While we could not find a lot of information for this particular species, this mayfly is part of the Flat-headed Mayfly family (Heptageniidae), a family whose larvae are known for their wide, flat heads. Their streamlined shape helps them cling to rocks or logs in flowing water, where they graze on algae and organic debris. Adults are most commonly found near cool, clean streams and rivers. This particular one was spotted along the Flathead River. 

When mayflies first emerge from the water they are commonly called a dun (scientifically called the subimago stage). As duns, mayflies physically resemble the final adult stage, but they are sexually immature and duller in color (the eyes, legs and genitalia are not fully developed). Depending on the species, within anywhere from a few minutes to a few days, the dun will molt a final time into the sexually mature adult (or imago). The mature adults lead ephemeral lives, normally living only a day or two, and with only vestigial mouthparts, they do not eat. View a succinct animation of the life cycle, created by the Watershed Education Network, here

Size: 5 – 20 mm for family, excluding tail  

Photo by: Roy Curet on 8/20/22 near Moiese, MT