Imitates a damselfly nymph. Extremely useful in lakes and slackwater portions of rivers. Damselfly nymphs are active throughout the spring. In July, they migrate to above-water objects such as standing timber, rocks, and the shore. Adults emerge out of the water. After July, most mature damselfly nymphs have hatched, so trout are no longer expecting to see them.
Variations

The most common body colors are olive and yellow-olive. A bead head is sometimes helpful, but in some waters and situations it will make the fly sink too fast.
How to Fish

Throughout the spring, count-down-and-retrieve, slow retrieve, or wind drift using an intermediate line. Damsel nymphs are not fast swimmers, so a leisurely retrieve is appropriate: a very slow but steady retrieve; or slowly strip in a foot of line, then pause a second or two and strip again. The fly should be somewhere between the weed tops and the top few inches of water.

Once a migration is underway, position yourself near an above-water object (one that damsel nymphs will be swimming towards), cast, and retrieve. Damselfly migrations are in the top few inches of water, just barely below the surface.