Pill Millipede

They are completely different animals that happen to share a trick. A "pill bug" or "roly-poly" in the everyday sense is usually Armadillidium vulgare, a terrestrial crustacean (an isopod, related to shrimps and crabs) that can roll into a ball. A pill millipede is a myriapod, in the same broad group as centipedes and other millipedes, and like all millipedes it has two pairs of legs on most body segments (a woodlouse has one pair per segment). Pill millipedes also have far fewer segments than long millipedes, a smooth shiny domed shell, and they are generally rounder and glossier than the more oval, textured woodlouse. Both evolved the ball-rolling defence (called conglobation in the woodlouse and volvation in the millipede) independently, because sealing yourself inside your own armour is an effective way to survive an ant swarm.

Its body plan is built for it. A typical millipede has dozens of nearly identical segments and hundreds of legs, and it curls into a loose flat spiral. A pill millipede instead has only about 11 to 13 segments, with the front and rear plates enlarged and shaped so that when the animal flexes, the plates slide over one another and lock into a closed sphere with no gap. The head, the delicate antennae, and all the legs fold into the hollow centre, and the last large plate caps the opening like a lid. The result is a seamless, surprisingly hard ball that a predator cannot easily open, bite through, or get a grip on, and it can be held for a long time.

The giant pill millipedes of the order Sphaerotheriida, found in Africa, Madagascar, India, Southeast Asia, and Australia, are the large ones. Rolled into a ball, the biggest species are 4 to 7 centimetres across, and a few reach around 10 centimetres — the size of a golf ball, a plum, or in the largest cases an orange, and weighing as much as a small mouse. Unrolled they are correspondingly long and broad. The European and North American pill millipedes (order Glomerida) are much smaller, from a few millimetres up to about 2 centimetres, roughly the size of the woodlouse pill bugs they resemble.

Giant pill millipedes, especially colourful Madagascan and Southeast Asian species, have become popular in the invertebrate hobby because they are large, harmless, slow, clean, and entertaining to watch roll and unroll. They need a humid tank with deep leaf litter and rotten wood to eat and burrow in, and they can live for many years. This popularity is a concern — several giant pill millipedes have very small ranges in threatened forests, particularly in Madagascar, and are collected in quantity for export with little regulation or knowledge of wild population sizes. Combined with rapid deforestation in their habitats, this means some species may be at risk even though the group as a whole has not been formally assessed.