Millipedes have many legs (milli= "1,000"; pede= "feet"). While no species discovered to date has 1,000 legs, hundreds of legs is the norm with two legs on each of several hundred body segments in more elongate species. Their numerous legs give them the power to burrow quickly into soil, but other defenses make the difference when cornered by a predator. Typically, a millipede has rows of pores (ozopores) that connect to glands that secrete toxic liquids. Millipedes that are brightly colored, or even fluorescent (such as Motyxia sp.), may be flashing a warning about their toxicity to predators. Some millipedes also can roll up into a defensive ball, exposing their hard external coverings (exoskeletons) and protecting their softer bellies. Despite their many defenses, millipedes are prey for some animals. Meerkats, for example, have overcome millipede defenses by developing immunity to their toxins and finding ways to clean them, such as rolling millipedes in sand before eating them.

Arthropods are the most successful group of organisms on Earth in terms of numbers, including almost half of all known species. The hallmark of their success is a body plan that makes them durable and adaptable. All arthropods have external skeletons made of hard material (chitin embedded in a protein matrix). The exoskeleton protects them from predators, weather conditions, and other threats. As they grow, arthropods typically shed the exoskeleton to reveal a bigger, fresh one underneath. The two lengthwise halves of an arthropod body are a mirror image of each other (bilateral symmetry) and typically divided into segments. Each segment has appendages that are specialized for the many activities of the arthropod. An arthropod may use appendages to feed (mouthparts), to breathe (gills, tracheae, book lungs), to reproduce (genitalia), and to move around (walking, swimming, flying). Having a modular body plan with multifunctional appendages has allowed arthropods to thrive in an impressive variety of environments.