Megabranchiellagen. nov. (Ephemeroptera: Baetidae) is established as a new baetid mayfly genus from northern Thailand. Two new species, Megabranchiella scutulatasp. nov. and Megabranchiella longusasp. nov., are described. This genus is distinguished from other Baetidae by abdominal segment I, bearing a pair of enlarged, ventrally oriented single gills, covering abdominal sternites II–V; other gills have normal size and are dorsolaterally oriented. The two new species Megabranchiella longusasp. nov. and Megabranchiella scutulatasp. nov. can be differentiated by the setation of femur dorsal margin and the shape of abdominal gill I. This mayfly genus was found in flowing water with cobble microhabitats in headwater streams of northern Thailand.

Following a series of biodiversity surveys conducted in Thailand, Acentrella fimbriata sp. nov. is described and illustrated based on the nymph, male subimago and egg stages. Our new species can be distinguished from other Acentrella species by the following combination of characteristics in the nymphal stage: a head capsule relatively densely covered with long, fine, simple setae, increasing in number on the genae and clypeus; the scape and pedicel densely covered with long, fine, simple setae along the inner margins; femoral margins with dense, irregular rows of long, simple setae; terga lateral margins with numerous fine, simple setae, increasing in number posteriorly; terga posterior margins with poorly-developed denticles, and with scattered ovoid and truncated translucent scales, increasing in number laterally; and gills on segments II–VII with abundant long, fine, simple setae apically. This species prefers cobble substrates in shallow and slow-moving water, where it is found covered with sediment. Extensive new reports of Acentrella (Liebebiella) vera (Müller-Liebenau, 1982) are given that show it to be a species widespread in Thailand and that provide basis for comparisons to our new species. Global systematics of Acentrella Bengtsson, 1912 are discussed.

The damselfly genus Bayadera Selys, 1853 comprises 17 species, but the larvae of only four species have been described. Here we describe the final stadium larva of another species—Bayadera serrata Davies & Yang, 1996. Larvae were collected from a headwater stream at Nam Nao National Park, Phetchabun Province, Thailand. The larva of B. serrata is distinguished from congener species following the presence of one to three distinct spines on the genae, two pairs of setae on the ligula, one pair of setae on the ventral side of prementum, the presence of three teeth of the distal end of the labial palp, the presence of a plate-like spine on the gonapophyses, the presence of a row of rod-like setae on the distal end of the tibial comb, the presence of a cluster of long simple setae on the abdominal terga S3–9, and the presence of short terminal filament of the caudal gills. The larvae of genus Bayadera have been compared with other known larvae from family Euphaidae. Key is provided to the genera with known Southeast Asian euphaeid larvae: Anisopleura, Bayadera, and Euphaea.

A new species of Baetidae, Alainites siamensissp. nov., is described from Thailand. This new species is closely related to Alainites lingulatus Tong & Dudgeon, 2000, Alainites laetificus (Kang & Yang, 1994) and Alainites yixiani (Gui & Lu, 1999). Species delimitation based on morphological and molecular (mitochondrial COI sequences) evidence is provided. The discovery of this species confirms the wide distribution of Alainites Waltz & McCafferty, 1994 in Southeast Asia.