I wonder if you can please indicate, if you can tell anyway, whether the individual is a full adult (imago), or a subimago ('adult' that has yet to go through its final molt)? I'd be curious of the answer on any and all of our mayfly images; or, please indicate how one can tell the difference. Thank you.

is that the wings are more opaque (covered with lots of anti-wetting hairs to help escape from the water) in the subimago than the imago (translucent wings), and the imago the genitalia are fully formed (best for males, this is a female), and the colors are brighter. It helps to have seen both stages, and from a photo I'm not always sure which stage it is. I think this is a subimago because the hindwing looks opaque, but the imago has a very similar bright yellow color, just slightly more transparent. Or it could be an imago.