The trout were sipping, gulping and leaping on these insects, but I and other anglers were unable to catch the fish. We were using very small Elk Hair Caddis patterns, but even these in size #18 and #20 seemed to big.
What sort of technique could a fly fisher use to attract a strike considering the tiny adult insects were NOT drifting on the surface, but dappling instead?
Sometimes, size does matter...:) Your considering a "micro-caddis" is more than likely dead on...The ovi-positing females occasionally get stuck to the surface when they attempt to free themselves from their eggs. The "bulging" rises were probably to these...Out west rather large fish in the Madison will do classic porpoising rises to these trapped caddis.
The important thing is the shear numbers during events like this. It doesn't make logical sense to us as observers that a fish would pass up a larger meal, your size 18, for something so small, but they are keyed into these bugs in an almost routine manner...Selective.
Dabbling, in my experience, works on small brook trout...Raise your rod high and skate the fly across the surface, then occasionally drop the rod and let the fly float...It takes a little practice to make it look life like and not put the nicer fish down. Palmered flies like the old Elk Hair work nice...It needs to ride on top.
Sometimes nature is just flat out better at it than we can ever hope to be...Sometimes the fish win.
Size to me is number one, then a design that closely imitates the naturals behavior...In this situation really small classic wets will work and spent caddis patterns...This imitates the flies that get stuck to the surface film and don't rise back up after.
Hope this helps?