, sexual dimorphism