The insect is big pretty big compared to other mayflies. It’s a size 10-12 and is like a T- bone steak compared to some of the other bugs they normally eat.
The nymph starts off as a brown color and turn black just before emergence. You’ll know they’re about to hatch when they get a whiteish split in their wing case. Like other mayfly hatches, trout will eat the emerging nymphs pre hatch. Then switch to eating the adults off of the surface. It’s sort of an awkward bug while it’s first trying to gain flight. You’ll see one tumbling through the current causing a trout to explode on it before making it into the air.
My favorite dry fly pattern for this hatch is the Hairwing Drake. It floats great and is a pretty good imitation. Use sizes in 10-12. Also an olive Stimulator works great.
For nymphs I like split case patterns in black, brown and olive. Again use size 10-12. Make sure to use bead head versions as a dropper behind your dry fly. This will catch those trout still focused on emergers.
I was in this cool film back in the Four Seasons days. It’s on the Middle Provo Green Drake hatch. Check it out.