Kelly devised this fly in the late 70's for the hexagenia hatch (hence the name); change colors to suit your supersized mayfly hatch. Original had bucktail underbody, deer hair used here; your choice.
hook - TMC 5212 #6
thread - UTC 140 hopper yellow
tail - moose mane
underbody - deer hair yellow
overbody - deer hair
wing - calf tail
hackle - brown/grizzly mix
Part 1
mash barb, start thread at 75% mark
clean, stack (good luck with this), measure (hook shank) a good clump of calftail
two soft loops, then really lean on it wrapping back (don't go in front of the initial wraps or all is lost); no place for wimpy thread here (UTC is the bomb), you don't want a spun calftail wing
trim the butts, brush a little Sally and smooth with thread wraps
push the hair up and back, creasing it with your thumbnail; make a thread dam in front with a few firm wraps