Millipedes are arthropods that have two pairs of legs per segment.
Harpaphe haydeniana reach a length of 4–5 centimetres [1.6–2.0 in] when mature, and may live for 2–3 years. The body is black, and is distinctively marked along the sides with patches of a yellowish colour. Harpaphe haydeniana has approximately twenty body segments, bearing a total of 30 [males] or 31 [females] pairs of legs. The difference between males and females is due to one pair of legs on the seventh segment being modified in the male to form gonopods used for sperm transfer. Harpaphe haydeniana is an important part of the forest ecosystem, breaking down leaf litter, and freeing up its nutrients for other organisms. Immature millipedes feed on humus. Harpaphe haydeniana has few predators, due to its aposematic coloration and its ability to secrete hydrogen cyanide when threatened. This behavior gives rise to the common names “cyanide millipede” and “almond-scented millipede”.