Start thread and tie lead dumbbell eyes on TOP of the shank one eye-length behind the hook eye so the fly rides point up and dives nose-first.
At the hook bend tie in 3-4 full-length Sili Legs so all the legs trail BEHIND the fly. This is the whole trick: a real crab sprints sideways/away from danger with its legs streaming behind it, unlike a Merkin whose legs stick out sideways.
Add two strands of Krystal Flash among the legs.
Tie in alternating clumps of tan and brown yarn or EP fiber with X-wraps across the shank, working from the bend forward to the dumbbell eyes. 6-8 clumps, packed tight.
Whip finish in front of the eyes and cement.
Trim the yarn body top and bottom into a flat, rounded crab disc, leaving the trailing legs and flash untouched. Taper it slightly thinner at the rear.