The genus Vietnamella Tshernova, 1972 is investigated in detail for the first time in Thailand. As a consequence, four species are recognized, namely Vietnamella maculosasp. nov., Vietnamella thani Tshernova, 1972, Vietnamella sp. B and Vietnamella sp. C. Herein, larvae and eggs of V. maculosasp. nov. are described and reported from Chiang Rai Province. The larva of Vietnamella sp. B from Tak Province is also described, but not named due to insufficient material, and the imaginal stages and eggs of V. thani Tshernova, 1972 are described and presented for the first time. Our morphological evidence is supported with COI data. The phylogeny showed that four different lineages of the genus Vietnamella occur in Thailand, one of them, viz., Vietnamella sp. C, only known from a couple of COI sequences retrieved from the Barcode of Life Data System (BOLD). Diagnoses for all known Oriental species are also presented.

A new species of Heptageniidae, Paegniodes sapanensissp. nov., is described based on larvae, subimagos, eggs, and COI data. The mayfly genus Paegniodes Eaton, 1881 is reported for the first time from Thailand. The larva of the new species can be distinguished from other known Paegniodes species by i) lamellae of gill I ca 1/4 of fibrilliform portion and ii) mandibles and basal segment of maxillary palp without dense setae on margin. The subimago characters useful to distinguish this new species from previously known species are i) the median stripes on abdominal terga and ii) shape of the female subgenital and subanal plates. The genetic distance between the new species and P. cupulatus (Eaton, 1871) was approximately 11%. The morphological characters of the new species are discussed and compared to other known species.

Cincticostella tornata, a new species of Ephemerellidae, is described from China. It belongs to the insolta-species group and is closely related to C. femorata. It is hitherto only known from its type locality in central China, more than 1000 km northern to the known distribution of C. femorata. In addition, the COI barcode of the new species is provided.

Larvae of the Notacanthella Jacobus & McCafferty, 2008 species N. commodema (Allen, 1971) and N. quadrata (Kluge & Zhou, 2004) from Thailand are compared and redescribed. Both species have serrations on maxillary canines, as does N. perculta (Allen, 1971) from Vietnam, and this contributes to synonymizing the subgenus Samiocca Jacobus & McCafferty, 2008 under Notacanthella s.s. (= Samiocca n. syn.). The Thailand species can be differentiated by the numbers of thoracic and abdominal tubercles. In addition, the chorionic structure of N. quadrata is investigated for the first time by scanning electron microscopic technique.

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.