Pill millipedes belong to the class Diplopoda. Their trunk segments are formed from fused pairs, which is why most bear two pairs of legs. Pill woodlice are isopod crustaceans, relatives of shrimp and crabs, with one pair of legs per segment and two pairs of antennae. The shared ability to conglobate, to roll into a sealed sphere, evolved independently in each lineage.
The convergence is functional. Rolling protects the soft underside and legs, presents only hardened plates outward, and reduces water loss, all valuable to a small litter-dwelling animal that cannot outrun anything. Pill millipedes generally roll into a tighter, smoother ball with fewer visible segments, and have a single pair of antennae.
Despite the name, no millipede has a thousand legs, and pill millipedes have among the fewest. The record-holder, the Australian species Eumillipes persephone described in 2021, was found with up to 1,306 legs, the first millipede known to exceed a thousand.