In answer to the above...

What purpose does the hackle serve then? In most dry flies, the wings are a small fraction of an inch taller than the hackle, which hides the lower part of the wing. How much "wing" do the fish actually see? In mayflies, the wings are held together above the fly as it rides down the water. How can split wings mimic this? A single post would do a better job.

I posit to you: the hackle ARE part of the wing to the fish, and are almost as tall as whatever wing one puts on the fly. I believe that the hackle serve the same purpose, rendering the wing unnecessary, except where I need an indicator. Otherwise, how could parachute styles of the same winged fly do as well?