When the big mayfly hatches in the rivers or the lakes in Norway, you have to be there. And your best card in this massive hatches could be to imitate the emerger stage, instead of putting on a adult dun pattern.
This one is made to make a lot of life, and it floats well, though it is some synthetic material in it
Hook : Mustad C53 SNP-BR
Thread : Semperfli Classic Waxed Thread, White 12/0
Tail : Marabou (use color matching your hatch)
Body : Marabou (use color matching your hatch)
Ribbing : Semperfli Straggle Legs, in this case I have used Copper Brown
Floatant : Foam, Grey
Legs/Movement : Grey or Natural CDC
Thorax : Build up thorax with some dubbing
Thorax 2 : Semperfli Straggle Legs, twisted together. I have used Copper Brown & Litchen
Mount the marabou in as a tail, put on some fibers of flash if you want. Then mount your ribbing. Use the rest of the marabou making the body, then rib the fly. Mount the floatant (foam). Then put CDC fibers in the split thread and bend the fibers backwards, it should imitate "some life" on the fly. Then do the rest and you have a very good imitations for the biggest mayflies.