The millipede body is separated into two main sections; an anterior head region and a long segmented trunk. Most of the trunk segments carry two pairs of legs (hence the name 'diplo'- two 'poda' - feet). This is because the apparent trunk ringed segments are actually 'diplosegments' formed by the fusion of two trunk segments. The first ring after the head (collum) is legless and the following three rings carry one leg pair each. The pre-anal ring is always legless and is situated at the proximal end of the body. Anal valves open for defecation. Most juvenile species have several legless rings at the end of their bodies.
Within each diplosegment, most millipedes have a pair of complex glands that secrete a volatile poisonous liquid (allomone) through openings (ozopores) alongside the trunk. These chemicals include quinazolinones, benzoquinones, aliphatic compounds, terpenoids, and Polydesmida secrete cyanide. Each allomone is chemically characteristic for the order or family. The chemistry of such secretions has recently been the focus of considerable interest due to the diversity of the compounds involved and their success in protecting millipedes from attack.
Millipedes have two sets of mouthparts; the mandibles (used for chewing) and the gnathochilarium (plate behind the mandibles). Antennae are present on all species and are made up of 6 or 7 visible articulates, the top segment containing sensory cones. Behind the antennae sockets lies the sensory Tömösváry organ which is either horseshoe or ringed shaped.
Like almost all arthropods, millipedes have a hard calcified exoskeleton which they have to moult in order to grow. The process of moulting is termed ecdysis.
The 'eyes' at the side of many millipedes head are made up of few to many individual ocelli which are grouped together in an ocular field. Many cave dwelling millipedes such as the Polydesmida have secondarily lost their ocelli.
(figure used with permission from Jeff Shultz)
Millipede sexes are separate, the third trunk segment bears the gonopores which produce the eggs or sperm. Adult male millipedes have distinct and extremely complicated genitalia which show great variation between species. Genitalia morphology is an extremely useful tool in for discriminating between different millipede taxa. In some species e.g. Glomeris marginata the male emits pheromones to attract the female. These are only effective over short distances. Millipede courtship involves the male walking along the females back, stimulating her with the rhythmic pulses of his legs. If the female agrees to copulate with the male, she will raise her anterior segments and allow the male to entwine himself around her. The male uses gonopods (modified legs of the seventh segment) to transfer sperm to the female via a spermatophore package. Fertilisation occurs internally. Spermatophores show great variation between species, believed to reduce hybridisation which would be a waste of valuable energy resourses blah. Parthenogenetic reproduction is known for a few species, such as Polyxenus lagurus and Proteroiulus fuscus. Females may lay up to 2,000 eggs depending upon her size and condition. Eggs are often laid in an underground nest, produced by the females by excreting soil they have eaten and using their anal folds to shape it into a structure. Female longevity varies between species, with some females dying after laying and others surviving many breeding seasons.
The offspring usually hatch with three pairs of legs and grow by adding segments onto the most posterior segment in a process known as anamorphosis.
(figure used with permission from David Banks)