Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
This spinner is Eurylophella rather than E. invaria. The long 9th abdominal segment of Eurylophella is often the easiest way to avoid confusing the duns and spinners with Ephemerella. The claspers and genitalia of this male are also representative of Eurylophella.
Eurylophella species comprised the "bicolor" group of Ephemerella until Allen elevated subgenera (like Drunella, Serratella, Attenella, etc.) in 1980. Although widespread and fairly common, they are infrequently mentioned in fly-fishing literature. In streams and rivers, Eurylophella nymphs usually inhabit slow, weedy water, but some are also found in lakes. Most Eastern/Midwestern species seem to emerge sometime in June/July. The dark-winged duns can also be mistaken for other species, like Ephemerella needhami.