I think the answers as noted above are indeed correct, generically speaking. The thing to keep in mind is that each fly no matter what the pattern has the opportunity to be improved upon for each situation. For example an Adams fly fished in low water which moves slowly could afford sparse materials whereas the same fly tied for use in a deep fast moving stream could stand a few more wraps....color variances, wing size, tail size are all open to manipulation depending on the circumstance. Here's a few pics of some flies I have tied and developed to meet my own situation where I fish here in New Brunswick Canada.
Little Brown Mayfly
Dark Quill
Adams
One of my Favourites I call the Whiting's Dun....Coq De Leon hen wings, Coq De Leon Cock neck tail, Whiting Dry Fly hackle, sorry Whiting the only material not Whiting is the hand stripped peacock hearl body....oh and the thread is Lagartun's 95 D in Iron Grey
Experiment, try out the basic pattern and do your homework on the actual hatch you are trying to imitate, in the stream you are fishing.
PS - for any photography buffs wondering, took these with my Pentax KR, 200MM ED lens, f/8, 1/5 sec, ISO 200 no flash just a light at my tying desk, camera fixed on a tripod, with a grey cardstock paper background....I'm just an Amateur trying to find my way....both in fly tying and photography.
Tight Lines!