Polydesmida (from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

''poly'' "many" and ''desmos'' "bond") is the largest

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

millipede Millipedes 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 resu ...

s, containing approximately 3,500 species, including all the millipedes reported to produce

hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on an ...

(HCN).

Description

Members of the order Polydesmida are also known as "flat-backed millipedes", because on most species, each body segment has wide lateral keels known as paranota. These keels are produced by the posterior half (metazonite) of each body ring behind the collum. Polydesmids have no eyes, and vary in length from . Many of the larger species show bright coloration patterns which warn predators of their toxic secretions. Adults usually have 20 segments, counting the collum as the first ring and the

telson The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on accou ...

as the last ring. Juveniles have from 7 to 19 rings. In species with the usual 20 segments, adult females have 31 pairs of legs, but in adult males, the eighth leg pair (the first leg pair of the 7th ring) is modified into a single pair of

gonopods Gonopods are specialized appendages of various arthropods used in reproduction or egg-laying. In males, they facilitate the transfer of sperm from male to female during mating, and thus are a type of intromittent organ. In crustaceans and milli ...

, leaving only 30 pairs of walking legs.

Variation

Many species deviate from the typical body plan. One striking and unique deviation occurs in adult males of the species ''Aenigmopus alatus'', which retain 31 pairs of walking legs and feature no gonopods. This millipede is the only species in the infraclass

Helminthomorpha Chilognatha is a subclass of the class Diplopoda, which includes the vast majority of extant millipedes, about 12,000 species. Taxonomy The classification of Chilognatha presented below is based on Shear, 2011, and Shear & Edgecombe, 2010 (extin ...

without gonopods. The most common deviation, however, is a reduction in the number of segments. Many species have only 19 segments (including the telson) as adults, including those in the genera ''

Brachydesmus ''Brachydesmus'' is a genus of millipedes belonging to the family Polydesmidae. Millipedes in this genus are found mainly in Europe. Species in this genus have 19 segments as adults (counting the collum, the telson, and the rings in between). Adu ...

'', ''Macrosternodesmus'', ''Poratia'', and ''Bacillidesmus''. In these species, adult females have only 29 pairs of legs, and adult males have only 28 pairs of walking legs. In a few species, including ''Hexadesmus lateridens'', ''Agenodesmus reticulatus'', and ''Eutynellus flavior'', adults have only 18 segments (including the telson), with a corresponding reduction in the number of leg pairs (27 in the adult female, 26 in the adult male, excluding the gonopods). Still other species exhibit

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

in segment number, for example, ''Prosopodesmus panporus'' (the usual 20 in adult females, but only 19 in adult males) and ''Doratodesmus pholeter'' (19 in adult females; 18 in adult males), with the expected number of leg pairs given the number of segments in each sex. Flat-backed Millipede (Polydesmida)

A few species deviate by having more than the usual number of segments, including those in the genus ''Devillea''. For example, in the species ''D. tuberculata'', adult females have 22 segments and adult males have 21 (including the telson), with a corresponding increase in the number of leg pairs (35 in adult females and 32 in adult males, excluding the gonopods). Some species in this genus also exhibit variation in segment number within the same sex, for example, in ''D. subterranea'', adult males can have as few as 19 segments or as many as 23 (including the telson). The most extreme outlier in segment number among polydesmids, however, is a cave-dwelling species discovered in Brazil, ''Dobrodesmus mirabilis'', with adult males found to have 40 segments (including the telson).

Ecology

Polydesmids are very common in

leaf litter Plant litter (also leaf litter, tree litter, soil litter, litterfall or duff) is dead plant material (such as leaves, bark, needles, twigs, and cladodes) that have fallen to the ground. This detritus or dead organic material and its constituent ...

, where they burrow by levering with the anterior end of the body. Some are preyed on by funnel-web spiders.

Classification

The c. 3500 species of Polydesmida are variously classified into four

suborders Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...

(names ending in "-idea"), and 29

families Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideall ...

, the largest (numerically) including

Paradoxosomatidae Paradoxosomatidae, the only family in the suborder Paradoxosomatidea (also known as Strongylosomatidea), is a family of flat-backed millipedes in the order Polydesmida. Containing nearly 200 genera and 975 species , it is one of the largest fami ...

Xystodesmidae Xystodesmidae is a family of millipedes. Its members often have very small distributional areas, with many species only known from a single locality. They are found across the northern hemisphere, with peak diversity in the Appalachian Mountains, ...

, and Chelodesmidae. ; Dalodesmidea Hoffman, 1980. 2 families *

Dalodesmidae Dalodesmidae is a family of millipedes in the order Polydesmida, containing at least 250 species found in the Southern Hemisphere. Distribution Species of Dalodesmidae are found in Australia (over 100 species), Madagascar, New Caledonia, New Ze ...

Cook, 1896 *

Vaalogonopodidae Vaalogonopodidae is a family of millipedes belonging to the order Polydesmida. Genera: * ''Phygoxerotes'' Verhoeff, 1939 * ''Vaalogonopus'' Verhoeff, 1940 References

{{Taxonbar, from=Q6448281 Polydesmida ...

Verhoeff, 1940 ; Leptodesmidea Brölemann, 1916. 13 families: * Chelodesmoidea Cook, 1895 ** Chelodesmidae Cook, 1895 * Platyrhacoidea Pocock, 1895 ** Aphelidesmidae Brölemann, 1916 ** Platyrhacidae Pocock, 1895 * Rhachodesmoidea Carl, 1903 ** Rhachodesmidae Carl, 1903 ** Tridontomidae Loomis & Hoffman, 1962 * Sphaeriodesmoidea Humbert & de Saussure, 1869 ** Campodesmidae Cook, 1896 ** Holistophallidae Silvestri, 1909 ** Sphaeriodesmidae Humbert & de Saussure, 1869 * Xystodesmoidea Cook, 1895 ** Eurymerodesmidae Causey, 1951 ** Euryuridae Pocock, 1909 ** Gomphodesmidae Cook, 1896 ** Oxydesmidae Cook, 1895 **

Cook, 1895 ;

Paradoxosomatidea Paradoxosomatidae, the only family in the suborder Paradoxosomatidea (also known as Strongylosomatidea), is a family (biology), family of flat-backed millipedes in the order Polydesmida. Containing nearly 200 genera and 975 species , it is one o ...

Daday, 1889. 1 family *

Daday, 1889 ;

Polydesmidea Polydesmidea is a suborder of flat-backed millipedes under the order Polydesmida Polydesmida (from the Greek ''poly'' "many" and ''desmos'' "bond") is the largest order of millipedes, containing approximately 3,500 species, including all the ...

Pocock, 1887. 12 families * Oniscodesmoidea Simonsen, 1990 ** Dorsoporidae Loomis, 1958 **

Oniscodesmidae Oniscodesmidae is a family of millipedes belonging to the order Polydesmida Polydesmida (from the Greek ''poly'' "many" and ''desmos'' "bond") is the largest order of millipedes, containing approximately 3,500 species, including all the millip ...

DeSaussure, 1860 * Pyrgodesmoidea Silvestri, 1896 ** Ammodesmidae Cook, 1896 ** Cyrtodesmidae Cook, 1896 **

Pyrgodesmidae Pyrgodesmidae is a family of flat-backed millipedes in the order Polydesmida. The family contains over 200 species distributed in tropics around the world. Some species are found only in ant colonies, and are considered obligate myrmecophiles ...

Silvestri, 1896 * Haplodesmoidea Cook, 1895 ** Haplodesmidae Cook, 1895 * Opisotretoidea Hoffman, 1980 ** Opisotretidae Hoffman, 1980 * Polydesmoidea Leach, 1815 ** Cryptodesmidae Karsch, 1880 **

Polydesmidae Polydesmidae is a family of millipedes in the order Polydesmida. These millipedes range from 4 mm to 30 mm in length. This family includes species notable for featuring sexual dimorphism in segment number: Adult females in the genus ''Perapolydes ...

Leach, 1815 * Trichopolydesmoidea Verhoeff, 1910 ** Fuhrmannodesmidae Brölemann, 1916 ** Macrosternodesmidae Brölemann, 1916 ** Nearctodesmidae Chamberlin & Hoffman, 1958 ** Trichopolydesmidae Verhoeff, 1910

References

External links

* *
External Anatomy of PolydesmidaNorth American Polydesmida - BugGuide
{{Taxonbar, from=Q282015 Millipede orders