In my experience they do. The biggest and best hatches around me are the BWOs, Sulfurs, Midges, Tricos, and Caddis. There are certain stages in each cycle of the bugs. I've fished sulfurs when they wouldn't nothing but comparaduns or parachutes, or classic-style dry flies. You can add emergers, cripples, etc. to that list. Through a hatch you are using only, say comparaun-style flies, there are always fish willing to eat, but not as many. You also need to consider spinners. You can make it as simple or complicated as you desire. JMO