Polydesmida

Polydesmida is an order of millipedes within the class Diplopoda. They are commonly called flat-backed millipedes due to their laterally expanded body segments (paranota) that create a flattened profile. With thousands of described species, Polydesmida is the largest order of millipedes, distributed worldwide in a variety of habitats.

Physiology and morphology: The body consists of a series of trunk segments; each segment bears two pairs of legs (a characteristic of diplopods), though legs can be reduced in the terminal segments. The paranota on most segments give a plate-like appearance. Most species lack eyes or have only simple ocelli; coloration and patterns vary. Defense: they possess repugnatorial glands (ozopores) that secrete benzoquinones or related chemicals when threatened.

Ecology and habitat: Polydesmids inhabit leaf litter, decaying wood, soil, and the understory of forests, but can also be found in grasslands or caves. They are detritivores and fungivores, contributing to decomposition and nutrient cycling in their ecosystems. They are generally slow-moving and prefer moist microhabitats, often being active at night or after rain.

Reproduction and development: Like other millipedes, Polydesmida reproduce sexually; males deposit spermatophores that are transferred to females. They undergo gradual metamorphosis; juveniles hatch with fewer segments and legs and gain more with subsequent molts, approaching the adult form over multiple molts.

Systematics and diversity: The order comprises several suborders and numerous families; the taxonomy is complex and subject to revision with new data. Polydesmida plays a key role in forest floor communities and is a common subject in studies of soil biodiversity and detritivore ecology.