Pill millipedes are any members of two living (and one extinct) orders of millipedes, often grouped together into a single superorder, Oniscomorpha. The name Oniscomorpha refers to the millipedes' resemblance to certain woodlice (Oniscidea), also called pillbugs or "roly-polies". However, millipedes and woodlice are not closely related (belonging to the subphyla Myriapoda and Crustacea, respectively); rather, this is a case of convergent evolution.
Pill millipedes are relatively short-bodied compared to most other millipedes, with only eleven to thirteen body segments, and are capable of rolling into a ball (volvation) when disturbed, as a defense against predators. This ability evolved separately in each of the two orders, making it a case of convergent evolution, rather than homology. They can also exude a noxious liquid, which may be both caustic and toxic, to repel predators. Pill millipedes are detritivorous, feeding on decomposing plant matter, usually in woodlands.
apparently all species are detritivores, feeding on dead organic matter such as leaves and wood on the forest floor. Like earthworms, they play an important role in decomposition; by breaking down decaying organic matter they release locked up nutrients back into the soil. Such recycling is essential for plant nutrition and accordingly for the entire ecology. It is possible that giant pill millipedes rely on special bacteria in their gut, much as termites do, to exploit the nutritional value of otherwise indigestible material such as lignin.
Like most millipedes, Sphaerotheriida inhabit mainly the leaf litter of humid forests. Some species, however, show an arboreal (tree-living) lifestyle, and in these the rolling-up reflex has been suppressed.
