So Many Legs, So Little Time

Two photos: one of long reddish black insects with many orange legs. The other of a bigger black insect with pointed green legs.

Left: millipedes of the order Polydesmida, the most species-rich order of the millipedes, with over 5,000 described species and many more to discover. Right: Scolopendra centipede. Both found in Vietnam.

What’s the difference between a centipede and a millipede? It’s more than just the number of legs in their strides.

What are arthropods?

Centipedes and millipedes are one of the five distinct groups of the arthropods, invertebrate animals with exoskeletons (sometimes just called creepy-crawlies). Members of this group have elongated bodies and many legs—not quite a thousand, but certainly many. Two groups of arthropods have fewer legs: spiders and their kin (the arachnids) run fast on eight legs, and the insects do fine with six legs (and many with two pairs of wings). The extinct trilobites had a variable number of legs, and so do crustaceans. Crustaceans, which live mainly in the oceans, are the only arthropods that are regularly eaten by humans.

While millipedes and centipedes are often confused, the two groups have some key differences:

Millipedes

Millipedes are mostly harmless herbivorous mulch munchers. With two leg pairs per body ring, they push and shove through the leaf litter, chewing on rotting leaves. The leggiest millipede features 375 leg pairs (that’s 750 legs!). Having so many limbs requires a lot of coordination (we stumble sometimes with just two feet).

Centipedes

On the other hand, centipedes have fewer legs: only one pair for each body ring. The other major difference is that centipedes are predators. Their first leg pair is modified for use as venomous claws, which they use to stab their prey. Only the largest of the centipedes, the formidable Scolopendra, can bite us humans. All others are harmless to us, but not to their insect prey. Everyone has to make living, right?