Subimago

Although not as abundant, the mayfly are still flying (and being eaten).  Unlike other insects with aquatic early stages, the mayfly undergoes two stages of metamorphosis. 
The first stage is nymph becoming a subimago - winged  but is not yet mature version of it's adult form. The mayfly spends less than 24 hours as a subimago  before moulting for the last time and becoming an adult (the imago - an  adult insect).
It is this final stage that I was privileged to see by the river this morning: