280 days of Urbpandemonium #34
I know almost nothing about mayflies, but I recognize them when I see them: the tapered body and long tails seem distinctive to me. I know that they breed in clean and/or moving water, and that fly-anglers have probably studied them more than almost anyone. The scientific name for the mayfly order is Ephemeroptera--a nod to the very short lifespan of the adult stage of the insect.
Knowing what little I do, I'm going to attempt an identification anyway. Short middle tail, dark brown body, dark front legs and other legs lighter. I'm leaning toward Leptophlebia cupida, the early brown spinner. This species looks right in my references, plus it is active in April and May, and found across the North American continent. As always, I'm eager to be contradicted by someone who knows the taxa better.