A nymph is a type of wet fly design to imitate the larvae of insects such as chironomids (midges), mayfly, caddis and stoneflies.
A nymph is typically weighted so it sinks to the bottom of the water column where the actual insects typically emerge.
However sometimes fly fishers use nymphs of different weights to fish different parts of the water column.
Bead head nymphs have a metal beat attached close to the eye of the hook and are designed to sink quickly. Using tungsten beads will make a nymph sink even quicker.
These nymphs are used in indicator nymphing and euro nymphing fly fishing techniques.