Millipedes are common terrestrial arthropods often found in damp environments, playing a significant role in breaking down decaying plant matter. They belong to the class Diplopoda, a name that hints at their defining physical trait. This guide describes the millipede’s body plan, leg arrangement, and the key features that set it apart from similar organisms.
Basic Body Structure and Size
The millipede’s body is elongated and segmented, typically cylindrical or slightly flattened, resembling a hard-shelled worm. Their outer covering is a tough, heavily calcified exoskeleton, often dark brown or black, which protects them and helps them push through soil and leaf litter. Millipedes vary widely in size, with some species reaching over 13 inches (33.5 cm). The distinct head capsule bears a pair of short, elbowed antennae used to probe the environment. Although some species are blind, many possess simple eyes, or ocelli, clustered on the sides of the head.
The Real Story Behind the Legs
The common name “millipede” literally means “thousand feet,” which is a significant misconception for most species, though one recently discovered species was found to have over 1,300 legs. The defining anatomical feature is the presence of “diplosegments,” formed by the fusion of two embryonic body segments into a single, double-legged ring. The majority of the trunk segments each bear two pairs of legs, resulting in four legs per visible body ring. This clustered leg arrangement is why they appear to have so many legs, typically ranging from 34 to around 400 legs in common species. These numerous, short legs are positioned directly beneath the body, creating a slow, wave-like motion that allows the millipede to push through debris and soil.
Key Differences from Centipedes
Distinguishing a millipede from a centipede relies on visual and behavioral traits. Millipedes have two pairs of legs per segment positioned underneath their rounded, cylindrical body. Centipedes have only one pair of legs per segment that stick out to the side, giving them a flatter appearance. Millipedes move slowly and deliberately, while centipedes are fast, agile predators. When threatened, the millipede curls into a tight spiral, protecting its soft underside, and some species secrete a foul-smelling fluid. The centipede’s defense is aggressive, using modified front legs called forcipules to deliver venomous bites or stings.