Abstract The males of almost all sepsid species have strongly modified forelegs that are used to clamp the female’s wingbase during mounting. Here, we describe a new species in the genus Perochaeta whose males have unmodified forelegs. We use DNA sequence data for ten genes to reconstruct the position of Perochaeta on the phylogenetic tree for Sepsidae, and reveal that the lack of foreleg armature in Perochaeta dikowi sp.n. is secondary. Through the study of the mating behaviour of the new species, we demonstrate that the loss of armature is correlated with a new mounting technique during which the males of P. dikowi do not use the foreleg to clamp the female’s wingbase. Instead, the male approaches the female from behind and bends his abdomen forwards in order to establish genital contact. Our study shows how data from morphology, phylogenetics, and behavioural biology can complement each other to yield a deeper understanding of how changes in morphology and behaviour are correlated.