Millipedes are long-bodied arthropods belonging to the class Diplopoda, a name that translates from Greek to “double foot.” They are part of the subphylum Myriapoda, which also includes centipedes. With an estimated 80,000 species worldwide, millipedes are generally found in dark, damp terrestrial environments across the globe.
Anatomy and the Myth of a Thousand Legs
The defining feature of a millipede is its segmented body structure. After the first few segments, the body is composed of “diplosegments,” which result from two embryonic segments fusing together. This fusion means that nearly every visible body ring bears two pairs of jointed legs instead of just one.
The common name “millipede” originates from the Latin words mille (thousand) and pes (foot), leading to the popular but inaccurate nickname “thousand-legger.” For centuries, no known species actually possessed 1,000 legs, with most having fewer than 100 legs. However, the recently discovered species Eumillipes persephone in Australia shattered this distinction, with individuals possessing up to 1,306 legs, making it the leggiest animal on Earth. Their numerous, short legs and cylindrical body shape necessitate a slow, wave-like movement through the soil and leaf litter.
Ecological Role as Decomposers
Millipedes are classified as detritivores, meaning their diet consists primarily of dead and decaying organic matter like leaf litter and other plant debris. They play a significant part in decomposition by physically fragmenting tough organic material. This action increases the surface area for bacteria and fungi to colonize, accelerating the final stages of decay.
This feeding activity aids in the cycling of nutrients back into the soil. Millipede waste products, excreted as tiny pellets, contain valuable nutrients like nitrogen and phosphorus that plants can readily use. Their presence is an indicator of healthy soil structure and moisture, as they require humid environments to prevent desiccation.
Defense Mechanisms and Toxicity
Millipedes are not aggressive and lack the ability to bite or sting, relying on passive defense mechanisms when threatened. Their first defense is their slow speed and tough, armor-like exoskeleton. When disturbed, a millipede typically stops moving and coils its body into a tight spiral to shield its vulnerable underside and delicate legs.
For a more potent defense, many species employ chemical warfare by secreting noxious fluids from microscopic glands along the sides of their bodies. These fluids contain various compounds, including hydrogen cyanide in some species, which can smell faintly of almonds. While this secretion is toxic to small animals and can stain human skin or cause minor irritation, the amount produced is harmless to people and pets. Millipedes are poisonous (harmful if ingested) but not venomous (they do not inject toxins).
Distinguishing Millipedes from Centipedes
The public often confuses millipedes with centipedes, but the two are easily differentiated by several key characteristics. Centipedes have a flattened body profile, allowing them to wedge into crevices, while millipedes are notably more cylindrical or rounded in shape. Millipedes have two pairs of legs per body segment, whereas centipedes have only a single pair of legs per segment.
The difference in locomotion and diet relates directly to their ecological roles. Centipedes are fast-moving predators that use their speed and modified first pair of legs, called forcipules, to inject paralyzing venom into prey such as insects and spiders. Millipedes, by contrast, are slow, non-venomous herbivores and detritivores. Centipedes represent a potential, though usually minor, threat due to their painful venom, while millipedes are generally considered harmless.