I started tying these this way in 1993 on a fishing trip with my father and my son, after watching a few thousand baby leeches being attacked by trout in about 2 ft of water along a weed bed beside the boat launch. It's been one of my go to's for many years. I have also used different colours of marabou mixed when tying these.
Hook - mustad #10 or #12
Thread - Sheer black 14/0
Tail - Mix of black and brown marabou
Rib - Copper wire
Body - Mix of black an brown marabou twisted before wrapping hook
Bead -copper or black