In regards too the space behind the eye on a Catskill style fly. Most people think that I t was used for a turle type knot. This is totally incorrect. If the space was for this knot why don't we see it on period wet flies and nymphs of the time. Check it is not there. HarryDarbee in his book gives a lengthy explanation of the reason why. He states that a Catskill style dry fly usually tied on a size 12 hook has the wing set back on the shank of the hook at the expense of a shortened body and leaving a bare shank at the head. This is done to better balance the fly and aid in flotation. Vince Marinaro used the same concept on his thorax style dun.