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Temporal range: 428–0MaLate Silurian to Recent
|Rusty millipede (Trigoniulus corallinus)|
De Blainville in Gervais, 1844
|16 orders, c. 12,000 species|
Temporal range: 428–0MaLate Silurian to Recent
|Rusty millipede (Trigoniulus corallinus)|
De Blainville in Gervais, 1844
|16 orders, c. 12,000 species|
Millipedes (class Diplopoda) are myriapodous arthropods that have two pairs of legs on most body segments. Each double-legged segment is a result of two single segments fused together as one—the name "Diplopoda" comes from the Greek words διπλοῦς (diplous), "double" and ποδός (podos), "foot". Most millipedes have very elongated cylindrical or flattened bodies with more than 20 segments, 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 the rare species Illacme plenipes has up to 750. Common species have between 36 and 400 legs. There are approximately 12,000 named species in ca. 140 families. The longest extant species is the giant African millipede (Archispirostreptus gigas).
Most millipedes are slow-moving detritivores, eating decaying leaves and other dead plant matter. However, they can also be minor garden pests, especially in greenhouses where they can cause severe damage to emergent seedlings.
Millipedes can be easily distinguished from the somewhat similar and related centipedes (Class Chilopoda) which move rapidly, are carnivorous, and have a single pair of legs for each body segment. The scientific study of millipedes is known as diplopodology, and a scientist who studies them is called a diplopodologist.
This class of arthropod is thought to be among the first animals to have colonised land during the Silurian geologic period. These early forms probably ate mosses and primitive vascular plants. The oldest known land creature, Pneumodesmus newmani, was a 1 cm (0.39 in) long millipede, and lived 428 million years ago. In the Upper Carboniferous ( ), Arthropleura became the largest known land invertebrate of all time, reaching lengths of up to 2.6 m (8 ft 6 in). Millipedes, centipedes, and other terrestrial arthropods attained very large sizes in comparison to modern species in the oxygen-rich environments of the Devonian and Carboniferous periods, and some could grow larger than one metre. As oxygen levels lowered through time, arthropods became smaller in size.
Millipedes range from 2 mm (0.079 in) to around 35 cm (14 in) in length, and can have as few as eleven to over one hundred segments. They are generally black or brown in colour, although there are a few brightly coloured species.
The millipede's most obvious feature is its large number of legs. Having many short legs makes millipedes rather slow, but the many legs pushing in unison provides powerful strength for burrowing.
The head contains a single pair of antennae with seven or eight segments and a group of sensory cones at the tip. Many orders also possess a pair of sensory organs known as the Tömösváry organs, shaped as small oval rings posterior and lateral to the base of the antennae. Their true function is unknown, but they also occur in some centipedes, and are possibly used to measure humidity or light levels in the surrounding environment.
Millipede eyes consist of a number of simple flat-lensed ocelli arranged in a group or patch on each side of the sides of the head. These patches are sometimes called ocular fields. Many species of millipedes, including the entire order Polydesmida and cave-dwelling millipedes such as Causeyella and Trichopetalum, have secondarily lost their eyes and are completely blind.
The body is flattened or cylindrical, composed of numerous metemeric segments, each with an exoskeleton consisting of a single chitinous plate above, one at each side, and two or three on the underside. In many millipedes, these plates are fused to varying degrees, sometimes forming a single cylindrical ring. The plates are typically hard, being impregnated with calcium salts. 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 first segment behind the head is legless and known as a collum. The second to fourth body segments bear a single pair of legs each and are known as "haplosegments", from the Greek haplo, "single" (the three haplosegments are sometimes referred to as a "thorax"). The remaining segments, from the fifth to the posterior, are properly known as diplosegments, or double segments. Each diplosegment bears two pairs of legs, rather than just one as in centipedes. This is because each diplosgment is formed by the fusion of two embryonic segments. In some millipedes the last few segments may be legless. The terms "segment" or "body ring" are often used interchangeably to refer to both haplo- and diplosegments. The final segment bears a telson, which consists of a legless preanal ring, pair of anal valves (closeable plates around the anus), and a scale below the anus.
Millipedes in several orders have keel-like extensions of the body-wall known as paranota, which can vary widely in shape and size. Paranota may allow millipedes to wedge more securely into crevices, protect the legs, or make the millipede more difficult for predators to swallow.
Millipedes breathe through two pairs of spiracles located ventrally on each segment. Each opens into an internal pouch, and connects to a system of tracheae. The heart runs the entire length of the body, with an aorta stretching into the head. The excretory organs are two pairs of malpighian tubules, located near the mid-part of the gut. The digestive tract is a simple tube with two pairs of salivary glands to help digest the food.
Millipedes show a diversity of mating styles and structures. In the basal order Polyxenida, males deposit spermatophores that are subsequently picked up by females. In all other millipede groups, males possess one or two pairs of modified legs called gonopods which are used to transfer sperm to the female during copulation. The location of the gonopods differs between groups: in males of the Pentazonia they are located at the rear of the body and known as telopods, while in Helminthomorpha – the vast majority of species – they are located on the 7th body segment. A few species are parthenogenetic, having few, if any, males.
Gonopod morphology is the predominant means of determining species among millipedes: the complex structures differ much between species but very little within a species.
The genital openings are located on the third segment, and are accompanied in the male by one or two penes which deposit the sperm packets onto the gonopods. In the female, the genital pores open into paired small sacs called cyphopods or vulvae, which are covered by a small hood-like cover, and are used to store the sperm after copulation. Millipede sperm is aflagellate (lacks a flagellum), a unique trait among myriapods.
Females lay between ten and three hundred eggs at a time, depending on species, fertilising them with the stored sperm as they do so. Many species simply deposit the eggs on moist soil or organic detritus, but some construct nests lined with dried faeces, and may protect the eggs within silk cocoons. In most species the female abandon the eggs after laying but some species in the orders Platydesmida and Stemmiulida provide parental care for eggs and young.
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. Some species moult within specially prepared chambers of soil or silk, which they may also use to wait out dry weather, and most species eat the shed exoskeleton after moulting. Millipedes live from one to ten years, depending on species.
In temperate zones, millipedes are most abundant in moist deciduous forests, but they also occur in coniferous forests, deserts, caves, and alpine ecosystems. Some species can survive freshwater floods and live submerged underwater for up to 11 months. A few species occur near the seashore and can survive in somewhat salty conditions.
Most millipedes are detritivores and feed on decomposing vegetation, faeces, or organic matter mixed with soil. Millipedes in the order Polyxenida graze algae from bark, and Platydesmida feed on fungi. A few species are omnivorous or occasionally carnivorous, feeding on insects, centipedes, earthworms, or snails Some species have piercing mouth parts that allow them to feed on plant juices.
Millipedes are preyed upon by a wide range of animals, including various reptiles, amphibians, birds, mammals, and insects. Mammalian predators such as coatis and meerkats roll captured millipedes on the ground to deplete defensive secretions and rub them off the body before consuming, while certain poison dart frogs are believed to incorporate the toxic compounds of millipedes into their own defenses. Several invertebrates have specialized behaviors or structures to feed on millipedes, including larval glowworm beetles, Probolomyrmex ants, chlamydephorid slugs, and the predaceous dung beetle Deltochilum valgum. A large subfamily of assassin bugs, the Ectrichodiinae with over 600 species, has specialized in preying upon millipedes.
Due to their lack of speed and their inability to bite or sting, millipedes' primary defence mechanism is to curl into a tight coil – protecting their delicate legs inside an armoured body exterior. Many species also emit various foul-smelling liquid secretions through microscopic holes called ozopores (the openings of "odoriferous" or "repugantorial glands"), along the sides of their bodies as a secondary defence. These secretions may include alkaloids, benzoquinones, phenols, terpenoids, and/or hydrogen cyanide, among many others. 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. Primates such as capuchin monkeys and lemurs have been observed intentionally irritating millipedes in order to rub the chemicals on themselves to repel mosquitoes. Some of these defensive compounds also show antifungal activity. The bristly millipedes (order Polyxenida) lack both an armoured exoskeleton and odiferous glands, and instead are covered in numerous bristles that in at least one species, Polyxenus fasciculatus, detach and entangle ants.
Some millipedes form mutualistic relationships with organisms of other species, in which both species benefit from the interaction, or commensal relationships, in which only one species benefits while the other is unaffected. Several species form close relationships with ants, a relationship known as myrmecophily, especially within the family Pyrgodesmidae (Polydesmida), which contains "obligate myrmecophiles"- species which have only been found in ant colonies. More species are "facultative myrmecophiles", being non-exclusively associated with ants, including many species of Polyxenida that have been found in ant nests around the world.
Many millipede species have commensal relationships with mites of the orders Mesostigmata and Astigmata. Many of these mites are believed to be phoretic rather than parasitic, which means they simply use the millipede host as a means of dispersal.
A novel interaction between millipedes and moss was described in 2011, in which the newly discovered Psammodesmus bryophorus was found to have up to 10 species living on its dorsal surface, in what may provide camouflage for the millipede and increased dispersal for the mosses.
Millipedes do not bite, and their defensive secretions are mostly harmless to humans – usually causing only minor discoloration on the skin – but the secretions of some tropical species may cause pain, itching, local erythema, edema, blisters, eczema, and occasionally cracked skin. Eye exposures to these secretions causes general eye irritation and potentially more severe effects such as conjunctivitis and keratitis. First aid consists of flushing the area thoroughly with water; further treatment is aimed at relieving the local effects.
Some millipedes are considered household pests, including Xenobolus carnifex which infests thatched roofs in India. Other species exhibit periodical swarming behaviour, which can result in home invasions, crop damage, train delays, or even train crashes and derailments.
Some of the larger millipedes in the orders Spirobolida, Spirostreptida, Sphaerotheriida are popular as pets. Some species commonly sold or kept include species of Archispirostreptus, Aphistogoniulus, Narceus, and Orthoporus.
Millipedes also appear in folklore and traditional medicine around the world. Many cultures ascribe millipede activity with coming rains. In the Yoruba culture of Nigeria, millipedes are used in pregnancy and business rituals, and crushed millipedes are used to treat fever, whitlow, and convulsion in children. In Zambia, smashed millipede pulp is used to treat wounds, and in the Bafia people of Cameroon millipede juice is used to treat earaches. In certain Himalayan Bhotiya tribes, dry millipede smoke is used to treat hemorrhoids. Native people in Malaysia use millipede secretions in poison-tipped arrows. The only reported usage of millipedes as food by humans comes from the Bobo people of Burkina Faso, who consume boiled, dried millipedes in tomato sauce.
Approximately 12,000 species of millipede have been described, but the true number of species on earth has been estimated at up to 80,000. Millipedes are considered a mega-diverse yet understudied group, with inconsistent taxonomic effort over time.
The living members of the Diplopoda are divided into sixteen orders in two subclasses. The basal subclass Penicillata contains small species whose exoskeleton is not calcified, and which are covered in setae or bristles. All other millipedes belong to the subclass Chilognatha, consisting of two infraclasses: the infraclass Pentazonia containing the short-bodied pill millipedes, and the infraclass Helminthomorpha (worm-like millipedes) containing the great majority of the species.
The higher-level classification of millipedes is presented below, based on Shear, 2011, and Shear & Edgecombe, 2010 (extinct groups). Recent cladistic and molecular studies have challenged traditional classification schemes, and in particular the position of the orders Siphoniulida and Polyzoniida is not yet well established. The placement and positions of extinct groups (†) known only from fossils is tentative and not fully resolved.
Class Diplopoda de Blainville in Gervais, 1844
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