Ostrich herl (legs) Black ostrich herl (One herl with the longest barbs you have)
Black permanent marker Black ostrich herl (To darken the front third of the shellback)
Bead (optional) Tungsten, any bead (One, if you want a heavier version)
How to tie it
Put about a dozen wraps of 0.015 lead-free wire over the middle of the shank, start black thread, build small thread dams front and back to smooth the transitions, then take the thread around the bend of the hook.
Tie in half a strand of light Z-lon well around the bend as a trailing shuck. Leave it long for now and use the butts to help fill the gap behind the weight.
Take the thread forward and tie in a strip of olive Thin Skin about half a hook gap wide, on top of the shank just in front of the weight. Bind it rearward with tight wraps, folding it slightly over the sides to keep it centered, going as far back as you want the body to run.
Tie in 3X nylon tippet at the back for the rib.
Wax the thread well and spin on a thin noodle of the olive/caddis-green dubbing blend. Wrap a slim body forward. It will take about two noodles, stopping short of the first third of the shank.
Tie in a black ostrich herl and wrap five to seven close turns to form a leggy thorax, then trim the excess and wet your fingers to sweep the barbs down and back.
Pull the Thin Skin forward over the top, keeping it centered and pulled tight, and hold it with a couple of temporary tight wraps at the front.
Wrap the mono rib forward in firm, evenly spaced turns, the first turn goes up under the Z-lon shuck, keeping tension while you tie it off behind the eye, then trim.
Pull the Thin Skin tight, snip it off at the front, and build a small nymph head over the stub. Whip finish, then trim the Z-lon shuck down to a tiny sliver.
Color the front third of the shellback, and a little way around the sides, with a black marker, then add a drop of head cement or UV resin. Singe any errant ostrich barbs if you want a cleaner look.