Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
Actually, from a Pennsylvanian perspective this beaver thing belongs on the Wisconsin contingent thread. PA was almost shed of the things (none had been seen in 15 years) when the Game Commission imported a mating pair from WI in 1915.
So, it's WI that has to answer for this...:)