Polydesmida is the largest order of millipedes, with more than 5,000 species, including all the millipedes reported to produce hydrogen cyanide (HCN). This order is also the most diverse of the millipede orders in terms of morphology. Millipedes in this order are found in all regions of the world other than Antarctica.
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, where one-third of the 300 or so species occur. They are particularly abundant in deciduous broadleaf forests in the Mediterranean Basin, Africa, Asia, Central and North America, and Russia. Information on basic taxonomy is scant for this family; for example, it is estimated that the genus Nannaria contains over 200 species, but only 25 were described as of 2006. By 2022, 78 species in Nannaria have been described.
Motyxia is a genus of cyanide-producing millipedes that are endemic to the southern Sierra Nevada, Tehachapi, and Santa Monica mountain ranges of California. Motyxias are blind and produce the poison cyanide, like all members of the Polydesmida. All species have the ability to glow brightly: some of the few known instances of bioluminescence in millipedes.
Orthomorpha is a genus of millipedes in the family Paradoxosomatidae containing approximately 50 species distributed in Southeast Asia.
Urochordeuma is a genus of millipedes in the order Chordeumatida and the only genus in the family Urochordeumatidae. Adult millipedes in this family have 30 segments. The two species are known from the U.S. state of Washington, where U. bumpusi is known from Pierce County, and U. porona from King County.
Parafontaria is a genus of "flat-backed" millipedes consisting of 13 species native to Japan, where they are referred to as train millipedes. This is because some species exhibit periodical swarming behavior during which large numbers congregate and can impact train passage when this congregation occurs on tracks. Documentation of this event goes back to 1920. Individuals vary from around 3.5 to 6 cm as adults, and feed on leaf litter as well as soil, making them comparable to earthworms in facilitating decomposition and soil nutrient cycling.
Haplodesmidae is a family of millipedes in the order Polydesmida. Species occur in East Asia, Southeast Asia, and Oceania, although some species have been introduced to the New world tropics. Species are small bodied, often with elaborate sculpturing on the tergites, and some species are capable of rolling into a near-complete ball.
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 Perapolydesmus have the usual 20 segments, but the adult males have only 19.
Ammodesmidae is a family of small millipedes endemic to Africa, containing seven species in two genera. Ammodesmids range from 1.4 to 5.0 mm long with 18 or 19 body segments in both sexes, and are capable of rolling into a tight sphere.
Glomeridesmidae is a millipede family of the order Glomeridesmida. This family includes two genera: The genus Glomeridesmus includes most species in this family; the genus Glomeridesmoides includes one species.
Fuhrmannodesmidae is a family of millipedes belonging to the order Polydesmida. Although some authorities deem Fuhrmannodesmidae to be a junior synonym for Trichopolydesmidae, others still regard the taxonomic status of Fuhrmannodesmidae as valid. The family Fuhrmannodesmidae includes over 50 genera.
Anthroleucosomatidae is a family of millipedes in the order Chordeumatida. These millipedes range from 3.5 mm to 28 mm in length. Adult millipedes in this family have 26, 28, 30, or 31 segments. This family includes Metamastigophorophyllon martensi, notable as the only chordeumatidan species with 31 segments. There are about 38 genera in Anthroleucosomatidae.
Craspedosomatidae is a family of millipedes in the order Chordeumatida. Most adult millipedes in this family have 30 segments, but some have only 28. There are at least 30 genera and 210 described species in Craspedosomatidae.
Metopidiotrichidae is a family of millipedes in the order Chordeumatida. These millipedes range from 4 mm to 17 mm in length. Adult millipedes in this family have 32 segments, not the 30 segments usually found in this order. Adult males in this family often feature a reduced or vestigial leg pair 10 as part of the gonopod complex, in addition to the two leg pairs typically modified into gonopods in this order. There are about 9 genera and at least 70 described species in Metopidiotrichidae.
Brachydesmus is a genus of millipedes belonging to the family Polydesmidae. The Czech zoologist Camill Heller first described this genus to contain the type species B. subterraneus. This genus now includes about 75 described species.
Trichopolydesmidae is a family of millipedes belonging to the order Polydesmida. This family includes two genera notable for featuring sexual dimorphism in segment number: adult females in these genera have the 20 segments usually found in this order, but adult males have only 19. This family also includes the species Deharvengius bedosae, notable for being among the very few species in this order to feature adults with only 18 segments rather than the 20 segments usually found in polydesmids.
Speophilosomatidae is a family of millipedes belonging to the order Chordeumatida. These millipedes are found in Japan and range from 4 mm to 6 mm in length. Adult millipedes in this family are notable for being among the few in this order with only 26 segments instead of the 30 usually found in chordeumatidans. The adult males in this family are also notable for involving three leg pairs in the gonopod complex rather than the two pairs typically modified into gonopods in this order.
Agenodesmus is a genus of millipedes in the family Fuhrmannodesmidae, which some authorities consider a junior synonym of Trichopolydesmidae. This genus is notable for being among the very few genera in the order Polydesmida to feature adults with only 18 segments rather than the 20 segments usually found in this order. The genus Agenodesmus contains only two species, A. reticulatus and A. nullus. The type species A. reticulatus is notable as the first polydesmidan millipede discovered with only 18 segments in adults, the smallest number recorded in the order Polydesmida. Before the discovery of A. reticulatus, polydesmidans were known to have only 19 or 20 segments in adults.
Opisthocheiron is a genus of millipedes in the family Opisthocheiridae. These millipedes are found in France and Spain. The French entomologist Henri Ribaut created this genus in 1913 to contain the newly discovered type species Opisthocheiron penicillatum. This genus also includes the cave-dwelling species Opisthocheiron canayerensis, notable as one of only a few species in the order Chordeumatida with only 26 segments in adults, four fewer segments than typically found in adults in this order. The adult female of this species has only 42 pairs of legs, and the adult male has only 40 pairs of walking legs, excluding two pairs of gonopods.
Opisthocheiron canayerensis is a species of millipede in the family Opisthocheiridae. This species is found in France, far inside deep caves. This millipede is notable as one of only a few species in the order Chordeumatida with only 26 segments in adults, four fewer segments than typically found in adults this order.
