The first global account of the diversity of the Ephemeroptera family Leptophlebiidae in New Guinea is provided, based on collections of nymphs collected throughout the island and in some neighboring islands by Michael Balke (Munich) and his collaborators mainly between 2006 and 2011. All specimens belong to the subfamily Choroterpinae and to the tribe Thraulini. All species can be accommodated in three genera: Thraulus Eaton, 1881, Nonnullidens Grant & Peters, 1993, and Kosminymphagen. nov. The genus Thraulus is divided into two subgenera, namely Thraulus and Masharikella Peters, Gilles & Edmunds, 1964, stat. nov. which is removed from synonymy with Thraulus. Thirty-one new species are described herein. The subgenus Thraulus is reported with nine new species: Thraulus (Thraulus) eloisaesp. nov., Thraulus (Thraulus) grantisp. nov., Thraulus (Thraulus) longinquussp. nov., Thraulus (Thraulus) nabiresp. nov., Thraulus (Thraulus) noesp. nov., Thraulus (Thraulus) ogeasp. nov., Thraulus (Thraulus) timorensissp. nov., Thraulus (Thraulus) wemalesp. nov., and Thraulus (Thraulus) sp. A. Five new species of the subgenus Masharikella are described: Thraulus (Masharikella) iterissp. nov., Thraulus (Masharikella) johannislucisp. nov., Thraulus (Masharikella) pascalaesp. nov., Thraulus (Masharikella) samuelisp. nov., and Thraulus (Masharikella) sp. B. Besides the seven known species placed in the genus Nonnullidens, 11 new species are described: Nonnullidens alvarezisp. nov., Nonnullidens angasp. nov., Nonnullidens boonsoongisp. nov., Nonnullidens cozzarolaesp. nov., Nonnullidens fuyugensissp. nov., Nonnullidens kaltenbachisp. nov., Nonnullidens marcelaesp. nov., Nonnullidens moiorumsp. nov., Nonnullidens silvaepumilorumsp. nov., and two unnamed species. Similar to Nonnullidens, the new genus Kosminymphagen. nov. is endemic to New Guinea and is currently restricted to the eastern part of Papua New Guinea. It is composed of six new species: Kosminympha asarorumsp. nov., Kosminympha balkeisp. nov., Kosminympha baruyasp. nov., Kosminympha kalamorumsp. nov., Kosminympha paulinaesp. nov., and Kosminympha sabrinaesp. nov.Nonnullidens mariae (Peters & Tsui, 1972), the type species of the genus Barba Grant & Peters, 1993 is redescribed, and the synonymy of Barba with Nonnullidens is followed but shown to be not completely satisfactory. A key for all currently known species of leptophlebiid nymphs occurring in the area is provided. Compared to the work of Cozzarolo et al. (2019) who investigated the genetic diversity of this fauna, our results appear to be more conservative, some molecular operational taxonomic units (MOTUs) being lumped together based on morphology. The affinities of this fauna with neighbouring areas of Southeast Asia and Australia are also discussed. Although this study reveals a large part of the hidden biodiversity of New Guinea, it is far from being complete and a huge number of new species of Leptophlebiidae await formal description.
