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, 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.

Xystodesmidae contains many colorful and distinctive species, including Apheloria virginiensis of the eastern U.S. and Harpaphe haydeniana of the western U.S. The Sierra luminous millipedes of the genus Motyxia exhibit the only known examples of bioluminescence in the Polydesmida. Species of Apheloria and Brachoria in the Appalachians exhibit Müllerian mimicry, in which unrelated species resemble one another where they co-occur.

This family also includes the cave-dwelling genus Devillea, notable for having more than the 20 segments (counting the collum as the first segment and the telson as the last) usually found the Polydesmida. For example, in the species D. tuberculata, adult females have 22 segments and adult males have 21, 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. The species D. doderoi has the maximum number of segments recorded in this family (29, including the telson).

The family Xystodesmidae was named by O. F. Cook in 1895, upon the description of Xystodesmus martensii, (previously Polydesmus martensii). Xytodesmidae is placed in the suborder Chelodesmidea within the order Polydesmida (the "flat-backed" or "keeled millipedes"). Xystodemids are characterized by a relatively broad and compact body shape and one or more spines on the second leg-segments (prefemoral spines) in most species.