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Polydesmus angustus, also known as the flat-backed millipede, is a millipede in the genus Polydesmus.
Flat-backed millipedes resemble centipedes. The bodies of the adults are flat, dark brown, with about twenty segments. They measure 14–25 millimetres (0.55–0.98 in) in length and are about 0.4 mm (0.016 in) wide. The plate segments covering the back are ridged along their lengths. The antennae and legs are longer than in most other millipedes.
The ridged body of this millipede helps it to push its way through the soil.
Mating occurs from late spring through the summer and then again in late summer through mid-fall. Males usually mate only once. The females store the sperm from a single mating and produce several batches of eggs. Young millipedes hatching earlier in the year reach adulthood in one year, while those hatching later require two.