The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
It's remarkable what thick hatches, and especially spinner flights, can come from a species few anglers have ever heard of. The genus Cinygmula is thought to produce unremarkable spinner flights, but this one would certainly have gotten the trout going if it weren't happening just before dark on a small stream where there isn't a lot of nocturnal feeding.