Fly Tyers world wide have dubbed Semperfli Dry Fly Polyyarn as the "floating yarn." This material is extremely fibrous, buggy, and easy to use material making them a must have for any tyer. As a dubbing on a rope it becomes ideal for tying dry flies, as it not only floats due to buoyancy but trapped air bubbles help to keep your fly on top of the water as well.
It has a specific gravity/ density number of .91g made of polyprpylene yarn. This means it is naturally bouyant due to the fact its density is lighter than waters 1.0 grams. Many companies claim to use polypropylene yarn however a simple test you can do at home to check if it is genuine polypropylene yarn is to cut a 1 inch / 25mm strip, scrunch it up in your fingers and drop it into a glass of water. If it is genuine polypropylene yarn then it should dip below the surface and then return to the surface. Indeed leave it for a day and it should still be floating as it is lighter than water and it is this property which makes it very, very special for dry flies.
There are a few fly tying polyyarns on the market today from different companies. Indeed Semperfli makes two versions of polyyarn - the original Semperfli Polyyarn that comes on cards and is ideal for , most commonly used as parachute posts, and dubbing wings and Dry Fly Polyyarn Caddis Collection, is a spool based product which is intended for bodies of flies or to create detached bodies.