Millipedes or millepedes (Class Diplopoda, previously also known as Chilognatha) are very elongated arthropods with cylindrical bodies that have two pairs of legs for each one of their 20 to 100 or more body segments (except for the first segment behind the head which does not have any appendages at all, and the few next which only have one pair of legs). Each segment that has two pairs of legs is a result of two single segments fused together as one. These animals are detritivores, slow and nonvenomous; unlike the somewhat similar and closely related centipedes (Class Chilopoda), which can be easily distinguished by their single pair of legs for each body segment. Most millipedes eat decaying leaves and other dead plant matter, moisturizing the food with secretions and then scraping it in with the jaws.

Not quite a thousand legs: The millipede's most obvious feature is its large number of legs. In fact its name is a compound word formed from the Latin roots milli ("thousand") and ped ("foot"). Despite their name, these creatures do not have a thousand legs, although some rare species have up to 750. However, common species have between 80 and 400 legs.

Short legs = slow: Having very many short legs makes millipedes rather slow, but they are powerful burrowers. Waving their body length and with the legs moving in a wavelike pattern, they easily force their way underground, head first. They also seem to have some engineering ability, reinforcing the tunnel by rearranging the particles around it.

The head contains a pair of sensory organs known as the Tömösváry organs. These are found just posterior and lateral to the antennae, and is shaped as small and oval rings at the base of the antennae. They are probably used to measure the humidity in the surroundings, and they may have some chemoreceptory abilities too.

Tight coil defense: Due to their lack of speed, millipedes' primary defense mechanism is to curl into a tight coil—protecting their delicate legs inside an armoured body exterior. Many species also emit a somewhat poisonous liquid secretion or hydrogen cyanide gas through microscopic pores along the sides of their bodies as a secondary defense. Some of these substances are acidic and can burn the exoskeleton of ants and other insect predators, and the skin and eyes of larger predators. As far as humans are concerned, this chemical brew is fairly harmless, although it should never be eaten or applied to the eyes. Because of this, caution should be used when handling millipedes. Lemurs have been known to intentionally irritate millipedes in order to rub the chemicals on themselves to repel insect pests, and possibly to produce a psychoactive effect. Some millipede species may be amphibious.

Millipedes stink when irritated - or crushed! Millipedes, especially if irritated or crushed, give off an offensive odor, often to the annoyance of homeowners. To rid millipedes from an indoor environment with minimal spread of odor, it is best to vacuum them and soon discard the bag so that the vacuum does not retain the smell of the foul odor.

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