Hexagenia mayflies are seldom seen, but talked about nonetheless. Large (2") and bright yellow, it is no wonder that they seek out the twilight to rise from their burrows and fly vertically into the evening. They are prime forage for aquatic and airborne predators. I saw rainbows, nighthawks and a large bat species feeding on them simultaneously. Had it been an hour earlier and these mayflies would never have survived the oslaught. They seek out muddy bottoms of alpine lakes and take at least two years to develop; as nymphs they are large- size 4 or 2's. As adult emergers, ridiculous. Check out my photos.
My oldest son & I spent two nights on Lost Lake, Mt. Hood Wilderness, trying to time the evening hatch and finding the window of opportunity quite narrow, to say the least. The hatch took off between 9:00 and 9:20pm both nights, leaving us scratching our heads and wondering whether we had missed something. But no, that was it. How ephemeral, how mayflyish.
The good news is that such alpine lakes harbor decent trout fishing during the other hours, as well as ample opportunity to enjoy the amenities of the lake for swimming and general carousing.