Myriapods are a major group of arthropods defined by their many legs and long, segmented bodies. The subphylum Myriapoda includes four classes: centipedes, millipedes, pauropods, and symphylans. You’ve almost certainly encountered the first two, but all four share a basic body plan of a head followed by a trunk made up of many similar segments, each bearing one or more pairs of legs. There are over 16,000 known species, found on every continent except Antarctica.
The Four Classes of Myriapods
Centipedes (class Chilopoda) are fast, flattened predators. Each body segment carries a single pair of legs that stick out visibly from the sides. They range from house centipedes a few centimeters long to tropical species exceeding 30 centimeters.
Millipedes (class Diplopoda) are rounder, slower, and built for pushing through soil and leaf litter. Their key anatomical difference from centipedes is that each body segment carries two pairs of short legs rather than one. Despite the name, no millipede was confirmed to have more than 750 legs until 2021, when researchers in Western Australia discovered Eumillipes persephone, a pale, thread-like species just under a millimeter wide that holds the record at 1,306 legs spread across 330 body segments.
Pauropods and symphylans are the two lesser-known classes. Both are tiny, translucent, and live entirely in soil. Symphylans (about 160 known species) look like miniature centipedes and have an unusual defense: glands at their rear end that release sticky threads, tangling the mouthparts of any predator that attacks them. Pauropods are even smaller and softer-bodied, rarely exceeding a couple of millimeters. Because both groups are so small and fragile, their fossil record is almost nonexistent, limited to specimens preserved in ancient amber.
How Myriapods Breathe
Like insects, myriapods breathe through a network of tiny tubes called tracheae that deliver oxygen directly to their tissues. Air enters through small openings called spiracles along the body. In most centipedes and millipedes, these spiracles are paired and sit along the sides of the trunk, but house centipedes and their relatives have an unusual arrangement: a single, unpaired spiracle on the top of certain segments. Some myriapods also carry hemocyanin, a copper-based molecule in their blood that helps transport oxygen, a feature more commonly associated with crustaceans and spiders.
This breathing system has a downside. Tracheae lose moisture easily, which is why the vast majority of myriapods live in damp environments: under logs, in soil, beneath leaf litter. They’re rarely found in deserts or other dry habitats.
What Myriapods Eat
The four classes split into two broad feeding strategies. Centipedes are predators. They use a pair of modified front legs called forcipules, which function like venomous fangs, to grab and subdue prey. Small centipedes eat mites, springtails, and insect larvae. Larger tropical species take down lizards, frogs, and even small rodents.
Millipedes, by contrast, are primarily decomposers. They chew through decaying leaves, rotting wood, and other organic matter, breaking it down into smaller pieces that fungi and bacteria can process further. This makes them important recyclers of nutrients in forest soils. Some millipede body types are specialized for different ways of moving through their environment. Juliform millipedes are cylindrical “bulldozers” that push through soil with brute force, while super-elongated species like E. persephone are squeezers and borers that thread their way into deep underground crevices. Other species are adapted to wedge under bark.
Defense Strategies
Centipedes rely on speed and venom for both hunting and self-defense. Millipedes take a different approach. Many species curl into a tight spiral when threatened, protecting their soft underbelly with a hard external skeleton. But their most distinctive defense is chemical. Certain millipede species secrete hydrogen cyanide and benzaldehyde through paired glands on their body segments. These chemicals are potent enough to repel fire ants and other aggressive predators. Some tropical millipedes produce compounds that can stain human skin brown and cause mild irritation.
Centipede Bites and Human Health
Millipedes don’t bite, but centipedes can. Their venom contains roughly 50 identified bioactive compounds that can affect muscles, the heart, and the nervous system. For most people, a centipede bite causes intense, burning pain at the site. The pain typically lasts anywhere from 30 minutes to three days, along with swelling, redness, and sometimes bruising. Some bites bleed heavily.
Serious reactions are uncommon but possible. Allergic reactions including anaphylaxis represent the most immediate danger. Other reported effects include headache, nausea, fever, and chills. Cardiovascular symptoms like palpitations or drops in blood pressure have been documented in rare cases. The larger the centipede, the more venom it delivers, which is why bites from large tropical species tend to be more painful and more likely to cause wider symptoms.
How They Grow
Myriapods grow in a way that sets them apart from insects. Instead of metamorphosing from larva to adult in distinct stages, many myriapods add new body segments each time they molt. This process is called anamorphosis, and it means a young millipede hatches with far fewer legs than it will eventually have, gaining more with each successive molt. Different groups use different variations of this process. Some add segments throughout their entire lives, while others stop adding segments after a certain point but continue to molt.
Certain centipede groups, including the large tropical species in the order Scolopendromorpha, take a different path called epimorphosis. They hatch with their full number of body segments already in place and simply grow larger with each molt. These two developmental strategies, adding segments versus hatching complete, evolved independently in multiple lineages, suggesting that the pressures of life underground and in leaf litter have pushed different groups toward similar solutions.
Ancient Giants
The most spectacular myriapod that ever lived was Arthropleura, an enormous relative of modern millipedes that roamed the swampy forests of the Carboniferous period around 300 million years ago. A fossilized fragment discovered in Northumberland, England allowed scientists at the Natural History Museum to estimate that Arthropleura reached about 2.6 meters long, roughly the length of a small car, and weighed up to 50 kilograms, comparable to a large dog. That makes it the largest land-dwelling arthropod ever known. The high oxygen levels of the Carboniferous atmosphere likely played a role in allowing arthropods to reach such sizes, since their tracheal breathing system becomes less efficient as body size increases.
Today’s myriapods are far more modest. The longest living centipede, Gonibregmatus plurimipes from Fiji, has 191 body rings and 382 legs. The leggiest millipede, Eumillipes persephone, is less than 10 centimeters long despite its record-breaking 1,306 legs. Both are reminders that in the world of myriapods, more segments and more legs don’t necessarily mean a bigger animal.