In lakes and slower moving rivers, the larvae of the chironomid midge make a large part of the trout’s diet. They vary in colour from pale green and brown to a bright blood red.

These red larvae, known by anglers as bloodworm, live the bulk of their lives in silt tubes, but when disturbed or migrating can move by a sinuous lashing motion. This action is almost impossible to replicate, but by adding a pinch of red marabou as a tail, enough action can be imparted to make an effective imitation.

The remainder of the fly is simply translucent red Nymph Glass, which is wound in touching turns along the hook.

Source: The Fly Tyers Bible, p.128-29