Overview
Millipedes are elongated, many-segmented arthropods in the class Diplopoda. They are best known for having numerous body segments and multiple pairs of legs, with most diplosegments bearing two pairs of legs. Millipedes occur in a wide range of terrestrial habitats and are important decomposers in many ecosystems. For broader context about their larger group, see general information about arthropods.
Anatomy and distinguishing features
A typical millipede has a head with simple mouthparts and antennae, followed by a trunk of repeated units. These trunk units—diplosegments—usually result from the fusion of two ancestral segments and commonly carry two pairs of legs each. The body can be cylindrical or dorsoventrally flattened and ranges from soft to heavily sclerotized (armored) in different groups. Legs are relatively short and move in coordinated waves to propel the animal. Many species have spiracles and a tracheal respiratory system adapted to life on land.
Diversity and classification
Millipedes are diverse: thousands of species have been scientifically described and many more remain undocumented. Modern accounts often cite roughly 12,000 described species, while global estimates suggest there may be many times that number, with tens of thousands of species yet to be discovered or formally described (estimated diversity). Millipedes are divided into numerous orders and families, including groups such as Julida, Polydesmida and Spirobolida, each with characteristic body forms and habits.
Habitat and ecological role
Most millipedes are detritivores that feed on dead plant material and associated microflora. By shredding fallen leaves, rotten wood and other organic matter they accelerate decomposition and contribute to soil formation and nutrient cycling. Typical food items include decaying leaves, humus and decomposing wood. Some species inhabit leaf litter, soil, rotting logs, caves, or the upper layers of the soil profile, while others are adapted to seasonal dry climates by sheltering in damp microhabitats.
Feeding and life cycle
Millipedes generally feed by rasping and ingesting decayed plant material and associated fungi and bacteria. Reproduction usually involves internal fertilization: males transfer sperm to females using modified legs called gonopods in many species. Females lay eggs in soil or hidden chambers and juveniles hatch with fewer segments than adults; they add segments and legs at successive molts, a process known as anamorphosis. Lifespans vary: some complete development in a year, while others live several years as adults.
Defenses and chemical ecology
Because millipedes move slowly, they rely on behavioral and chemical defenses. The classic response to disturbance is to coil into a tight spiral, which shields the soft underside and head—a behavior often cited as the primary defense mechanism that protects delicate organs. Many species possess glands that release odorous, irritating or toxic compounds to deter predators; in a few groups these secretions include reactive chemicals such as hydrogen cyanide or quinones. Such secretions can irritate skin or trigger allergic responses in sensitive people (irritant secretions). Other millipedes protect themselves with hardened plates or detachable bristles that can entangle or irritate small predators (bristle defenses).
Behavior and locomotion
Millipede locomotion is characterized by coordinated waves of leg movement, producing a steady, often slow progression suited to moving through litter and soil. Many species are nocturnal, reducing exposure to predators and desiccation. Aggregations can form seasonally or after rains, and some species enter houses or buildings in large numbers, particularly when conditions change outdoors.
Interactions with other animals and humans
Most millipedes are harmless to people and pets. They are not venomous like centipedes, but dense swarms or large individuals can be considered a nuisance where they invade buildings, gardens or stored thatch. In greenhouse or nursery situations millipedes occasionally feed on seedlings or tender roots and can be perceived as pests under those conditions (greenhouse issues) though they are generally not serious agricultural pests (pest status). Some tropical species are locally notable for congregating in thatch or structures; for example, reports cite Xenobolus carnifex in thatched roofs in parts of Asia.
Distinction from centipedes
Millipedes are commonly confused with centipedes, but they have clear differences: centipedes move more quickly, typically have one pair of legs per trunk segment, and are active predators that capture prey, whereas millipedes usually have two pairs of legs per diplosegment and feed mainly on decaying plant matter. A few millipede species are omnivorous or opportunistic scavengers, but they lack the venom claws and rapid predatory behavior typical of centipedes (centipede differences).
Research, conservation and fossil record
Specialists who study millipedes are known as diplopodologists, and the field is sometimes called diplopodology. Millipedes have an extensive fossil history: fossil myriapods interpreted as millipede-like forms are among the earliest arthropods to show adaptations for terrestrial life. One well-known fossil species, Pneumodesmus newmani, is interpreted as an early air-breathing myriapod dating back hundreds of millions of years, highlighting the long evolutionary history of terrestrial decomposers. Many living species remain poorly known, and habitat loss threatens specialized or range-restricted millipedes; conservation attention is increasing for taxa with small distributions or unique ecological roles.
Further reading and resources
For taxonomy, species lists and general information consult institutional guides, regional field keys and authoritative databases. General overviews and taxonomic resources can be found through educational and museum pages on arthropods and specific species lists described species or diversity estimates estimated diversity. For more on diet and decomposition see resources on decaying leaves, and for defense chemistry consult material on chemical defenses. Practical notes on human encounters and management include pages about pest status, greenhouse problems, and reports of species such as Xenobolus carnifex. Further reading about coiling and defensive behavior is available under entries on the rolling defense and protective anatomy that shields vital organs. For more on bristly millipedes and related defenses see notes on bristle defenses and for comparisons with centipedes consult resources summarizing centipede differences.