Quote Originally Posted by Byron haugh View Post
Surely we know the location of the wing base on most mayfly duns??
I'm not sure we do. The postitions of wings in comparison to the body varies widely among mayflies. Look at the trico dun on this page:

http://hipwader.com/2004/10/sunday-morning-tricos/

(the wing pretty much covers the whole length of the body)

and compare it with the bwo on this page:

http://www.troutnut.com/specimen/491

where it's pretty much just the front quarter of the body.

We may know the wing location on any species, but I'm not sure there's a position that applies to "most" species.

What's more, beside tying something that looks like the natural, people desing flies for mechanical properties, like not falling flat on their face. An individual tyer might also put the wing a bit futher back to leave more room for a bushier hackle, or to leave a gap of bare hook for a turle knot. (Not that most of us use a turle knot, but it is kinda a tradition in Catskill flies.)

What I really like about the tool you show is that it makes it easier to tie flies with consistent proportions.