The Perdigon nymphs feature a glass bead in the front with hackle fibers that gives the fly a realistic look. Tied on a barbless hook. These are designed to sink faster, swim deeper and stay deeper than regular nymphs. Obviously by running this nymph right in front of trout you have a good chance to catch more trout. These can be fished as a dropper below a dry fly such as the Mr. Rapidan or as a single nymph by itself.