Let's start with the basics: Millipedes are not worms, nor are they insects. Insects have six legs, and obviously millipedes have many more. Exactly how many more?
If you think the answer is 994, you'd be wrong, my friend.
It turns out scientists did not choose the Latin prefix in their name, "milli-," meaning 1,000, as a way to convey the precise number of legs these organisms have. Their colloquial nickname "thousand leggers" is not accurate either.
"I guess the people who were seeing these things and coming up with common names said, 'Man that's a lot of legs,'" says Derek Hennen, a Ph.D. candidate in entomology at Virginia Tech who studies millipedes. "It's a bit of confusing, and it's the same with centipedes. It's just getting at the fact that they are very leggy."
Millipedes fall within a phylum of organisms called arthropods, a category that includes invertebrates as diverse as spiders, scorpions and butterflies. All arthropods have in common a hard exoskeleton, segmented bodies and jointed legs. What distinguishes millipedes is that for each segment of their body, they have two pairs of legs, instead of one pair as you would see with, say, centipedes. This gave way to the more accurate name used by scientists: diplopoda, meaning "double foot."
Having so many legs is part of what makes millipedes so adaptive. Fossil evidence suggests that millipedes were among the first-ever terrestrial animals, which means they have been around for about 400 million years.
Back in the coniferous period, about 300 million years ago, a millipede species known as Arthropleura grew to an enormous 6.6 feet (2 meters) long and 1.6 feet (0.5 meter) wide. Today, millipedes range in size from 0.1 inch (3 millimeters) to about 11 inches (0.3 of a meter). So far, scientists have discovered about 12,000 species on every continent except Antarctica, but estimate there could be as many as 80,000 species.
"Each state has some millipedes that occur only there, and nowhere else," says Hennen who loves them so much he runs a Twitter account (@DearMillipede) on all things millipede.
Why Millipedes Have So Many Legs
Millipedes feast on leaf litter that, although not rich in nutrients, is plentiful around the world. This means millipedes spend most of their time in soil, under leaves and rocks. And what makes them so effective at getting around in this dense habitat? Their many mighty legs, of course.
"The collum [first segment] acts like a bulldozer, and having so many legs gives it power to push and burrow into the dirt," Hennen says.
When millipedes hatch, they only have a few pairs of legs. Then, just like many other creatures such as crabs and spiders, they grow through a process called molting. This process involves shedding their exoskeleton and growing a new one. Each time they do this, millipedes also grow a new segment and therefore two new pairs of legs. Some millipedes stop molting when they reach adulthood, and others molt their whole lives – which on average is about two years.
So, how many legs do millipedes have? It depends on the group, and Hennen says it ranges between 24 and 750, at most. In fact, most millipede species have under 100 legs.
Differences Between Millipedes and Centipedes
Millipedes have some defensive mechanisms, but they don't bite or sting. (They have very poor eyesight — some species have no eyes at all — and mostly just use antennae to find their way.) A millipede's best move when it feels threatened is to curl up in a ball, and secrete chemicals that ward off predators. The chemicals they secrete vary, but are released in such tiny quantities that they are generally not hazardous to humans. Some research describes how in certain tropical regions, monkeys actually seek out millipedes to use the chemicals they secrete as mosquito repellent.
Centipedes, by contrast, can bite using small fangs that secrete venom. Even then, while a centipede bite can be painful, it usually won't cause other harmful effects.
Leg wise, centipedes' legs tend to spread out, while millipedes' legs point downward. Centipedes only have one pair of legs per segment, while millipedes have two pairs. If you're not keen on getting close enough to check, Hennen says observe the creature's behavior. If it runs away quickly, it's likely a centipede. If it just curls up, it's probably a millipede.