The mayfly and its life cycle provides a good bit of flyfishing's raison d'etre as well as a good bit of its vocabulary.
Egg, nymph, subimago, and imago are the four stages that a mayfly goes through. "Dun" is another word for subimago. This mayfly, at the dun stage of its life, can fly, which it does from the water surface to a sheltered spot along the stream where its wings can dry and harden a bit. The wings sometimes have an olive tint to them at this stage. It is now an adult, but thought to be sexually immature. At some point shortly after surviving this flight from water to shelter, it sheds its skin one last time. This mayfly is now fully mature and is known as a spinner.
And there you have it, the sex life of bugs and the fish that eat them. It's a tough life.