Polydesmida

Polydesmida (from the Greek poly 'many' and desmos 'bond') is the largest order of millipedes, containing approximately 3,500 species, including all the millipedes reported to produce hydrogen cyanide (HCN). 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 3 to 130 mm (0.12 to 5.12 in). Including the telson, adults have 19 or 20 rings, while juveniles may have from 7 to 19 rings. Mature males have a single pair of gonopods consisting of the modified forward leg pair of the 7th segment. Many of the larger species show bright coloration patterns which warn predators of their toxic secretions. Polydesmids are very common in leaf litter, where they burrow by levering with the anterior end of the body. The c. 3500 species of Polydesmida are variously classified into four suborders (names ending in '-idea'), and 29 families, the largest (numerically) including Paradoxosomatidae, Xystodesmidae, and Chelodesmidae.