I remember the first time I saw a mayfly hatch when I was a kid. We were visiting my grandparents' cottage on an island in a like in SW lower Michigan. It got dark and was time to leave. We put the car on the ferry and when we got to the shore-side, there was a street light to light the landing, and a phone booth (for you kids, google "phone booth"). The street light and phone booth were completely covered with mayflies. So thick that the light was darkened, and if there was somebody in the booth, you couldn't have seen them. The road was covered as well. Left a lasting memory in my brain.