Nymphs are aquatic insects that are still in their underwater stage, not having reached their adult, or flying stage of life yet. For those of you that are familiar with dry flies, a nymph is just a younger version of those same kinds of flies. The younger version stays under water while they are developing. Nymphs are smaller and incomplete in their development. Nymphs work throughout the different seasons and are some of the best flies for targeting trout when water temperatures are cold.