Thread Orange or olive (match body) 140 denier / 6-0
Materials
Weight Lead or lead-free wire wraps (or dumbbell eyes) to sink it and ride it hook-up (10-12 wraps)
Antennae / claws Barred rubber legs and a pair of marabou or Zonker-strip claws, orange or olive (2 claws + 2 antennae)
Body / abdomen Dubbing (rabbit or SLF), orange or olive. Buggy (pinch)
Rib Copper wire or mono, to segment and reinforce the shell (6 in)
Shell back / carapace Thin Skin, Scud Back, or mottled feather, orange/olive over the body (1 strip)
Legs Barred rubber legs or palmered soft hackle, orange/olive (several)
How to tie it
Weight the hook with lead wire (or mount small dumbbell eyes near the hook eye so the fly rides hook-point-up like a crayfish backing away). Cover with thread.
At the bend (which becomes the FRONT of the crayfish, crayfish flee backward), tie in two claws (marabou tufts or thin Zonker strips) splayed in a V, plus two rubber antennae, in orange or olive.
Tie in the shell-back strip (Thin Skin/Scud Back) and the copper rib wire at the same point, left hanging.
Dub a fat, buggy body forward over the weighted shank to about the 80% point.
Pull the shell-back strip flat over the top of the body to form the carapace, tie it down, then counter-wrap the copper wire forward in 5-6 open turns to rib/segment the shell and reinforce it.
Add a few barred rubber legs at the sides (or palmer a soft hackle) for the walking legs, then build a small head at the hook eye and whip finish.
Optional: coat the shell back with a thin layer of UV resin for durability.