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Temporal range: 428–0MaLate Silurian to Recent
|An assortment of millipedes|
(not to scale)
De Blainville in Gervais, 1844
|16 orders, c. 12,000 species|
Temporal range: 428–0MaLate Silurian to Recent
|An assortment of millipedes|
(not to scale)
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 34 and 400 legs. There are approximately 12,000 named species classified into sixteen orders and around 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.
Millipedes are among the first animals to have colonised land during the Silurian geologic period. Early forms probably ate mosses and primitive vascular plants. There are two major groups of entirely extinct millipedes: the Archipolypoda ("ancient, many-legged ones") which contain the oldest known terrestrial animals, and Arthropleuridea, which contain the largest known land invertebrates. The oldest known land creature, Pneumodesmus newmani, was a 1 cm (0.39 in) long archipolypodan that lived 428 million years ago in the upper Silurian, and has clear evidence of spiracles (breathing holes) attesting to its air-breathing habits. During 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 also include the earliest evidence of chemical defense, as some Devonian fossils have defensive gland openings called ozopores. 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 come in a variety of body shapes and sizes, ranging 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.
Body styles vary greatly between major millipede groups. In the basal subclass Penicillata, consisting of the tiny bristle millipedes, the exoskelton is soft and uncalcified, and is covered in prominent setae or bristles. All other millipedes, belonging to the subclass Chilognatha, have a hardened exoskeleton. The chilognaths are in turn divided into two infraclasses: the Pentazonia, containing relatively short-bodied groups such as pill millipedes, and the Helminthomorpha ("worm-like" millipedes), which contains the vast majority of species, and the long, many-segmented body types familiar to most people.
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 also called ocular fields or ocellaria. 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.
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Millipede bodies may be flattened or cylindrical, and are composed of numerous metemeric segments, each with an exoskeleton consisting of five chitinous plates: a single plate above (the tergite), one at each side (pleurites), and a plate on the underside (sternite) where the legs attach. 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 (from the Latin for neck or collar). The second, third, and 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 is known as the telson, which consists of a legless preanal ring, a pair of anal valves (closeable plates around the anus), and a small 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, size, and texture. Paranota may allow millipedes to wedge more securely into crevices, protect the legs, or make the millipede more difficult for predators to swallow.
The legs are composed of seven segments, and attach on the underside of the body. The legs of an individual are generally rather similar to each other, although often longer in males than females, and males of some species have a reduced or enlarged first pair of legs. The most conspicuous leg modifications are involved in reproduction, and are discussed below.
Millipedes breathe through two pairs of spiracles located ventrally on each segment near the base of the legs. 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 (bristle millipedes), mating is indirect: males deposit spermatophores onto webs they secrete with special glands, and the spermatophores 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 and may also function in grasping females, while in the Helminthomorpha – the vast majority of species – they are located on the seventh body segment. A few species are parthenogenetic, having few, if any, males.
Gonopods occur in a diversity of shapes and sizes, and in the range from closely resembling walking legs to complex structures quite unlike legs at all. In some groups the gonopods are kept retracted within the body, while in others they project forward parallel to the body. Gonopod morphology is the predominant means of determining species among millipedes: the structures may differ greatly between closely related species but very little within a species. The gonopods develop gradually from walking legs through successive moults until reproductive maturity.
The genital openings (gonopores) of both sexes are located on the underside of the third body segment (near the second pair of legs) and may be 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. The cyphopod morphology can also be used to identify species. Millipede sperm is aflagellate (lacks a flagellum), a unique trait among myriapods.
In all millipedes except the bristle millipedes, copulation occurs with the two individuals facing one another. Copulation may be preceded by male behaviors such as tapping with antennae, running along the back of the female, offering glandular secretions which the female consumes, or in the case of some pill-millipedes, stridulation or "chirping". During copulation in most millipedes, the male positions his seventh segment in front of the female's third segment, and may insert his gonopods to extrude the vulvae before bending his body to deposit sperm onto his gonopods and reinserting the "charged" gonopods into the female.
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. The adult stage- when individuals become reproductively mature- is generally reached in the final molt stage, which varies between species and orders, although some species continue to molt after adulthood. Furthermore, some species alternate between reproductive and non-reproductive stages after maturity, a phenomenon known as periodomorphosis, in which the reproductive structures regress during non-reproductive stages. Millipedes may live from one to ten years, depending on species.
Millipedes occur on all continents except Antarctica, and occupy almost all terrestrial habitats, ranging as far north as the Arctic Circle in Iceland, Norway, and Central Russia, and as far south as Santa Cruz Province, Argentina. Millipedes are typically forest floor dwellers, occurring leaf litter, dead wood, or soil, with a preference for humidity. In temperate zones, millipedes are most abundant in moist deciduous forests, and may reach densities of over 1,000 individuals per square meter. Other habitats include 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.
The majority of millipedes are detritivores and feed on decomposing vegetation, faeces, or organic matter mixed with soil. They often play important roles in the breakdown and decomposition of leaf litter: estimates of consumption rates for individual species range from 1 to 11 percent of all leaf litter, depending on species and region, and collectively millipedes may consume nearly all the leaf litter in a region. The leaf litter is fragmented in the millipede gut and excreted as pellets of leaf fragments, algae, fungi, and bacteria, which facilitates decomposition by the microorganisms. Some millipedes are herbivorous, feeding on living plants, and some species can become serious pests of crops. 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 predaceous dung beetles of the genera Sceliages and Deltochilum. 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 exoskeleton. 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 mosses was described in 2011, in which individuals of the newly discovered Psammodesmus bryophorus was found to have up to ten species living on its dorsal surface, in what may provide camouflage for the millipede and increased dispersal for the mosses.
Millipedes generally have little impact to human economic or social well-being, especially in comparison with insects, although locally can be a nuisance or agricultural pest. 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 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 can infest thatched roofs in India, and Ommatoiulus moreleti, which periodically invades homes in Australia. Other species exhibit periodical swarming behaviour, which can result in home invasions, crop damage, train delays, or even train crashes and derailments when the tracks become slippery with the crushed remains of thousands of millipedes. Some millipedes can cause significant damage to crops: the spotted snake millipede (Blaniulus guttulatus) is a noted pest of sugar beets and other root crops, and as a result is one of the few millipedes with a common name.
Some of the larger millipedes in the orders Spirobolida, Spirostreptida, and 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 secretions of Spirobolus bungii have even been reported to inhibit division of human cancer cells. 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. In popular music (including names of albums, songs, and artists) millipedes are poorly represented compared to other arthropods.
Millipedes have also inspired and played roles in scientific research. The locomotion and anatomy of millipedes have inspired the design of experimental robots, in particular when heavy loads are needed to be carried in tight areas involving turns and curves. In biology, some authors have advocated millipedes as model organisms for the study of arthropod physiology and the developmental processes controlling the number and shape of body segments.
The science of millipede biology and taxonomy is called diplopodology: the study of diplopods. Approximately 12,000 millipede species have been described, but estimates of the true number of species on earth range from 15,000–20,000 to as high as 80,000.
The living members of the Diplopoda are divided into sixteen orders in two subclasses. The basal subclass Penicillata contains a single order, Polyxenida (bristle millipedes). 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 history of scientific millipede classification began with Carl Linnaeus, who in his 10th edition of Systema Naturae, 1758, named seven species of Julus as "Insecta Aptera" (wingless insects). In 1802, the French zoologist Pierre André Latreille proposed the name Chilognatha as the first group of what are now the Diplopoda, and in 1840 the German naturalist Johann Friedrich von Brandt produced the first detailed classification. The name Diplopoda itself was coined in 1844 by Henri Marie Ducrotay de Blainville. In the following decades, millipede taxonomy was driven by relatively few researchers at any given time, with major contributions by Carl Attems, Karl Verhoeff and Ralph V. Chamberlin, who each described over 1,000 species, as well as Orator F. Cook, Filippo Silvestri, R. I. Pocock, and Henry W. Brölemann. The 50-year period from 1890 to 1940, when the seven researchers above were working, was a period when the science of diplopodology flourished: rates of species descriptions during this period was on average the highest in history, sometimes exceeding 300 per year.
In 1971, Dutch biologist C. A. W. Jeekel published a comprehensive listing of all known millipede genera and families described between 1758 and 1957 in his Nomenclator Generum et Familiarum Diplopodorum, a work credited as launching the "modern era" of millipede taxonomy. In 1980, American biologist Richard L. Hoffman published a classification of millipedes which recognized the Penicillata, Pentazonia, and Helminthomorpha, and the first phylogenetic analysis of millipede orders using modern cladistic methods was published in 1984 by Henrik Enghoff of Denmark. A 2003 classification by American myriapodologist Rowland Shelley is similar to classification originally proposed by Verhoeff, and remains the currently accepted classification scheme (shown below), despite more recent molecular studies which propose a number of conflicting relationships. A 2011 summary of millipede family diversity by William A. Shear placed the order Siphoniulida within the larger group Nematomorpha.
In addition to the 16 living orders, there are 9 extinct orders and one superfamily known only from fossils. The relationship of these to living groups and to each other is controversial. The extinct Arthropleuridea was long considered a distinct myriapod class, although work in the early 21st century established the group as a subclass of millipedes. Several living orders also appear in the fossil record. Below are two proposed arrangements of fossil millipede groups. Extinct groups are indicated with a dagger (†). The extinct order Zosterogrammida, a chilognath of uncertain position, is not shown.
|Alternate hypothesis of fossil relationships |
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 the traditional classification schemes, above 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. After each name is listed the author citation: the name of the person who coined the name or defined the group, even if not at the current rank.
Class Diplopoda de Blainville in Gervais, 1844
Illacme plenipes has up to 750 legs...
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