The shell back is thin skin. The over wrap is the body material which is a couple strands of fine yarn.
You tie in the shell back and the yarn at the hook bend. Take a couple yarn wraps under then one over to create the segments.....couple under.....one over etc. use the left over shell back to create the head and wing case after forming the thorax. There's no real reason to add the antennae or long legs. But it looks good to me. Use any coarse dubbing for the legs.
There's a variation I have that uses heavy flourescent green mono eyes tied in at the head. Very sexy eyes.
There's lots simpler patterns for this bug. Here's a couple more I tie.
The plump body segments, shell back and dark head are the only real necessities. I'm sure Mother Nature varies the colors of the larvae as well.
And, yes, I do use sharpies to color some.
There's a thousand and one ways to tie a fly.
Rg