Female imagoes usually with intensely colored leading edge of the forewing, males may have this coloration less pronounced or lacking; subimagoes typically have dull grey, brown, or tan wings with a network of pale veins (based on Lloyd Gonzales's comment).
"Following mating, female imagoes of genera Callibaetis and Cloeon seek a safe place to remain stationary for five days or so, until their eggs have matured. Once their eggs are mature, the female imago flies off to land on the water, and deposit her load of eggs, which immediately start hatching. This behavior is unique to genera Callibaetis and Cloeon, as the eggs of other mayfly genera require up to two weeks following oviposition to begin hatching.
" (Roger Rohrbeck's comment here
New World, more diverse in the Neotropics(2)