Casters are the chrysalis of flies. The shell protects the maggots during the early stages of the insects life cycle.
Anglers using casters would usually use a 16 or 18 hook, which should be inserted into the top of the blunt wide end of the caster.
Casters are quite cheap and can be bought from most tackle shop, sold by either ½ pints or pints and can be kept in the fridge for several days.
Casters can be used to catch Roach, Tench and most other fish.