Thank you for the ID. Is it the eye spacing that is diagnostic of the sex? Do all mayflies in the imago stage act erratic like this one was?

In most (but not all) mayfly families, the males have much larger eyes than the females, often covering most of the facial area. (Leptohyphidae, Caenidae, and Potamanthidae are examples of exceptions to this.) This eye-size difference can often be seen in the mature nymph, as well as the subimago and imago. In addition, the adult males have greatly elongated forelegs (for grasping the female during mating), very long tails, and visible terminalia (claspers) at the end of the abdomen.

The subimago usually seems docile, conserving energy as it awaits the final molt and mating. The imago is much more active, and they often seem to be fooled by some surfaces (paved roads, for example).