The butterfly genus Forsterinaria Gray is the only strictly montane representative of the diverse Neotropical subtribe Euptychiina (Nymphalidae, Satyrinae), with 24 described species. Recent research in some of the most isolated and highly diverse Andean regions, such as central Peru, show that its total species richness is still underestimated. An example is the new species described here, Forsterinaria emo n. sp., which is particularly interesting because of an unusual structure discovered in its male genitalia which consists of a bunch of bristle-like processes, composing a fringe-like formation on the dorsum of the tegumen. No similar, homologous structure was found in any congener, nor indeed, in any species of diurnal Lepidoptera. Scanning electron microscope studies revealed that the microstructure of the processes resembles a membrane lining the tegumen. Its function is unknown but two hypotheses are discussed based on a comparative study with other genital structures of butterflies. We argue that it may help stabilizing the partners in the process of mating or it may serve as a 'mating plug', preventing the female from multiple copulations.
Keywords: Morphology; Satyrinae; systematics; taxonomy; tegumen.