What's the Difference Between a Centipede and a Millipede?

No doubt about it, centipedes and millipedes are creepily similar with their elongated, wormlike bodies and too many legs to count. In fact, for a lot of us, their names are nearly interchangeable. But these multi-legged creepy-crawlies are more different than you might think.

Knowing what separates them is a fascinating study in Mother Nature's diversity. But it can also help you decide whether to let them stay in your garden and home (both are vital contributors to ecosystem health) or whether to send them packing. Here's how to do a proper ID.

Shape and Size

Centipedes and millipedes aren't insects, but they're both part of the same group called "arthropods," meaning they have multiple body segments and jointed legs, as outlined by the University of California Statewide Integrated Pest Management Program.

Centipedes have brown, flattened bodies divided into numerous segments. They're usually at least an inch or two long, and often much longer. One heart-stopping species, the Amazonian giant centipede (pictured), regularly grows up to a foot or more in length, according to the Metropolitan Oceanic Institute & Aquarium. They tend to move quickly.

Millipedes, on the other hand, have multi-segmented cylindrical or slightly flattened brownish bodies, making them look more cylindrical and worm-like. Most species range from a half-inch to a few inches long. They move slowly and like to burrow.

Legginess

Sometimes called "hundred-leggers," centipedes sport two legs per body segment, but few have exactly 100 legs. Most range anywhere from 30 to about 350. Their legs are attached to the side of their bodies and are typically longer and more visible than millipedes' legs. House centipedes, which you may spy at home, are especially quick runners, faster than garden centipedes.

By contrast, millipedes have four tiny bristle-like legs on most body segments. They're attached underneath and undulate in a wave-like manner when they move, making millipedes slower than centipedes. Likewise, their nickname, "thousand-leggers," is a misnomer since most millipede species actually average fewer than 100 legs. Of course, there are a few stomach-churners, like the giant African millipede, which sports over 250 legs (and a body that can reach 15 inches). When disturbed, a millipede will curl up into a little ball, whereas a centipede does not.

Digs

Both like dark, moist environments, mainly outdoors. In nature, centipedes are found across the planet—everywhere from forests and savannas to deserts and caves. Most prefer hiding by day in damp places including under stones, logs, and leaf litter. Millipedes also make their home around the globe, and seek out damp, dark spots—typically burrowed in the soil or under plant debris on forest floors.

Meal Plan

Centipedes are nocturnal carnivores that prey on insects by injecting paralyzing venom from their fangs. Some of the heftier ones, like the eight-inch giant redheaded centipede, prefer heartier meals such as toads, lizards, rodents, and snakes.

Millipedes, on the other hand, are mostly detritivores; that is, they munch on rotting leaves, wood, and other moist, decaying vegetation. In fact, these scavengers function as important plant decomposers in nature, recycling nutrients back into the soil like earthworms. They may eat roots and leaves of small seedlings. They are "ecologically esteemed as agents of microbial decomposition and soil nutrient cycles."

Playing Defense

Of the two, it is centipedes that should give you more pause. Most are shy and beat a super-speedy retreat into dark cracks or small hidey-holes when provoked. But many can bite if handled. Mega species, in particular (like the red-headed centipede pictured above), can inflict some major pain.

Millipedes are generally pretty harmless to humans. Because they're slow moving, most defend themselves by curling into a tight ball. They don't bite or carry venom. However, many species give off a stinky secretion when bothered. In some, this substance can irritate, burn, or discolor skin temporarily.

Both have poor vision and rely on senses like vibration to detect what's happening in their surroundings.

In Your Home

House centipedes are the only species that can live and reproduce indoors. They typically turn up in damp places like basements, garages, and bathrooms, especially in spring and fall. Despite their abnormally long, hair-strand-like legs, these small invaders are generally harmless, do not eat household plants, and in fact can be helpful in keeping down annoying populations of flies, silverfish, cockroaches, and other indoor pests. Most centipedes are too fast to catch and release outside. So if you're creeped out at the thought of sharing your home and you're not into toxic pesticides, keep rooms aired out or dry, deny them a food source by getting rid of other pests, and seal up cracks and openings so they can't get in.

Millipedes also occasionally venture into homes. Most common are small greenhouse, or garden, millipedes that may pay a visit during mass migrations after heavy spring rains. Like centipedes, they're harmless and usually seek out moist spaces on lower floors (though they can occasionally take a liking to potted plants). Many don't live long inside if conditions aren’t moist enough and there’s not enough forest-style plant foods. Often you can sweep them up and release them outside since they're not as fast-moving. As with centipedes, keep things dry and seal up your house.

In the Garden

As predators, centipedes can be beneficial garden buddies by keeping down unwanted invaders that harm plants. If you find too many in your garden or yard, remove their hiding places such as moist mulch, leaf litter, and other organic matter.

Millipedes can also be helpful in your garden as nutrient recyclers. They munch on rotting vegetation to help it break down. However, if their population explodes due to mass migrations, overmulching, or overwatering, they may begin feeding on garden plants. Discourage them by removing mulch and other organic matter and cutting back on water.