Polydesmida

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 1.4 to 134 mm. Adults usually have 20 segments, Juveniles have from 7 to 19 rings. This millipede is the only species in the infraclass Helminthomorpha 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, Macrosternodesmus, and Bacillidesmus. 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). == Development ==

Polydesmids grow and develop through a series of molts, adding segments until they reach a fixed number in the adult stage, which is usually the same for a given sex in a given species, at which point the molting and the addition of segments and legs stop. Millipedes in this order typically develop through a series of eight stages, hatching with only 7 segments (including the telson) and 3 pairs of legs, then molting seven times. These eight stages feature 7, 9, 12, 15, 17, 18, 19, then finally 20 segments. The adults then reproduce and die without another molt. Species with fewer than the usual 20 segments as adults go through the same stages observed in other polydesmids until reaching maturity, which occurs one molt earlier for 19 segments or two molts earlier for 18 segments. Species with more than the usual 20 segments are thought to add these extra segments through additional molts, adding one segment per molt. ==Ecology==

Polydesmids are very common in leaf litter, where they burrow by levering with the anterior end of the body. == Classification ==

The species of Polydesmida are variously classified into four suborders (names ending in "-idea"), and 29 families, the largest (numerically) including Paradoxosomatidae, Xystodesmidae, and Chelodesmidae. ;Dalodesmidea Hoffman, 1980. 2 families • Dalodesmidae Cook, 1896 • Vaalogonopodidae 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 • Xystodesmidae Cook, 1895 ;Paradoxosomatidea Daday, 1889. 1 family • Paradoxosomatidae Daday, 1889 ;Polydesmidea Pocock, 1887. 12 families • Oniscodesmoidea Simonsen, 1990 • Dorsoporidae Loomis, 1958 • Oniscodesmidae DeSaussure, 1860 • Pyrgodesmoidea Silvestri, 1896 • Ammodesmidae Cook, 1896 • Cyrtodesmidae Cook, 1896 • Pyrgodesmidae Silvestri, 1896 • Haplodesmoidea Cook, 1895 • Haplodesmidae Cook, 1895 • Opisotretoidea Hoffman, 1980 • Opisotretidae Hoffman, 1980 • Polydesmoidea Leach, 1815 • Cryptodesmidae Karsch, 1880 • Polydesmidae Leach, 1815 • Trichopolydesmoidea Verhoeff, 1910 • Fuhrmannodesmidae Brölemann, 1916 • Macrosternodesmidae Brölemann, 1916 • Nearctodesmidae Chamberlin & Hoffman, 1958 • Trichopolydesmidae Verhoeff, 1910 ==References==