Millipedes, named from the Latin words mille ("thousand") and pes ("foot"), are a group of arthropods. They are scientifically called the class Diplopoda, a name that comes from their feature of having two pairs of legs on most body segments. Each of these double-legged segments forms when two single segments join together. Most millipedes have long, round or flat bodies with more than 20 segments, while pill millipedes are shorter and can roll into a tight ball. The name "millipede" suggests "thousand feet," but no species had 1,000 legs until 2020, when Eumillipes persephone was discovered. This species can have over 1,300 legs. There are about 12,000 known millipede species, grouped into 16 orders and around 140 families. This makes Diplopoda the largest class of myriapods, a group of arthropods that also includes centipedes and other multi-legged creatures.

Most millipedes move slowly and eat decaying leaves and other dead plant material. Some eat fungi or drink plant fluids. They are usually harmless to humans but can sometimes become pests in homes or gardens, especially in greenhouses where they may harm young plants. Millipedes defend themselves by releasing chemicals from body pores. Tiny bristle millipedes have detachable bristles for protection. Their main defense is curling into a tight coil, which shields their legs and other body parts with a hard outer shell. Most millipedes reproduce using special male legs called gonopods, which transfer sperm to females.

Millipedes first appeared during the Silurian period, making them some of the oldest known land animals. Prehistoric species like Arthropleura could grow over 2 meters (6 and a half feet) long, while modern species reach lengths of 27 to 38 centimeters (10 and a half to 15 inches). The longest living species is the giant African millipede (Archispirostreptus gigas).

Millipedes are traditionally thought to be closely related to tiny pauropods, though some studies suggest otherwise. They are different from centipedes (Chilopoda), which move quickly, are venomous, eat meat, and have only one pair of legs per body segment.

The scientific study of millipedes is called diplopodology, and a scientist who studies them is called a diplopodologist.

Etymology and names

The word "millipede" is commonly found in both everyday and scientific writings. In North America, scientists sometimes use the term "milliped" (without the final letter "e"). Other names people use for these creatures include "thousand-legger" or "diplopod." The scientific study of millipede biology and classification is known as diplopodology.

Classification

Approximately 12,000 types of millipedes have been identified. Scientists estimate that the actual number of millipede species on Earth may range from 15,000 to 80,000. Most millipede species are not found in many places; they move slowly on land and need wet environments. These traits have led to separate genetic groups and the quick creation of new species, resulting in many types of millipedes with limited areas where they live.

Living millipedes are divided into 16 groups within two larger categories. The oldest category, Penicillata, includes only one group, Polyxenida (bristle millipedes). All other millipedes belong to the category Chilognatha, which includes two smaller categories: Pentazonia (short-bodied pill millipedes) and Helminthomorpha (worm-like millipedes), which contains most millipede species.

Millipede classification is based on studies by Shear (2011) and Shear & Edgecombe (2010), including extinct groups. Recent research has questioned older classification systems, especially the placement of the groups Siphoniulida and Polyzoniida. The positions of extinct groups, known only from fossils, are not fully understood. After each name is listed the scientist who first described the group.

Class Diplopoda de Blainville in Gervais, 1844

• Subclass Penicillata Latreille, 1831 Order Polyxenida Verhoeff, 1934
• Subclass † Arthropleuridea (placed in Penicillata by some authors) Order † Arthropleurida Waterlot, 1934 Order † Eoarthropleurida Shear & Selden, 1995 Order † Microdecemplicida Wilson & Shear, 2000
• Subclass Chilognatha Latreille, 1802 Order † Zosterogrammida Wilson, 2005 (Chilognatha incertae sedis ) Infraclass Pentazonia Brandt, 1833 Order † Amynilyspedida Hoffman, 1969 Superorder Limacomorpha Pocock, 1894 Order Glomeridesmida Cook, 1895 Superorder Oniscomorpha Pocock, 1887 Order Glomerida Brandt, 1833 Order Sphaerotheriida Brandt, 1833 Infraclass Helminthomorpha Pocock, 1887 Superorder † Archipolypoda Scudder, 1882 Order † Archidesmida Wilson & Anderson 2004 Order † Cowiedesmida Wilson & Anderson 2004 Order † Euphoberiida Hoffman, 1969 Order † Palaeosomatida Hannibal & Krzeminski, 2005 Order † Pleurojulida Schneider & Werneburg, 1998 (possibly sister to Colobognatha) Subterclass Colobognatha Brandt, 1834 Order Platydesmida Cook, 1895 Order Polyzoniida Cook, 1895 Order Siphonocryptida Cook, 1895 Order Siphonophorida Newport, 1844 Subterclass Eugnatha Attems, 1898 Superorder Juliformia Attems, 1926 Order Julida Brandt, 1833 Order Spirobolida Cook, 1895 Order Spirostreptida Brandt, 1833 Superfamily † Xyloiuloidea Cook, 1895 (Sometimes aligned with Spirobolida) Superorder Nematophora Verhoeff, 1913 Order Callipodida Pocock, 1894 Order Chordeumatida Pocock 1894 Order Stemmiulida Cook, 1895 Order Siphoniulida Cook, 1895 Superorder Merocheta Cook, 1895 Order Polydesmida Pocock, 1887

Millipedes were among the first animals to live on land during the Silurian period. Early millipedes likely ate mosses and simple plants. Two major groups of millipedes are now extinct: Archipolypoda ("ancient, many-legged ones"), which include the oldest known land animals, and Arthropleuridea, which include the largest known land invertebrates. Pneumodesmus newmani is the earliest known millipede, from the late Silurian period around 428 million years ago or early Devonian period around 414 million years ago. It was found in a 1 cm long piece of fossil and had clear signs of breathing holes, showing it could breathe air. Other early millipede fossils include Kampecaris obanensis and Archidesmus sp. from 425 million years ago. During the Carboniferous period, Arthropleura was the largest known land invertebrate, growing over 2 meters long. Scientists are not sure why Arthropleura grew so large; some think high oxygen levels in the air helped, while others believe lack of competition was the reason. Millipedes also show the earliest evidence of chemical defense, as some Devonian fossils had openings for defensive glands called ozopores.

The study of millipede classification began with Carl Linnaeus, who in 1758 named seven species of Julus as "Insecta Aptera" (wingless insects). In 1802, Pierre André Latreille proposed the name Chilognatha for the first group of what are now called Diplopoda. In 1840, Johann Friedrich von Brandt described the group Chilognatha. In 1844, the name Diplopoda was created by Johann Friedrich von Brandt. In 1844, the name Diplopoda was created by Johann Friedrich von Brandt. In 1844, the name Diplopoda was created by Johann Friedrich von Brandt. In 1844, the name Diplopoda was created by Johann Friedrich von Brandt. In 1844, the name Diplopoda was created by Johann Friedrich von Brandt

Characteristics

Millipedes come in many different body shapes and sizes, from as small as 2 mm (1/16 inch) to as large as 35 cm (14 inches). They can have as few as 11 or as many as over 300 body segments. Most millipedes are black or brown, but some have bright colors. A few species have warning colors to show they are poisonous. Some species, like those in the group Motyxia, make cyanide as a defense and can glow in the dark.

The body shapes of millipedes vary among different groups. In the group Penicillata, which includes tiny bristle millipedes, the outside covering (exoskeleton) is soft and not hardened with calcium. These millipedes have many bristles. All other millipedes, in the group Chilognatha, have a hardened exoskeleton. Chilognatha is divided into two groups: Pentazonia, which includes short-bodied millipedes like pill millipedes, and Helminthomorpha, which includes most millipede species with long, many-segmented bodies.

Millipedes have lost a gene that helps make a hormone called juvenile hormone, which controls development and reproduction in other arthropods like insects, centipedes, and crustaceans.

The head of a millipede is rounded on top and flat on the bottom. It has a pair of large mandibles in front of a plate called the gnathochilarium. The head has one pair of antennae with seven or eight segments and a group of sensory cones at the tip. Some millipede groups also have sensory organs called Tömösváry organs, which are small oval rings near the base of the antennae. These organs are also found in some centipedes, but their exact purpose is unknown.

Millipede eyes are made of simple, flat-lensed structures called ocelli, which are grouped in patches on each side of the head. These patches are called ocular fields or ocellaria. Many millipede species, including those in the orders Polydesmida, Siphoniulida, Glomeridesmida, Siphonophorida, Platydesmida, and cave-dwelling species like Causeyella and Trichopetalum, have lost their eyes and are blind.

Millipede bodies can be flat or round and are made of many body segments. Each segment has an exoskeleton with four chitinous plates: one on top (tergite), two on the sides (pleurites), and one on the bottom (sternite) where legs attach. In some millipedes, like Merocheta and Juliformia, these plates are joined together, forming a single ring. The plates are usually hard and contain calcium. Because millipedes cannot close their spiracles (tiny openings for breathing) and most lack a waxy outer layer, they lose water easily and must live in moist environments.

The first body segment after the head is legless and called a collum. The next three segments each have one pair of legs and are called haplosegments. These are sometimes called the thorax. The remaining segments, from the fifth to the end, are diplosegments, formed by the joining of two embryonic segments. Each diplosegment has two pairs of legs, unlike centipedes, which have one pair per segment. Some millipedes have legless segments near the end. The terms "segment" or "body ring" are often used to describe both haplo- and diplosegments. The final segment is called the telson and includes a legless preanal ring, a pair of anal valves (plates around the anus), and a small scale below the anus.

Some millipede orders have keel-like body-wall extensions called paranota, which can be shaped like lobes, ridges, spines, or notches. These features may help millipedes fit into crevices, protect their legs, or make them harder for predators to swallow.

Millipede legs have seven segments and attach to the underside of the body. Most legs are similar, though males often have longer legs than females. In some species, males have modified first legs. The most noticeable leg changes are related to reproduction. Until 2021, no millipede was known to have 1,000 legs. Most species have between 34 and 400 legs, but the species Eumillipes persephone has up to 1,306 legs, the most of any creature on Earth.

Millipedes breathe through two pairs of spiracles on each body segment, near the base of the legs. Each spiracle leads to an internal pouch connected to a system of tracheae (tubes). The heart runs the length of the body, with an aorta extending into the head. Excretory organs called Malpighian tubules are located near the middle of the gut. The digestive tract is a simple tube with two pairs of salivary glands to help break down food.

Reproduction and growth

Millipedes have many different ways of mating and body structures. In the basic group Polyxenida (bristle millipedes), males do not directly mate with females. Instead, they leave behind small packages of sperm called spermatophores on webs made by special glands. Later, females pick up these spermatophores. In all other millipede groups, males have one or two pairs of special legs called gonopods. These legs are used to transfer sperm to females during mating. The placement of gonopods varies: in some groups, like Pentazonia, they are at the back of the body and called telopods, which may also help males hold onto females. In most millipede species, gonopods are found on the seventh body segment. A few species do not need males to reproduce because females can create offspring without mating.

Gonopods come in many shapes and sizes, ranging from ones that look like regular legs to ones that are very different. In some species, gonopods are hidden inside the body, while in others, they stick out forward. Scientists often use the shape of gonopods to identify millipede species, as they can look very different between closely related species but are similar within the same species. Gonopods develop slowly over time, changing from regular legs through a series of molts until the millipede is ready to reproduce.

The openings for reproduction, called gonopores, are located on the underside of the third body segment (near the second pair of legs) in both males and females. In males, one or two structures called penes may help deposit sperm onto the gonopods. In females, the openings lead to small sacs called cyphopods or vulvae, which are covered by small lid-like structures. These sacs store sperm after mating. The shape of cyphopods can also help identify millipede species. Millipede sperm do not have flagella, a feature unique to this group of animals.

In most millipede species, males and females face each other during mating. Before mating, males may use behaviors like tapping with their antennae, running along the female’s back, or offering food-like secretions. Some pill-millipedes make chirping sounds. During mating, the male positions his seventh body segment near the female’s third segment, inserts his gonopods to open the vulvae, and then bends his body to place sperm onto the gonopods before transferring them to the female.

Females lay between 10 and 300 eggs at a time, depending on the species. They use stored sperm to fertilize the eggs as they are laid. Many species lay eggs on moist soil or organic material, but some build nests lined with dried feces and protect the eggs in silk-like cocoons. In most cases, females leave the eggs after laying them, but some species in the groups Platydesmida and Stemmiulida care for their eggs and young.

After hatching, young millipedes have only three pairs of legs and up to four legless segments. As they grow, they molt repeatedly, adding new segments and legs each time. This process is called anamorphosis. Some species prepare special chambers in soil or silk for molting and may stay in these chambers during wet weather. Most species eat their old exoskeleton after molting. The adult stage, when millipedes can reproduce, is reached during the final molt. However, some species continue to molt after becoming adults. A few species alternate between reproductive and non-reproductive stages after reaching maturity, a process called periodomorphosis. Millipedes can live from one to ten years, depending on the species.

Ecology

Millipedes live on every continent except Antarctica and can be found in nearly all land habitats. They live as far north as the Arctic Circle in places like Iceland, Norway, and Central Russia, and as far south as Santa Cruz Province in Argentina. Most millipedes live on the forest floor, hiding in leaf litter, dead wood, or soil. They prefer damp areas. In temperate regions, they are most common in moist deciduous forests, where their numbers can reach over 1,000 per square meter. They also live in coniferous forests, caves, and alpine areas. Some millipedes live in deserts, like the species Orthoporus ornatus, which has a waxy outer layer and can absorb water from dry air. A few species can survive underwater for up to 11 months, and some live near the ocean and tolerate salty conditions.

Millipedes have body segments that help them dig underground. Nearly all millipedes live mostly below the ground. They use three main ways to dig: bulldozing, wedging, and boring. Members of the orders Julida, Spirobolida, and Spirostreptida push forward with their heads, using the collum (front part of their body) to move through soil. Flat-backed millipedes in the order Polydesmida wedge their front ends into cracks and widen them by pushing upward with their legs. Members of the order Polyzoniida use their wedge-shaped bodies to widen cracks as they move forward. Some millipedes live above ground and do not dig. This may happen if they are too small to dig, too large to bother, or if they move quickly and hunt other animals.

Most millipedes eat decaying plants, animal waste, or soil mixed with organic matter. They help break down leaf litter, with some species eating up to 11% of leaf litter in an area. Their digestion breaks leaf litter into small pieces, which helps microorganisms decompose it. In tropical forests where earthworms are rare, millipedes play a key role in this process. Some millipedes eat living plants and can harm crops. Others eat algae, fungi, or are omnivores, eating both plants and animals. A few species drink plant juices, and cave-dwelling millipedes filter tiny particles from water.

Millipedes are eaten by many animals, including reptiles, amphibians, birds, mammals, and insects. Some mammals roll millipedes to remove their defensive chemicals before eating them. Poison dart frogs may use millipede toxins for their own protection. Some insects, like glowworm beetles and certain ants, have special ways to eat millipedes. Assassin bugs and other predators also hunt millipedes. Parasites like nematodes and fungi live on or inside millipedes.

Because they cannot run fast or bite, millipedes curl into tight coils to protect their legs. Many also release smelly liquids through small holes called ozopores. These liquids contain chemicals like alkaloids, benzoquinones, and hydrogen cyanide, which can harm predators. Some animals, like capuchin monkeys, rub millipede chemicals on their skin to repel mosquitoes. These chemicals also fight fungi.

Bristly millipedes (order Polyxenida) do not have armor or smelly glands. Instead, they have bristles that can detach and trap ants. Some millipedes form helpful relationships with other species. For example, certain millipedes live with ants, and some mites ride on millipedes to move to new areas. In 2011, scientists found mosses living on millipedes, which may help the millipedes hide and help the mosses spread.

Interactions with humans

Millipedes usually do not greatly affect human economic or social life, especially compared to insects. However, in some areas, they can become a nuisance or harm crops. Millipedes do not bite, and their defensive secretions are mostly harmless to humans, often causing only slight discoloration on the skin. However, secretions from some tropical species may cause pain, itching, redness, swelling, blisters, eczema, or cracked skin. If these secretions get into the eyes, they can cause irritation, and in some cases, more serious issues like conjunctivitis or keratitis. This condition is called millipede burn. First aid involves washing the affected area thoroughly with water, and further care focuses on reducing local symptoms.

Some millipedes are considered household pests. For example, Xenobolus carnifex can infest thatched roofs in India, and Ommatoiulus moreleti sometimes enters homes in Australia. Other species swarm in certain times of the year, leading to problems such as home invasions, crop damage, and train delays when crushed millipedes make tracks slippery. Some species, like the spotted snake millipede (Blaniulus guttulatus), harm crops such as sugar beets and are one of the few millipedes with a common name.

Larger millipedes from the orders Spirobolida, Spirostreptida, and Sphaerotheriida are sometimes kept as pets. Commonly sold or kept species include those from the genera Archispirostreptus, Aphistogoniulus, Narceus, and Orthoporus.

Millipedes appear in folklore and traditional medicine worldwide. In some cultures, millipede activity is linked to rain. In Zambia, crushed millipede pulp is used to treat wounds, and the Bafia people of Cameroon use millipede juice for earaches. In certain Himalayan Bhotiya tribes, dry millipede smoke is used to treat hemorrhoids. In Malaysia, millipede secretions are used in poison-tipped arrows. The secretions of Spirobolus bungii have been observed to stop human cancer cells from dividing. The only known use of millipedes as food comes from the Bobo people of Burkina Faso, who boil and dry millipedes from the families Gomphodesmidae and Spirostreptidae and serve them with tomato sauce.

Millipedes have also influenced scientific research. In 1963, a 36-legged walking vehicle was designed, inspired by studies of millipede movement. Experimental robots have also been modeled after millipedes, especially for carrying heavy loads in tight spaces. In biology, some scientists suggest using millipedes as models to study arthropod physiology and how body segments develop.

Like worms in vermicomposting, millipedes can help turn plant material into compost, a process called millicomposting. This method improves compost quality.