Casters are the chrysalis stage of the common fly & it's their final transition from maggot to fly. Most fish have a real fondness for casters, there is something about the soft crunchy shell and gooey inside which makes fish feed confidently.
Barbel, bream, carp & tench will happily root around on the bottom in search of this crunchy snack and roach & rudd will aim to intercept this bait as it falls through the water.
Casters used in conjunction with hemp as part of a loose feed mix really encourages fish to root around in the swim, this is because the fine grains of hemp and casters have a knack of falling in between gravel & vegetation on the bottom, which helps to keep the fish interested & feeding in your swim for longer as they root out every last morsel.
The tan colour of a caster gives a good indication of its age - The lighter the caster, the fresher they are and the quicker they sink, the darker they become also shows a sign of their development. Casters with a black or very dark brown appearance usually float and are not eaten by fish as eagerly as the lighter brown casters.
Always place your casters in a bait box out of direct sunlight and covered with water when fishing, not only will this help keep them fresh, it will also allow you to select the sinkers from the floaters - If casters are allowed to sit in the sun or if stored in humid conditions, it will speed their development and they will quickly lose their effectiveness.