This nymph has been tied from highly valued material by many fly tiers - the catgut. I caught on this pattern in the last day of the trout season several trouts on small river nearby to me.
Catgut this is surgical thread used in veterinary medicine, which has an interesting feature visible after soaking: becomes slightly transparent and slightly swells.
The first feature enables to tie underbody from colored thread (best seen in natural color catgut). Due to the second feature does not need to tie each wrap of the catgut close to next wrap, we save a little bit not so cheap material
Hook: Hanak 400BL no. 12
Thread: UTC Fl. Yellow
Bead: tungsten 2.8mm in copper
Tag: pearl UNI tinsel
Tail: Coq de Leon
Body: Catgut, in brown size M