Pill Bug

Not an insect

The pill bug (also called pill woodlouse, roly-poly, armadillo bug, or slater) is not an insect — it’s a terrestrial isopod crustacean, more closely related to crabs and shrimps than to beetles or ants. It’s one of the few crustaceans to have successfully colonised dry land, but it retains gills and requires moist conditions to breathe.

The pill bug (Armadillidium vulgare) is distinguished from the similar pill millipede by its segmented armour and its ability to roll into a complete sphere — pill millipedes are not true woodlice and have more legs.

The roll

The defensive roll is the pill bug’s defining behaviour — when touched or threatened, it tucks its antennae and legs in and curls its overlapping dorsal plates into a nearly perfect sphere. This exposes only the hard dorsal armour to a predator, protecting the softer underside and legs. The ball can be rolled away by the wind, further removing it from danger.

Decomposition

Pill bugs are detritivores — their primary role is consuming dead plant material (leaves, rotting wood) and breaking it into smaller particles that bacteria and fungi can process further. They also consume their own moult (shed exoskeleton) to recycle minerals. A healthy population of pill bugs in compost significantly accelerates decomposition.

The “woodlouse”

In Britain, “woodlouse” (plural woodlice) is the standard common name for all terrestrial isopods, with the pill bug being the most common species. The ability to roll into a ball distinguishes Armadillidium from the non-rolling Oniscus and Porcellio woodlice.