Millipedes are arthropods that have two pairs of legs per segment. Each segment that has two pairs of legs is a result of two single segments fused together as one. Most millipedes have very elongated cylindrical bodies, although some are flattened, while pill millipedes are shorter and can roll into a ball, like a pillbug.
The name "millipede" is a compound word formed from the Latin roots mille ("thousand") and pes ("foot"). Despite their name, no known millipede has 1,000 legs, although a rare species has up to 750. Common species have between 36 and 400 legs.
Millipedes are slow moving. Most millipedes eat decaying leaves and other dead plant matter, moisturizing the food with secretions and then scraping it in with their jaws. However, they can also be minor garden pests, especially in greenhouses where they can cause severe damage to emergent seedlings. Signs of millipede damage include the stripping of the outer layers of a young plant stem and irregular damage to leaves and plant on the very top.
Millipedes can be easily distinguished from the somewhat similar and related centipedes, which move rapidly, and have a single pair of legs for each body segment. Millipedes range from 0.079 to 11 inches in length, and can have as few as eleven, to over a hundred segments. They are generally black or brown in color.
The head of a millipede is typically rounded above and flattened below and bears large mandibles. The body is flattened or cylindrical. Because they lack a waxy cuticle millipedes are susceptible to water loss, and must spend most of their time in moist or humid environments. The millipede's most obvious feature is its large number of legs. Having very many short legs makes millipedes rather slow, but they are powerful burrowers. With their legs and body length moving in a wavelike pattern, they easily force their way underground head first.
The head contains a pair of sensory organs, and are shaped as small and oval rings at the base of the antennae. They are used to measure the humidity in the surroundings.
According to Guinness World Records the African giant black millipede can grow to 15.2 inches. Most millipedes are herbivorous, and feed on decomposing vegetation or organic matter mixed with soil. A few species are omnivorous or carnivorous, and may prey on small arthropods, such as insects and centipedes, or on earthworms. Some species have piercing mouth parts that allow them to feed on plant juices.
Females lay between ten and three hundred eggs at a time, depending on species, fertilizing them with the stored sperm. Many species simply deposit the eggs on moist soil or organic material. The young hatch after a few weeks, and typically have only three pairs of legs, followed by up to four legless segments. As they grow, they continually moult, adding further segments and legs as they do so. Millipedes live from one to ten years, depending on species.
Due to their lack of speed and their inability to bite or sting, millipedes' primary defense mechanism is to curl into a tight coil — protecting their delicate legs inside an armored body exterior. Many species also emit poisonous liquid secretions or hydrogen cyanide gas through microscopic pores along the sides of their bodies as a secondary defense. Some of these substances are caustic and can burn the exoskeleton of ants and other insect predators, and the skin and eyes of larger predators. Animals such as Capuchin monkeys have been observed intentionally irritating millipedes in order to rub the chemicals on themselves to repel mosquitoes. As far as humans are concerned, this chemical is fairly harmless, usually causing only minor effects on the skin, the main effect being discoloration, but may also include pain, itching, blisters, eczema, and occasionally cracked skin. Eye exposures to these secretions causes general eye irritation and potentially more severe effects such as conjunctivitis. First aid consists of flushing the area thoroughly with water; further treatment is aimed at relieving the local effects.