Millipedes are a class of arthropods distinguished by the presence of two pairs of jointed legs on most body segments; the name Diplopoda is derived from this trait. Each double-legged segment is made up of two single segments that have been fused together. The majority of millipedes are slow-moving detritivores that feed on decaying leaves and other dead plant materials. Some consume fungus or plant juices, while others are predatory. Millipedes are typically harmless to people, though some can become pests in the home or garden. Millipedes can be a nuisance, especially in greenhouses where they can inflict significant harm to emergent seedlings. Although the small bristle millipedes are covered with tufts of detachable bristles, most millipedes defend themselves with a range of substances produced from pores around the body. Its principal defence strategy is to curl into a tight coil, shielding its legs and other vitally important sensitive portions of the body behind a hard exoskeleton. In most species, reproduction is carried out by gonopods, which are modified male legs that deliver sperm packets to females. Millipede diplosegments have evolved in tandem with their burrowing tendencies, and nearly all millipedes live mostly underground. They burrow using three methods: bulldozing, wedging, and boring. Millipedes have a wide range of body forms and sizes, with as few as eleven to over three hundred segments. They are mostly black or brown in colour, with a few vividly coloured species with aposematic colouring to warn that they are dangerous.