Telling the difference between a centipede and a millipede should be a pretty straightforward process: Just get one to stand still, and count the legs. A hundred is a centipede; a thousand is a millipede. Right?
If only it were so easy. Unfortunately the names “centipede” and “millipede” aren’t to be taken so literally as meaning “hundred” or “thousand” legs; both are informal names that are just meant to highlight that the creatures have a lot of legs.
But even if counting the legs won’t do it, there are several other things to look for to distinguish between a centipede and a millipede:
1. Both of these arthropods have segmented bodies. If you look closely, the centipede has one pair of legs per segment. The millipede has two pairs of legs per segment.
2. The legs of a centipede extend noticeably to the side. The legs of a millipede are more directly underneath its body.
3. Look at the back end of the creature. If it has legs trailing behind it coming from its last segment, it’s a centipede. If the legs emanating from its last segment are no different from those of any other segment and don’t trail behind it, it’s a millipede.
4. Centipedes have long antennae that extend far in front of them; millipedes have much shorter antennae.
5. Centipedes can scurry along pretty quickly when they need to. Millipedes are slow, methodical walkers.
6. You probably won’t get a chance to watch their eating habits, but the centipede is primarily a predator that uses venomous bites on its prey, mostly other arthropods. (Centipedes aren’t the most pleasant things in the world to have in your house, but when you consider that they eat things like cockroaches, you may feel less urgency about getting rid of them.) Millipedes are primarily scavengers that eat decomposing plant material.
7. Some of the larger centipedes can give you a pretty nasty bite. If you pick up one of these creatures to look more closely for one of these signs as to whether it’s a centipede or a millipede, and then you drop it in pain when it responds by chomping your finger, you needn’t inquire any further: it’s a centipede.
Neither centipedes nor millipedes are insects by the way (too many legs, for one thing-insects have only six), though they’re all arthropods, as are arachnids (e.g., spiders), and crustaceans (e.g., crabs). Nor are they snakes or worms, which are legless and move via a very different kind of locomotion. For that matter, they aren’t even as closely related to each other as you might guess, being of different classes within the arthropods.
Finally, one might ask, if centipedes don’t have a hundred legs, and millipedes don’t have a thousand, then how many do they have?
Unfortunately there’s no simple answer to that. Unlike most species of animal, neither has a set number of legs. Centipedes and millipedes shed their skin regularly throughout their adult life, and every time they do, they add another segment, and with each segment comes more legs-one pair for the centipede and two pairs for the millipede. Both centipedes and millipedes can have anywhere from about 30 legs to several hundred legs, depending on how long they’ve survived and how many sheddings they’ve been through.
Debbie Hadley, “How to Tell the Difference Between a Centipede and a Millipede: Comparing Chilopoda and Diplopoda.” About.com.
Laura Jesse, “Centipedes and Millipedes.” Iowa State University, University Extension, Horticulture & Home Pest News.
Renée Lizotte, “Centipedes & Millipedes.” Arizona-Sonora Desert Museum.
“Millipedes, Centipedes & Pill Bugs.” Backyard Nature.