The Ephemeroptera get their name from the short lifespan of the adult fly which can be as little as 30 minutes.  Adults have only vestigial mouthparts so their sole purpose as an adult is to reproduce before their energy runs out.

Cloeos dipterum - Nottinghamshire, England - August 2011

Baetis sp. - Derbyshire, England - April 2010

One unique feature of the Ephemeroptera life-cycle is that the fully developed nymph does not directly give rise to the adult fly (imago).  Instead it emerges as a sub-imago.  Although this stage is fully winged it is not quite yet fully developed and, after a period of rest, has to undergo a further moult to reach full adulthood.

Centroptilum luteolum - Nottinghamshire, England - May 2012