See Acerpenna pygmaea for details. It is the only species known to be of even minor importance to anglers and was previously known as Baetis pygmaeus.
Where & when
In 38 records from GBIF, adults of this genus have been collected during July (37%), June (26%), May (18%), April (8%), September (5%), and August (5%).
In 52 records from GBIF, this genus has been collected at elevations ranging from 3 to 4692 ft, with an average (median) of 1713 ft.
This is a very rare mayfly in western Montana. The shape of the last gill is a key to its identification. I have collected this species in only one location.