The Butterfly is a popular fly for Atlantic Salmon. I should really bring up East Coast flies more often. They have some unique character that you can only find out east. Interesting enough, it was a Florida man by the name of Maurice Ingalls who developed the fly and brought it to fish the mighty Miramichi in New Brunswick. The fly is now popular across the Maritimes and can be found being flung on rivers all over. The spilt wing design creates a pulsing action and is said to drive the fish nuts.
Follow the link to view the step-by-step tutorial for tying the Butterfly – http://www.novascotiafishing.com/om/article.asp?id=186.
Check out some of the variations found at W.W. Doak in New Brunswick. Butterfly Variations