Nymphs

Nymphs are  macroinvertebrates-Caddis flies, stoneflies, mayflies and midges to name a few. 

Nymph fishing is a highly effective technique in fly fishing, as it allows you to imitate the underwater stage of these insects. Since approximately 90% of a trout's diet consists of nymphs, presenting a well-presented and realistic nymph fly can often lead to successful day on the water..

In addition to the natural nymphs there is another category of flies called "junk flies" that are commonly used in fly fishing. These flies imitate non-insect food sources that trout may feed on, including eggs, worms, and mop flies.