RESUMO
Bess beetles (Passalidae) are known to maintain biological interactions with a diverse gut symbiotic community, which is far from being well described since these communities from most of the species of Passalidae have not been studied. In order to improve our knowledge of these communities, we describe a new species of Nematoda of the genus Hystrignathus (Hystrignathidae) associated with Passalus interruptus, a bess beetle species of the tribe Passalini collected in Hunuco, Peru, in the Peruvian Amazonia. Hystrignathus nunashae n. sp. is characterized by the presence of cuticular spines beginning just after the end of the first cephalic annule, a posterior cephalic ring dilated and longer than the anterior ring, as well as ornamented eggs. The new species has a didelphic-amphidelphic reproductive system, lateral alae extending from the posterior end of procorpus almost reaching the anus, and a short subulate tail. The external and internal morphological traits of the new species are described, and additionally, based on nuclear ribosomal DNA, we investigated the phylogenetic relationships of the new species.