http://www.wiflyfisher.com/hex-limbata-mayfly-hatch.asp
“The nymphs of Hexagenia limbata live in the muck, sand, and silty bottoms of pools and back waters of many Wisconsin streams and lakes. At dusk, in late June to mid-July the nymphs begin to come out of their burrows in the mucky bottoms and wiggle and swim towards the surface. These partially emerging duns are extremely vulnerable to eagerly waiting trout, smallmouth bass or other fish. At this time, a wiggle nymph or a large emerger pattern can be very effective. Once they shuck their nymphal case the large yellowish Hex duns pop to the surface and try to become airborne quickly before being devoured.”
The Wisconsin Flyfisher has some great pics of several mayflies. http://www.wiflyfisher.com/fly-hatches.asp
SwittersB & Exploring 