Midges are small, very small but at times they make up a significant part of a trout's diet. Of all midge imitations, the Griffith's Gnat is arguably the most well known. Although larger sizes work well, I'm going to try a Griffith's Gnat here on a size 20 #305 Dai-Riki dry fly hook. Because the fly is fairly simple, tying it on really small hooks isn't that bad.
After securing the hook in the vise, I'm going to load a bobbin with UTC 70 Denier black thread. Start the thread just behind the hook eye. After about 4 or 5 turns, snip off the tag end and select a single peacock herl. Starting an inch or two down from the tip, secure the herl to the hook shank working it to the far side of the hook as you go then pull or snip off the brittle tip.
For the hackle, I'm going to use a size 20 grizzly hackle from a Whiting's 100's pack. With the dull side facing toward you, snip a few fibers from either side of the stem to give your thread some purchase. Then strip a few more fibers from the top of the stem to ensure that it wraps with the dull side facing forward. Tie the hackle in at the hook bend and then wind your thread forward to just behind the eye.
With the peacock herl, make adjacent wraps all the way up the shank of the hook. You need to watch out for the hook point so it doesn't nick and break the delicate herl. It's very important to keep light but consistent tension as you advance the herl. Make one wrap underneath the hook shank, in front of the thread then make one wrap of tying thread in front of the herl, one behind, and another one in front then snip the remainder of the herl free. You can then begin palmering the hackle forward around the peacock herl. And repeat the same tie off procedure as before.
With the hackle tip snipped off, do a 3-4 turn whip finish, trying not to trap hackles as you go. Then snip or cut your thread free. Whether trout mistake it for a single midge or a clump of midges, who knows. All that really matters is that the Griffith's Gnat has, over many years, proven itself as a very effective pattern. Try it in sizes from as large as 14 all the way down to 22.