Catgut, a natural absorbable suture is obtained by longitudinally slitting the intestinal submucosa of sheep and goat, twisting the ribbons and joining them in wet condition giving monofilament finish. Catgut have 2 colors: natural or plain (ivory -cream color) and Chromic (dark brown color).For a long period, catgut was the most common material for the strings of harps, violins, and as well as other stringed musical instruments.
Catgut is a great fly tying material and lies tied with catgut looks very realistic.
Here is a few pictures with nymphs tied on size #10 and #12
Flies tied with size #16 and #18: