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Millipedes are long, cylindrical, segmented, many-legged terrestrial arthropods in the class Diplopoda, in total comprising about 10,000 species. The common name of these animals is derived from the Latin word for "thousand legs," although most species actually have fewer than 200 legs, and some as few as about 60.

Millipedes have an elongate, almost cylindrical body form, with two short legs on each segment, except for the first three, anterior (head), segments, which do not have legs. Most species of millipedes have small, compound eyes, consisting of bundles of optical units known as simple eyes, or ocelli. Millipedes have a pair of large chewing mandibles at the head end, which they use to break up their food—usually decaying vegetation or the flesh of dead animals.

Millipedes are slow-moving, deliberate animals which usually live in damp and dark places, often under some sort of cover. Some species of millipedes feed on living plants, and when these animals are abundant they can cause significant damage. For example, Oxidus gracilus, a common millipede found in greenhouses, is sometimes considered to be a pest. A few species of millipedes are predators of other invertebrates.

When millipedes are disturbed, some species curl up into a tight spiral, with their head in the center. Some species also exude a foul, dark fluid from pores in the sides of their body when they are disturbed. This excretion can kill some types of insects when they are closely confined with these millipedes.

Millipedes have internal fertilization, with the male using specialized, modified legs on its seventh body segment to pass sperm to the female. Openings of the reproductive tracts of both sexes are located at the front of the body, between the second and third pairs of legs. Millipedes lay clutches of small, whitish eggs in damp places. The newly hatched millipedes have only three pairs of legs, with the other parts being added as the animals grow and molt.

The largest species of millipede in North America is Narceus americanus, a dark-brown animal with narrow, red, transverse rings on its body, occurs in moist coniferous forests of the west coast. This beautiful millipede can reach an impressive body length of 4 in (10 cm), while species of millipedes in the tropics can reach the length of a foot (30 cm).

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