A yellowish mayfly with brown chevrons pointing toward the tail. Body is about 2.5 cm, and the cerci are a bit shorter. The wings are transparent and colorless (or very faintly colored) except for a very pale yellowish anterior border and fine dark markings. I believe that the coloration is consistent with the subimago (aka dun) stage of development, rather than imago stage (aka, spinner or sexually mature adult); please feel free to correct me on this.
Found on the side of Cosby High School on my way in to work this morning.
1. Based on what I read on BugGuide.net and elsewhere, Hexagenia limbata is extraordinarily difficult to distinguish from H. rigida. One of the main difficulties in distinguishing the species is that the appearance of H. limbata is highly variable. As far as I can see, BugGuide doesn't have any pictures ID'd as H. rigida--makes me think that either a photo isn't sufficient for unambiguous ID of rigida, or a reclassification has occurred and they've been lumped together. 2. Hexagenia is often used as a model organism to determine the health of aquatic ecosystems. In particular, Hexagenia nymphs very efficiently accumulate organic mercury compounds.