This is the same thread that I use in tying the very popular Utah Killer Bug because of it's intense shade of color, as you can see.
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100% Filament Silk from Japan. Very thin but, because it's silk, it's strong. Doesn't FUZZ like synthetics. Fits most bobbins, but the end caps are removable to make it narrower.
Closely equivalent in size to Pearsall's Gossamer Silk (see comparison photo), so it lays well on the hook and works great for small flies. I recommend that you use either a "Bees Wax" (which also helps hold materials in place), head cement or both when tying.
Try a very thin dubbing twisted over the silk and applied sparingly to the body. When wet, the highly colorful thread shows through the fuzzy dubbing creating a translucent effect.
NOTE: Not for salt water use.
Not particularly cheap per spool compared to synthetics, but each spool contains 200 Meters (218 Yards) of beautiful, high quality, natural pink silk! Spooled in the US. I've used it now professionally for six full seasons, and am very happy with it. Sometimes hard to get.