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Polydesmida (from the Greek language poly "many" and desmos "bond") is the largest order of , 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.
Adults usually have 20 segments, counting the collum as the first ring and the telson 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, leaving only 30 pairs of walking legs.
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. 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 Agenodesmus reticulatus, Deharvengius, and Doratodesmus hispidus, 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 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. Even more unusual are two species, Ammodesmidae and Ammodesmidae, in which adults in each sex can have 18 or 19 segments. A few species deviate by having more than the usual number of segments, including those in the cave-dwelling 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).
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.
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