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.
Keywords: COI; Ephemerelloidea; integrative taxonomy; mayfly.