Polydesmida is the largest order of millipedes, containing approximately 3,500 species, including all the millipedes reported to produce hydrogen cyanide (HCN). Polydesmids grow and develop through a series of moults, 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 moulting and the addition of segments and legs stop. This mode of development, known as teloanamorphosis, distinguishes this order from most other orders of millipedes, which usually continue to moult as adults, developing through either euanamorphosis or hemianamorphosis.
Tridontomidae is a small family of millipedes. Its members are endemic to Guatemala. These millipedes range from 22 mm to 28 mm in length and are uniformly grayish in color; their legs and antennae are unusually long and slender. This family includes the remarkable species Aenigmopus alatus, in which adult males feature no gonopods. This millipede is the only species in the infraclass Helminthomorpha without gonopods.
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.
Haplogona is a genus of chordeumatidan millipedes and the only genus in the family Verhoeffiidae. Adult millipedes in this family have 30 segments. These species occur in the southern Alps of Europe, from Genoa, Italy to the Istrian peninsula.
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.
Caseyidae is a family of millipedes in the order Chordeumatida. Adult millipedes in this family have 30 segments. There are about 7 genera and at least 40 described species in Caseyidae.
Cleidogonidae is a family of millipedes in the order Chordeumatida. Adult millipedes in this family have 28, 29, or 30 segments. This family includes the genus Tianella, notable for featuring adult millipedes with 29 segments, a number not found in the adults of any other chordeumatidan species. All Tianella species have adults with 29 segments except for two in which adults have only 28 segments. In the Tianella species with 29 segments, adult females have 48 pairs of legs, as one would expect in adult female chordeumatidans with one segment fewer than the 30 usually found in this order. There are seven genera and at least 140 described species in Cleidogonidae.
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.
Rhiscosomides is a genus of millipedes in the order Chordeumatida with seven described species, and is the only genus in the family Rhiscosomididae. Adult millipedes in this family have 30 segments.
Striariidae is a family of millipedes in the order Chordeumatida. Adult millipedes in this family have 30 segments. There are at least 3 genera and about 13 described species in Striariidae.
Trichopetalidae is a family of millipedes in the order Chordeumatida. Adult millipedes in this family have 28 or 30 segments. There are about 5 genera and at least 30 described species in Trichopetalidae.
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.
Mastigophorophyllidae is a family of millipedes belonging to the order Chordeumatida. Adult millipedes in this family have 30 segments.
Chordeumatidae is a family of millipedes belonging to the order Chordeumatida. These millipedes range from 7 mm to 18 mm in length and are found in Europe. Adult millipedes in this family have either 28 or 30 segments. This family features distinctive sex-linked modifications to the legs in adults: In the adult female, a legless sternite replaces the third pair of legs, and in the adult male, five pairs of legs are modified in the gonopod complex. These modifications are more extensive than those found in other adult males in this order, which often have only two leg pairs modified into gonopods. With the more extensive modifications to the legs in this family, species with the usual 30 segments feature adult females with only 49 leg pairs and adult males with only 45 pairs of walking legs, and species with only 28 segments feature adult females with only 45 leg pairs and adult males with only 41 pairs of walking legs.
Apterouridae is a family of millipedes belonging to the order Chordeumatida. Adult millipedes in this family have 30 segments.
Hoffmaneumatidae is a family of millipedes belonging to the order Chordeumatida. This family includes two genera, one (Hoffmaneuma) found in the Russian Far East and the other (Japanoparvus) found in Japan. Millipedes in this family range from 4 mm to 6 mm in length. Adult millipedes in this family have only 28 segments, not the 30 segments usually found in this order. Adult males in this family feature a reduced leg pair 10 in addition to the two pairs normally modified into gonopods in this order. In the adult male of the species Hoffmaneuma exiguum, for example, the gonopod complex derives from all three leg pairs rather than from just the usual two.
Attemsiidae is a family of millipedes belonging to the order Chordeumatida. Adult millipedes in this family have 30 segments.
Trachygonidae is a family of millipedes belonging to the order Chordeumatida. Adult millipedes in this family have 28 or 30 segments.
Lusitaniosomatidae is a family of millipedes belonging to the order Chordeumatida. Adult millipedes in this family have 30 segments.
Heterolatzeliidae is a family of millipedes belonging to the order Chordeumatida. Adult millipedes in this family have 30 segments.
