An emerger is a hatching insect (ie caddis or mayfly) that is at the phase of its life where it is between a nymph and a dun (the term can also refer to a fly tied to imitate this phase such as a Klinkhammer).
It is the stage of the insect’s life cycle when it is transitioning from an aquatic nymph or larva to a winged, adult form.
The emerger stage occurs just before the insect fully emerges from the water and molts into its final, flying adult stage. During this stage, the insect is partially emerged from the water, with its wings and body still partially submerged.
This is an important stage for aquatic insects, as it makes them very vulnerable to predation by fish and other aquatic predators. Emerging insects are often targeted by feeding fish, as they are in a transition phase and cannot easily escape.
For anglers and fly fishermen, the emerger stage is an important feeding period for fish, and imitating the appearance and behavior of emerging aquatic insects is a key technique for effective fly fishing.
Emerger patterns in fly fishing are designed to mimic the distinctive silhouette and positioning of aquatic insects during this critical transitional phase.