First report of the genus Tenuibaetis (Ephemeroptera, Baetidae) from Thailand revealing a complex of cryptic species.

A new species of the genus Tenuibaetis Kang & Yang, 1994 is described from Thailand and the genus is reported for the first time from this country. Tenuibaetispanhai sp. nov. is easily distinguished from other known Tenuibaetis by the complete absence of hindwing pads. Molecular evidence based on COI confirmed the validity of the new species. Additional putative species of Tenuibaetis based on molecular evidence only are considered as Molecular Operational Taxonomic Units (MOTUs) without description. The morphological characters of the new species and its closely related species are discussed; a key to the Oriental species is provided.