A new species of Acentrella Bengtsson (Ephemeroptera: Baetidae) from New York and New England (USA), redescription of the nymph of A. parvula (McDunnough), and key to known adult males of Nearctic Acentrella

A new species Acentrella rallatoma (type locality: Connetquot River, Oakdale, NY, USA) is described from nymphs and reared adults. Additional specimens were studied from southeastern New York, southeastern New Hampshire, coastal Connecticut, and Martha's Vineyard, MA, USA. Nymphs can be separated from those of other Nearctic species by: scraper-like outer incisors of mandibles, cerci alternately banded from bases to tips, and row of robust bristle-like setae (with modified tips) on dorsal edge of femora. Male imagos can be separated from those that lack hind wings by: distinctive colour pattern of abdominal terga, and genitalia with pigmented sclerotised penes cover. Comparative analysis of A. rallatoma with other Nearctic species revealed a lack of comparative data on nymphs of its presumptive sister species A. parvula. To facilitate future studies of this group, the nymph of A. parvula is redescribed and regional differences are discussed. Abdominal colour patterns of A. parvula that vary over its range from Florida, USA to Labrador, Canada are illustrated. Problems concerning genus-level diagnostic characters for Acentrella nymphs and adults are discussed to facilitate placement of future taxa. To complement existing keys to Nearctic nymphs of Acentrella a provisional key to the known adult males is given.