I've seen Hexagenia hatches on some lakes in the Florida panhandle where the big bluegills were as "selective" as any snooty trout I've ever been around. Once the Hex hatch got going in full swing, there were a few weeks when you could only get the occasional fish on the standard poppers or bugs during the day, and not at all when the flies were hatching. When the sun went down, the hex started coming off and the big bluegills and bass would be feeding like crazy, but they were keyed on the emergers. The locals actually quit fishing during that time because they couldn't catch them on poppers or crickets. We caught extra large bluegills till our arms were tired and we couldn't see anymore.