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Millipede

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Millipedes
Temporal range: 427–0 Ma Late Silurian to Present
An assortment of millipedes (not to scale)
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Myriapoda
Class: Diplopoda
Blainville in Gervais, 1844 
Subclasses
Diversity
16 orders, c. 12,000 species

Millipedes (originating from the Latin mille, "thousand", and pes, "foot")[1][2] 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.[3] 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 (e.g. forest litter) and other dead plant matter (e.g. coarse woody debris); however, some eat fungi or suck plant fluids. 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 (volvation), 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 2 m (6+1⁄2 ft); the largest modern species reach maximum lengths of 27 to 38 cm (10+1⁄2 to 15 in). 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

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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.[4][5] Other vernacular names include "thousand-legger" or simply "diplopod".[6] The study of millipede biology and taxonomy is called diplopodology.[7]

Classification

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Diversity
Approximate relative diversity of extant millipede orders, ranging from ca. 3,500 species of Polydesmida to 2 species of Siphoniulida[8]

Approximately 12,000 millipede species have been described. Estimates of the true number of species on earth range from 15,000[9] to as high as 80,000.[10] 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 (strong phylogeographic structure) and rapid speciation, producing many lineages with restricted ranges.[11][12]

The living members of the Diplopoda are divided into sixteen orders in two subclasses.[8][13] 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.[14][15] A recent phylogenomic analysis using 311 orthologous genes placed 15 orders in a phylogenetic context.[13]

Outline of classification

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The higher-level classification of millipedes is presented below, based on Shear (2011),[8] and Shear & Edgecombe (2010)[16] for 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.[10] The placement and positions of extinct groups (†) known only from fossils is tentative and not fully resolved.[10][16] 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

Evolution

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Millipedes are among the first animals to have colonised land during the Silurian period.[18] Early forms most likely ate mosses and primitive vascular plants.[19] 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,[20] and Arthropleuridea, which contain the largest known land invertebrates.[21] The earliest known body fossil of millipedes is from the Wenlock Epoch, dated to approximately 427.0 ± 4.5 million years ago.[22] Other early millipedes from the Ludlow Epoch are represented by Kampecaris obanensis and Archidesmus sp. from 425 million years ago.[23] Pneumodesmus newmani was once argued to be the earliest member of the millipedes from the late Wenlock epoch of the late Silurian around 428 million years ago,[24][25] 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.[26][27] During the Carboniferous, Arthropleura became the largest known land-dwelling invertebrate on record, with a length exceeding 2 m (6+1⁄2 ft). 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 considered that other factors were more probable.[28] Millipedes also exhibit the earliest evidence of chemical defense, as some Devonian fossils have defensive gland openings called ozopores.[16]

Living groups

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Octoglena sierra
Octoglena sierra (Colobognatha, Polyzoniida)
Anadenobolus monilicornis
Anadenobolus monilicornis (Juliformia, Spirobolida)
Harpaphe haydeniana
Harpaphe haydeniana (Polydesmida)

The history of scientific millipede classification began with Carl Linnaeus, who in his 10th edition of Systema Naturae, in 1758, named seven species of Julus as "Insecta Aptera" (wingless insects).[29] 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.[10] 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.[9]

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.[30][31] In 1980, the American biologist Richard L. Hoffman published a classification of millipedes which recognized the Penicillata, Pentazonia, and Helminthomorpha,[32] and the first phylogenetic analysis of millipede orders using modern cladistic methods was published in 1984 by Henrik Enghoff of Denmark.