The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
The question for fly fishermen seeking big trout is: "Which insects provide the best opportunity for catching such fish?" My list would be: Giant Orange Sedge (Dicosmoecus sp.), Salmon Fly (Pteronarcys californica, a stonefly), and the Michigan Mayfly (Hexagenia limbata). Dicosmoecus is the most important -- and the contest is not even close.