Silk thread has been a part of tying fishing flies for many decades, if not more than a century. When I first started tying in the 1970's, I used black silk thread. It was a pain to work with. It broke easily and that, of course, increased my frustration level.
But some traditional patterns require silk as an integral component of tying the fly. Here, I'm alluding to North Country fly patterns. These so called soft hackles or spiders, have traditionally used silks, such as Pearsall's silk thread to create the thin, delicate bodies. Unfortunately, Pearsall's is no longer manufactured, and can only be found occasionally on sites like eBay. But alternatives do exist.
Alan Petrucci of Small Stream Reflections has written a number of articles on YLI silk thread. About the time he started introducing folks to this line of thread, I discovered some in my local fabric and crafts store. I bought a few colors and began using it in some of my fly patterns.
Now, some years later, I still use YLI silk instead of standard tying thread for many of my favorite patterns. YLI isn't as strong as modern synthetic fly tying threads, but due to its thread size, it builds bodies or heads quicker, has a wonderful natural sheen obtained only with 100% silk threads. It also holds its color well when wet. Some of my favorite colors are bright red #202, black (there is no # designation for black), yellow #229, and pink #240.
If you've never tied with silk, you ought to give it a try. Try YLI Silk Thread 100 weight. It's wonderful thread; there's just something about silk!