The species in this genus were formerly classified in Stenonema. See the genus Maccaffertium for details. Only one species, Stenacron interpunctatum, is important to fly fishermen. See its page for details.
Where & when
In 197 records from GBIF, adults of this genus have mostly been collected during May (27%), June (24%), April (19%), July (16%), and August (5%).
In 143 records from GBIF, this genus has been collected at elevations ranging from 3 to 9236 ft, with an average (median) of 2707 ft.
I know I'm a little out of season, butI was curious if you had any shots of Stenacron Interpunctatum next to your hook quide, or could just give me a size range. I was thinking they were 14-16 but could be wrong. Also you have written that Stenacron Interpunctatum were rarely called cahills. If so what mayfly is it that i'm after (that is common name "cahill"). The stenacron look the same to me, but i have also never kept and memorized any specimens.