Nymph and shrimp flies imitate the sub-surface food of trout & grayling. They are the larval form of flies so you can get Olive nymph or agile darters imitated by Pheasant Tail Nymphs, Mayflies and Damsels also have their own nymphs imitated by fly fishermen, These patterns are regularly used on stillwaters or many other species of fly that emerge while we are fishing using fishing flies. Some nymphs may take one or 2 years to develop into adults