Flat-backed Millipede

The flat-backed millipede gets its name from the lateral extensions, or paranota, that project from each body segment, flattening the animal's cross-section and giving it a ridged, armored look very different from the cylindrical shape of most other millipedes. Members of the order Polydesmida, these millipedes are among the most frequently encountered in gardens, compost heaps, and forest leaf litter across temperate regions.

Like all millipedes, they are detritivores that play an important role in breaking down dead plant material and returning nutrients to the soil. They lack eyes in many species and rely on antennae and chemical cues to navigate the dark, humid microhabitats they prefer. Their slow, deliberate crawl and tendency to curl into a tight spiral when disturbed are hallmark behaviors of the group.

• Body is noticeably flattened rather than round in cross-section, with keel-like plates (paranota) extending outward from each segment
• Coloration ranges from dull brown to grayish-tan, sometimes with darker mottling
• Typically 20 or fewer body segments, each bearing two pairs of short legs
• Antennae are short and clubbed at the tip
• Curls into a tight flat spiral (not a ball) when threatened, unlike pill millipedes
• Lacks the smooth, glossy, cylindrical body of common garden millipedes like Julidae

Found under logs, stones, bark, and leaf litter in gardens, woodlands, and compost piles throughout much of North America and Europe. They favor consistently moist, shaded ground and are most active after rain or during humid nights, retreating deeper into soil or debris during dry spells.

Flat-backed millipedes feed on decaying leaves, wood, and other plant debris, occasionally supplementing their diet with fungi or very soft, decomposing organic matter. They are almost entirely nocturnal, spending daylight hours hidden in cool, damp refuges. As decomposers, they contribute to nutrient cycling and soil formation, and they serve as a food source for ground beetles, birds, shrews, and other predators. When disturbed they may coil defensively or release a mild secretion from segmental glands as a deterrent.

Females lay small clutches of eggs in moist soil or decaying wood, sometimes surrounding them with a protective chamber of chewed debris. Hatchlings emerge with only a few segments and legs, gaining additional segments and leg pairs with each successive molt in a process called anamorphosis. It can take one to several years and numerous molts to reach full adult segment count, and many species live two to three years, overwintering as juveniles or adults in sheltered soil layers.