The Griffith's Gnat is a midge-cluster pattern. In cold water, midges hatch and mate in knots of dozens, and trout will key on those clusters instead of picking off single insects. This fly fakes that knot, and a cluster is far easier to see and fish than a single size 24 midge. The body is peacock herl (the buggy, iridescent green) wrapped from bend to eye, with a grizzly hackle palmered the full length. The hackle tips stand in for tangled legs and wings and ride the fly flush in the surface film like a real cluster.
Tie and fish it small: sizes 18-24, peacock the only color you need. The bigger sizes cover a visible cluster of bugs; drop to 22 or 24 when fish are picky on flat water.
This is a cold-water, clear-water fly. It earns its keep at 40-60F on low, slow flows - winter and early-spring tailwaters and spring creeks where midges are the only game. Fish it dead-drift on long, fine tippet (6X-7X), cast upstream, and watch for a quiet sip rather than a splash. If you can't see the fly, watch the spot where it should be and set on any rise nearby.