I don't know much about mayflies of the Pacific Northwest. But I've been seeing them lately hanging out on the outside of my greenhouse. According to Wikipedia, they belong to a group of insects called Ephemeroptera. They're aquatic insects, which in the adult stage have a very short lifespan, from a few minutes to a few days. The nymphs live longer than a year, in a body of water. The adults' primary function is to reproduce. They have vestigial mouthparts, which apparently are not used for eating (what a bummer that would be), and a digestive system full of air.
Not as pretty as butterflies or dragonflies, but then, they hold still longer.