Rolling thru my mayfly photos I was able to take many pictures at one of the Spring Jams two years ago. I wanted to demonstrate the differences between spinner (imago) male and female. These two coffin flies (Ephemera guttulata ) attached show these differences.
Male (left photo)
Rear claspers or forceps at the rear of the body
Eyes on a male tend to be larger and more bulbus
Female (right photo)
Short fore wing
Forceps do not exist
Smaller flatter eyes