A new species of Acentrella Bengtsson, 1912 (Ephemeroptera: Baetidae) from 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 IIVII 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 (Mller-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.