Millipedes (originating from the Latin , "thousand", and , "foot") are a group of arthropods that are characterised by having two pairs of jointed legs on most body segments; they are known scientifically as the class Diplopoda, the name derived from this feature. Each double-legged segment is a result of two single segments fused together. Most millipedes have very elongated cylindrical or flattened bodies with more than 20 segments, while pill millipedes are shorter and can roll into a tight ball. Although the name "millipede" derives from Latin for "thousand feet", no species was known to have 1,000 or more until the discovery in 2020 of Eumillipes persephone, which can have over 1,300 legs. There are approximately 12,000 named species classified into 16 orders and around 140 families, making Diplopoda the largest class of myriapods, an arthropod subphylum which also includes centipedes and other multi-legged creatures.
Most millipedes are slow-moving detritivores, eating decaying leaves and other dead plant matter; however, some eat fungi 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 greenhouses 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 gonopods, 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 pauropods, although some molecular studies challenge this relationship. Millipedes can be distinguished from the somewhat similar but only distantly related centipedes (class Chilopoda), which move rapidly, are venomous, 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. Other vernacular names include "thousand-legger" or simply "diplopod". 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 to as high as 80,000. Few species of millipede are at all widespread; they have very poor dispersal abilities, depending as they do on terrestrial locomotion and humid habitats. These factors have favoured genetic isolation and rapid speciation, producing many lineages with restricted ranges.
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: Pentazonia, containing the short-bodied pill millipedes, and Helminthomorpha (worm-like millipedes), containing the great majority of the species. A recent phylogenomic analysis using 311 orthologous genes placed 15 orders in a phylogenetic context.
Outline of classification
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 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
Evolution Millipedes are among the first animals to have colonised land during the Silurian period. Early forms most likely ate mosses and primitive vascular plants. There are two major groups of millipedes whose members are all extinct: the Archipolypoda ("ancient, many-legged ones") which contain the oldest known terrestrial animals, and Arthropleuridea, which contain the largest known land invertebrates. The earliest known body fossil of millipedes is from the Wenlock Epoch, dated to approximately 427.0 ± 4.5 million years ago. Other early millipedes from the Ludlow Epoch are represented by Kampecaris obanensis and Archidesmus sp. from 425 million years ago. Pneumodesmus newmani was once argued to be the earliest member of the millipedes from the late Wenlock epoch of the late Silurian around , but recent studies indicate that the dating method was erroneous, and that the taxon most likely originated during the Pridoli Epoch of the late Silurian or the Lochkovian of the early Devonian. During the Carboniferous, Arthropleura became the largest known land-dwelling invertebrate on record, length exceeding . The reason that Arthropleura was able to achieve this size is not clearly known; early studies posited that it was a result of high atmospheric oxygen levels, while later studies consider that the lack of competition is more probable. Millipedes also exhibit the earliest evidence of chemical defence, as some Devonian fossils have defensive gland openings called ozopores.
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). 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 the French zoologist Henri Marie Ducrotay de Blainville. From 1890 to 1940, millipede taxonomy was driven by relatively few researchers at any given time, with major contributions by Carl Attems, Karl Wilhelm Verhoeff and Ralph Vary Chamberlin, who each described over 1,000 species, as well as Orator F. Cook, Filippo Silvestri, R. I. Pocock, and Henry W. Brölemann. 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. In 1980, the 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 the American myriapodologist Rowland Shelley is similar to the one originally proposed by Verhoeff, and remains the currently accepted classification scheme (shown below), despite more recent molecular studies proposing conflicting relationships. A 2011 summary of millipede family diversity by William A. 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. 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.
Relation to other myriapods
Although the relationships of millipede orders are still a 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 centipedes (class Chilopoda) as well as the lesser-known pauropods (class Pauropoda) and symphylans (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. Both groups of myriapods share similarities, such as long, multi-segmented bodies, many legs, a single pair of antennae, and the presence of postantennal organs, but have many differences and distinct evolutionary histories, as the most recent common ancestor of centipedes and millipedes lived around 450 to 475 million years ago in the Silurian. The head alone exemplifies the differences; millipedes have short, geniculate (elbowed) antennae for probing the substrate, a pair of robust mandibles and a single pair of maxillae fused into a lip; centipedes have long, threadlike antennae, a pair of small mandibles, two pairs of maxillae and a pair of large poison claws.