I stumbled into this little bit of knowledge the hard way about a month ago. I was piddle fishing on a local stream when a midge hatch bloomed out of nowhere. Swarms of the little guys were cruising dangerously above the water. The little rainbows went bananas. They began to launch themselves out of the water in chase of the flies fluttering about six inches above the water. At anytime I could spot five to ten rises upstream or downstream.
I tied on my go to dry for midge fishing, the Griffith's Gnat, and immediately ... didn't catch anything? Dry fly fishing a midge hatch is all about two things. First is size, second is color. If you're like me and can't stand tying flies smaller than a 20 then at least be prepared to match midge color even if what you've got is to big. That day I didn't have either, and the fish didn't care. I immediately whipped out some tan and olive Griffith's Gnats the next chance I got and they've already come in handy. Putting a few in your box may just put few more fish in your hands.