Dave Whitlock's Red Fox Squirrel Nymph is one of the modern classics - and deserves a place in every trout angler's fly box:
Red Fox Squirrel Nymph
Hook: Nymph hook 3XL #14
Thread: Benecchi 12/0, tobacco
Rib: Tinsel, gold fine oval
Tail: Red Fox Squirrel guard hairs - sparse
Abdomen: Red Fox Squirrel; SLF Finesse, pale olive; amber antron - mixed
Thorax: Red Fox Squirrel - in split thread
Video here - please view in HD, if able:
http://www.youtube.com/watch?v=kaHOHV4hRA8
Cheers,
Hans W