The dry fly fishing was very good as long as I stayed with small size 18 or smaller flies. Unfortunately I was not anticipating this type fishing. Most of the dry flies I had with me were size 14 and larger.
Eventually I lost my last small fly and put on a Parachute Adams size 14 Dry Fly. I fished this carefully over excellent water for the next half hour but I did not get a single strike so I decided to cut my size 14 Adams down to make it as small as possible.
First I removed 3/4 of the tail fibers and shortened those remaining down to 1/8 inch. Next I trimmed most of the parachute hackles off and cut those remaining back to 1/4 inch. I trimmed most of the wing post out and shortened those remaining to 1/8 inch long. The fly looked strange to me but the trout took it readily and I had great fishing with it for the rest of the day.
This happened to me one day while fishing the Firehole (pictured above). After pruning my dry fly I had excellent trout fishing.