The subphylum Myriapoda represents a diverse group of terrestrial arthropods known for having many legs. The name Myriapoda is derived from Greek words meaning “countless feet.” While often grouped with insects and crustaceans, myriapods occupy their own distinct evolutionary branch. The exact number of legs is not fixed, varying dramatically by species, age, and class.
What Defines a Myriapod?
Myriapods are classified within the phylum Arthropoda, which includes all organisms with jointed limbs and an exoskeleton. This subphylum includes four extant classes: Chilopoda (centipedes), Diplopoda (millipedes), Pauropoda, and Symphyla. They are exclusively terrestrial, primarily inhabiting moist environments like soil, leaf litter, and decaying wood.
The myriapod body plan is relatively simple, consisting of two main regions: a distinct head and an elongated trunk. The head typically features a single pair of antennae and, in most species, simple eyes or light-sensitive organs called ocelli. Respiration occurs through a tracheal system, with air entering the body through tiny external openings along the trunk called spiracles.
The trunk is composed of numerous segments, each bearing one or two pairs of jointed legs. This repetitive, segmented structure is a defining trait, distinguishing them from insects (three pairs of legs) and most arachnids (four pairs). This morphology allows them to function as either fast-moving predators or slow-moving decomposers.
The Actual Range of Leg Counts
The common misconception that myriapods possess a fixed number of legs, such as the “thousand” suggested by the name millipede, is far from reality. The adult leg count for the subphylum ranges from as few as 16 legs to over 1,300 legs. This massive variability is directly tied to segmentation and development.
Many myriapods exhibit a post-embryonic growth pattern known as anamorphosis, where new body segments and corresponding leg pairs are added with each molt. Juveniles hatch with fewer segments and legs than the adult form, progressively increasing their count throughout their lifespan. This mechanism explains why smaller, younger individuals have fewer legs than their mature counterparts.
The record-holder is the recently discovered millipede species, Eumillipes persephone, which can possess up to 1,306 legs. Conversely, the smallest myriapods, such as some Pauropoda and Symphyla, have a consistent, small number of legs, establishing the lower end of the count.
Leg Counts Across the Four Major Classes
The four classes of Myriapoda are distinguished by how their legs are arranged on their body segments. The class Diplopoda (millipedes) is characterized by diplosegments—fused pairs of segments that each bear two pairs of legs. Millipedes typically range from 48 to 400 legs, though the record-holding species reaches 1,306 legs.
In contrast, the class Chilopoda (centipedes) has only one pair of legs per segment. Centipedes are known for their speed and predatory lifestyle. Their leg count is almost always an odd number of pairs, ranging from a minimum of 15 pairs (30 legs) up to 191 pairs (382 legs). The Geophilomorpha (soil centipedes) possess the highest counts, while the common house centipede has a fixed 15 pairs.
The two less-known classes are substantially smaller in size and leg count, reflecting their micro-habitat in soil and humus. The Symphyla (garden symphylans or pseudocentipedes) typically have 12 pairs of legs (24 legs total), though a few species have only 11 pairs. They are small, pale, and resemble miniature centipedes but lack venomous fangs.
The class Pauropoda is composed of even smaller myriapods. Most adults have nine pairs of legs (18 legs total). Some species range from eight to eleven pairs of legs, establishing the absolute minimum for adult myriapods. Their small, consistent number of legs and microscopic size allow them to navigate tight spaces within soil particles and leaf mold.