Diplopoda is the main class of millipedes.
Millipedes are arthropods in the class Diplopoda, which is characterized by having two pairs of jointed legs on most body segments. Each double-legged segment is a result of two single segments fused together as one. Most millipedes have very elongated cylindrical bodies with more than 20 segments, while pill millipedes are shorter and can roll into a ball. Although the name "millipede" derives from the Latin for "thousand feet", though no known species has 1,000, the closest record being 750 legs, belonging to Illacme plenipes. There are approximately 12,000 named species classified into sixteen orders and around 140 families, making Diplopoda the largest class of myriapods, an arthropod group which also includes centipedes and other multi-legged creatures.
Most millipedes are slow-moving detritivores, eating decaying leaves and other dead plant matter. Some eat fungi or suck plant fluids, and a small minority are predatory. Millipedes are generally harmless to humans, although some can become household or garden pests, especially in greenhouses where they can cause severe damage to emergent seedlings.
Most millipedes defend themselves with a variety of defensive chemicals secreted from pores along the body, although the tiny bristle millipedes are covered with tufts of detachable bristles.
Reproduction in most species is carried out by modified male legs called gonopods, which transfer packets of sperm to females. Millipedes are some of the oldest known land animals, first appearing in the Silurian period. Some members of prehistoric groups grew to over 2 m (6 ft 7 in), while the largest modern species reach maximum lengths of 27 to 38 cm (11 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 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.
The scientific name "Diplopoda" comes from the Greek words διπλοῦς (diplous), "double" and ποδός (podos), "foot", referring to the appearance of two legs on most segments, as described below. The common name "millipede" is a compound word formed from the Latin roots mille ("thousand") and ped ("foot"). 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".
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 earliest known land creature, Pneumodesmus newmani, was a 1 cm (0.4 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 (340 to 280 million years ago), Arthropleura became the largest known land-dwelling invertebrate of all time, reaching lengths of at least 2 m (6 ft 7 in). Millipedes also include the earliest evidence of chemical defence, 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. 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 favored genetic isolation and rapid speciation, producing many lineages with restricted ranges.
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