Troutnut.com User ReAlizm

This particular picture is slightly blurry because the nymph is swimming, but it shows an important characteristic of Hexagenia nymphs which is potentially important to fly tiers and is evident in the videos as well: Hexagenia nymphs hold their gills splayed almost straight out to the side while they're swimming, while Ephemera nymphs keep their gills closer to vertical. The difference is such that when I'm looking through my tub of critters from the stream trying to pick out which things to photograph, I can tell the two genera apart more easily by looking at their profile as they swim than by looking at their abdominal markings or frontal prominence. The angle of the gills in the swimming nymphs is probably one of the most noticeable differences between these genera from the trout's perspective.

Hexagenia limbata (Hex) Mayfly Nymph from unknown in Wisconsin