Sorta laid up following a surgery. Using all the down time to re-read some books on tying.
Picked up LaFontaine's "The Dry Fly". LaFontaine was one of the few tiers/authors who applied science to the subject. He did a lot of scuba diving to observe and record the trouts' behavior towards insects and fly imitations.
Anyway, he relates how some guys were fishing a PMD hatch on the Henry's Fork one day. The PMD's were size 16 and all were catching. Then the insect hatching became smaller. The fishermen switched to size 18 fly from the previous size 16.
The fish no longer took the flies.
Gary's observation/conclusion was that the wing trigger on the size 18 was so small that it no longer was an effective trigger......
His suggestion: tie the size 18 with a size 16 wing.
Since all the books written which explain what the trout sees ( through underwater photography, slant tanks, etc.) I think I will try it when the condition presents itself.
The fly on the left is a 16; on the right is an 18. Measured the wings to both be size 16.