Polydesmida is the largest order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 of millipede

Millipede

Millipedes are arthropods that have two pairs of legs per segment . Each segment that has two pairs of legs is a result of two single segments fused together as one...

s, with more than 2,700 species in more than 170 genera, including all the millipedes reported to produce hydrogen cyanide (HCN).

Description

Members of the order Polydesmida are known as "flat-backed millipedes", because each body segment has wide lateral keels. These keels are produced by the posterior half (metazonite) of each body ring behind the collum

Collum

Collum is a surname, and may refer to:*Hugh Collum , British businessman*Jackie Collum , American Major League Baseball pitcher*Jason Paul Collum , American film maker*John Collum , American child actor...

. Polydemids have no eyes, and vary in length from 3 millimetre. Including the telson

Telson

The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...

, adults have 19 or 20 rings, while juveniles

Juvenile (organism)

A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...

 may have from 7 to 19 rings.

Ecology

Polydesmids are very common in leaf litter, where they burrow by levering with the anterior end of the body.