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.
Key features I discerned with a microscope include that the femoral flange is blunt (later note: on nymphs of this size, it's somewhat ambiguous and the femora can be damaged in the collection process), the postero-lateral spines have a longer anterior, the first gill is not extended, all gills are tracheated, and the femora have a distinctive brown spot on the center while all the other spots are gray.
This mayfly was collected from unknown in Wisconsin on February 7th, 2004 and added to Troutnut.com by Troutnut on January 25th, 2006.