A New Species of Vietnamella Tshernova 1972 (Ephemeroptera: Vietnamellidae) from Thailand

SIMPLE SUMMARY: The monogeneric family Vietnamellidae is endemic in the Oriental region and found only in the Indomalayan biogeographic realm. Seven nominal species have previously been described, but three are considered to be junior synonyms. However, some provisional species designations have been established. In this study we described formally Vietnamella sp. C as a new species, V. nanensis sp. n., based on the specimens used for initial DNA data and our expanded set of materials from Nan Province, Thailand. Based on morphology, the larva of the new species can be distinguished from those of other congeners. Additional mitochondrial cytochrome oxidase subunit I gene data for the new species are also provided. A key to larvae of all known species in the genus is provided. ABSTRACT: The larva, male subimago, female imago, and eggs of V. nanensis sp. n. are described based on specimens from Mae Hong Son and Nan provinces, Thailand. The female subimago is described based on a photograph of a specimen reared to the imago stage. The species previously was distinguished only by DNA barcode data and designated as Vietnamella sp. C. Based on morphology, the larva of the new species can be distinguished with the following combination of characteristics: (i) pattern of serration on the ventral margin of the forefemur, (ii) posterolateral margins of abdominal terga with pairs of acute tubercles, especially terga VI and VII, (iii) a well-developed pair of median ridge projections on tergum X, (iv) the second segment of the maxillary palp being about 1.3× the length of the third segment, and (v) females containing eggs with prominent protuberances on the chorionic surface. A key to larvae of all known species in the genus is provided.