Millipedes (originating from the Latin mille, "thousand", and pes, "foot")[ There are approximately 12,000 named species classified into 16 orders and around 140 families, making Diplopoda the largest class of , an arthropod subphylum which also includes and other multi-legged creatures.
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Most millipedes are slow-moving , eating decaying leaves and other dead plant matter; however, some eat fungus or drink plant fluid. Millipedes are generally harmless to humans, although some can become household or garden pests. Millipedes can be an unwanted nuisance particularly in where they can potentially cause severe damage to emergent seedlings. Most millipedes defend themselves with a variety of chemicals secreted from pores along the body, although the tiny bristle millipedes are covered with tufts of detachable bristles. Its primary defence mechanism is to curl into a tight coil, thereby protecting its legs and other vital delicate areas on the body behind a hard exoskeleton. Reproduction in most species is carried out by modified male legs called , which transfer packets of sperm to females.
First appearing in the Silurian period, millipedes are some of the oldest known land animals. Some members of prehistoric groups, such as Arthropleura, grew to over ; the largest modern species reach maximum lengths of . The longest extant species is the giant African millipede ( Archispirostreptus gigas).
Among myriapods, millipedes have traditionally been considered most closely related to the tiny , although some molecular studies challenge this relationship. Millipedes can be distinguished from the somewhat similar but only distantly related (class Chilopoda), which move rapidly, are , carnivorous, and have only a single pair of legs on each body segment.
The scientific study of millipedes is known as diplopodology, and a scientist who studies them is called a diplopodologist.
Etymology and names
The term "millipede" is widespread in popular and scientific literature, but among North American scientists, the term "milliped" (without the terminal e) is also used.
[ The study of millipede biology and taxonomy is called diplopodology.]
Classification
Approximately 12,000 millipede species have been described. Estimates of the true number of species on earth range from 15,000
The living members of the Diplopoda are divided into sixteen orders in two subclasses.
Outline of classification
The higher-level classification of millipedes is presented below, based on Shear, 2011,[ (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.]
Class Diplopoda de Blainville in Gervais, 1844
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Subclass Penicillata Latreille, 1831
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Subclass †Arthropleuridea (placed in Penicillata by some authors)
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Subclass Chilognatha Latreille, 1802
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Order †Zosterogrammida Wilson, 2005 (Chilognatha incertae sedis)
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Infraclass Pentazonia Brandt, 1833
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Infraclass Helminthomorpha Pocock, 1887
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Superorder †Archipolypoda Scudder, 1882
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Order †Pleurojulida Schneider & Werneburg, 1998 (possibly sister to Colobognatha)
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Subterclass Colobognatha Brandt, 1834
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Subterclass Eugnatha Attems, 1898
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Superorder Juliformia Attems, 1926
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Superorder Nematophora Verhoeff, 1913
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Superorder Merocheta Cook, 1895
Evolution
Millipedes are among the first animals to have colonised land during the Silurian.[
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Living groups
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).[ This was a period when the science of diplopodology flourished: rates of species descriptions were on average the highest in history, sometimes exceeding 300 per year.]
In 1971, the 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.[ A 2011 summary of millipede family diversity by William Shear placed the order Siphoniulida within the larger group Nematophora.]
Fossil record
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.[ Extinct groups are indicated with a dagger (†). The extinct order Zosterogrammida, a chilognath of uncertain position,][ is not shown.
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Relation to other myriapods
Although the relationships of millipede orders are still the subject of debate, the class Diplopoda as a whole is considered a monophyletic group of arthropods: all millipedes are more closely related to each other than to any other arthropods. Diplopoda is a class within the arthropod subphylum Myriapoda, the myriapods, which includes (class Chilopoda) as well as the lesser-known Pauropoda (class Pauropoda) and (class Symphyla). Within myriapods, the closest relatives or sister group of millipedes has long been considered the pauropods, which also have a collum and diplosegments.
Distinction from centipedes
The differences between millipedes and centipedes are a common question from the general public.+ Millipede and centipede differences | Trait
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Characteristics
Millipedes come in a variety of body shapes and sizes, ranging from to around in length,
Body styles vary greatly between major millipede groups. In the basal subclass Penicillata, consisting of the tiny bristle millipedes, the exoskeleton is soft and uncalcified, and is covered in prominent 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 , and the Helminthomorpha ("worm-like" millipedes), which contains the vast majority of species, with long, many-segmented bodies.
They have also lost the gene that codes for the JHAMTl enzyme, which is responsible for catalysing the last step of the production of a juvenile hormone that regulates the development and reproduction in other arthropods like crustaceans, centipedes and insects.
Head
The head of a millipede is typically rounded above and flattened below and bears a pair of large mandibles in front of a plate-like structure called a gnathochilarium ("jaw lip").[ The head contains a single pair of antennae with seven or eight segments and a group of sensory cones at the tip.]
Millipede eyes consist of several simple flat-lensed ocelli arranged in a group or patch on each side of the head. These patches are also called ocular fields or ocellaria. Many species of millipedes, including the entire orders Polydesmida, Siphoniulus, Glomeridesmida, Siphonophorida and Platydesmida, and cave-dwelling millipedes such as Causeyella and Trichopetalum, had ancestors that could see but have subsequently lost their eyes and are blind.[
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Body
Millipede bodies may be flattened or cylindrical, and are composed of numerous metameric segments, each with an exoskeleton consisting of four plates: a single plate above (the tergite), one at each side (), and a plate on the underside (sternite) where the legs attach. In many millipedes, such as Merocheta and Juliformia, these plates are fused to varying degrees, sometimes forming a single cylindrical ring. The plates are typically hard, impregnated with calcium salts.
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" (the three haplosegments are sometimes referred to as a "thorax"[
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Millipedes in several orders have keel-like extensions of the body-wall known as paranota, which can vary widely in shape, size, and texture; modifications include lobes, papillae, ridges, crests, spines and notches.[ 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 may have a reduced or enlarged first pair of legs.[ The most conspicuous leg modifications are involved in reproduction, discussed below. Despite the common name, no millipede was known to have 1,000 legs until 2021: common species have between 34 and 400 legs, and the record is held by Eumillipes persephone, with individuals possessing up to 1,306 legs – more than any other creature on Earth.]
Internal organs
Millipedes breathe through two pairs of spiracles located ventrally on each segment near the base of the legs.[
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Reproduction and growth
Millipedes show a diversity of mating styles and structures. In the basal order Polyxenida (bristle millipedes), mating is indirect: males deposit onto webs they secrete with special glands, and the spermatophores are subsequently picked up by females.
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; 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 cyphopod morphology can also be used to identify species. Millipede sperm lack flagellum, a unique trait among myriapods.][
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In all except the bristle millipedes, copulation occurs with the two individuals facing one another. Copulation may be preceded by male behaviours such as tapping with antennae, running along the back of the female, offering edible glandular secretions, or in the case of some pill-millipedes, stridulation or "chirping".
Females lay from ten to three hundred eggs at a time, depending on species, fertilising them with the stored sperm as they do so. Many species 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 abandons the eggs after they are laid, 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 ecdysis, adding further segments and legs as they do so, a mode of development known as anamorphosis.[ Millipedes may live from one to ten years, depending on species.][
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Ecology
Habitat and distribution
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.[ Deserticolous millipedes, species evolved to live in the desert, like Orthoporus ornatus, may show adaptations like a waxy epicuticle and the ability of water uptake from unsaturated air.]
Burrowing
The diplosegments of millipedes have evolved in conjunction with their burrowing habits, and nearly all millipedes adopt a mainly subterranean lifestyle. They use three main methods of burrowing; bulldozing, wedging and boring. Members of the orders Julida, Spirobolida and Spirostreptida, lower their heads and barge their way into the substrate, the collum leading the way. Flat-backed millipedes in the order Polydesmida tend to insert their front end, like a wedge, into a horizontal crevice, and then widen the crack by pushing upwards with their legs, the paranota in this instance constituting the main lifting surface. Boring is used by members of the order Polyzoniida. These have smaller segments at the front and increasingly large ones further back; they propel themselves forward into a crack with their legs, the wedge-shaped body widening the gap as they go. Some millipedes have adopted an above-ground lifestyle and lost the burrowing habit. This may be because they are too small to have enough leverage to burrow, or because they are too large to make the effort worthwhile, or in some cases because they move relatively fast (for a millipede) and are active predators.[
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Diet
Most millipedes are and feed on decomposing vegetation, feces, or organic matter mixed with soil. They often play important roles in the breakdown and decomposition of plant 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.[ Where earthworm populations are low in tropical forests, millipedes play an important role in facilitating microbial decomposition of the leaf litter.][ 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.][ Worldwide distribution of cave-dwelling Chelodesmidae (Diplopoda, Polydesmida)]
Predators and parasites
Millipedes are predation by a wide range of animals, including various , , , , and .
Defence mechanisms
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 "repugnatorial glands"), along the sides of their bodies as a secondary defence. Among the many irritant and toxic chemicals found in these secretions are , , phenols, , and hydrogen cyanide.
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.
Other inter-species interactions
Some millipedes form mutualistic relationships with organisms of other species, in which both species benefit from the interaction, or Commensalism, 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", 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 of the orders Mesostigmata and Astigmata. Many of these mites are believed to be Phoresy rather than parasitic, which means that they 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.
Interactions with humans
Millipedes generally have little impact on human economic or social well-being, especially in comparison with insects, although locally they can be a nuisance or agricultural pest. Millipedes do not bite, and their defensive secretions are mostly harmless to humans — usually causing only minor discolouration on the skin — but the secretions of some tropical species may cause pain, itching, local erythema, edema, , Dermatitis, and occasionally cracked skin.
Some millipedes are considered household pests, including Xenobolus carnifex which can infest thatching in India,[
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Some of the larger millipedes in the orders Spirobolida, Spirostreptida, and Sphaerotheriida are popular as pets.[ In certain Himalayan Bhotiya tribes, dry millipede smoke is used to treat .][ The secretions of Spirobolus bungii have been observed to inhibit division of human cancer cells.]
Millipedes have also inspired and played roles in scientific research. In 1963, a walking vehicle with 36 legs was designed, said to have been inspired by a study of millipede locomotion.[
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Similar to vermicompost, millipedes can be used to convert plant matter into compost in what has been named millicomposting, which improves the quality of the compost.[ Millicompost: an alternate biocompost for forest nurseries]
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