Somewhere in southern Australia, a train once slid to a stop because the tracks were covered in millipedes. Not leaves. Not ice. Millipedes — thousands of them, squashed flat under the wheels, turning steel rails into a skating rink. The animal responsible is the Portuguese millipede, Ommatoiulus moreleti, a creature barely longer than your thumb that has, through sheer force of numbers, derailed commuter schedules, stained a continent’s carpets, and earned itself a permanent spot in the pest hall of fame. It is also, in its day job, a humble recycler of dead leaves. This guide is about both halves of that résumé.

And if you are reading this in Florida because you just found a pile of little black worms on your patio after a rainstorm — stay with us. Your patio millipedes and this one are cut from the same cloth, and everything in the “what to do” section works for yours too. The Portuguese millipede is simply the most famous member of the family, the one that took a quiet detritivore lifestyle and turned it into an international incident.

What it looks like

Forget everything the word “millipede” makes you picture — this is not the giant, armored, tropical monster from a nature documentary. The Portuguese millipede is small, smooth, and almost boringly neat. An adult is 20 to 45 millimeters long (about 0.8 to 1.8 inches, call it one to one-and-a-half inches for the metrically allergic), slate-grey to black, and shaped like a tiny length of shiny black cable. Run your eyes along it and you will count up to around 50 body segments, each one carrying two pairs of legs — that is the millipede’s defining trick, and the fastest way to tell it from a centipede, which carries only one pair per segment. Do the leg math and you land somewhere around 180 to 200 legs, which sounds like a lot until you remember the animal is named for having a thousand and delivers roughly a fifth of that. False advertising, but make an exception: it is still a lot of legs.

The body is smooth and cylindrical, with no ridges, keels, or bumps — that smoothness is a genuine field mark, and it matters in the look-alikes section below. There is a small but distinctive projection on the tail end, a little spike that helps separate this species from other smooth black millipedes if you are the kind of person who photographs millipede rear ends. No judgment. We are that kind of person now.

Young ones look nothing like the adults, which is half the confusion. One-year-old immatures are light brown with a darker stripe down the middle. They only turn black after about two years, when they reach their tenth or eleventh growth stage and finally become adults. So a pale brown “millipede” and a black one on the same patio can easily be the same species at different ages — the teenager and the grown-up, if teenagers took two years to get dressed.

The name game (a two-letter scandal)

You will see this animal spelled two ways: Ommatoiulus moreleti and Ommatoiulus moreletii. Both refer to the same animal. The double-i version is the currently accepted spelling; the single-i version is the older spelling you will find in most of the Australian pest literature, because that is where most of the writing about this animal happened. We use moreleti in this guide because that is the name under which it became famous, and because half the internet would fail to find it otherwise. Same millipede, two spellings, zero drama — unless you are a taxonomist, in which case we apologize for nothing.

The taxonomy, for the nerds in the back: kingdom Animalia, phylum Arthropoda, class Diplopoda (the millipedes), order Julida, family Julidae, genus Ommatoiulus. The Julida are the classic cylindrical millipedes — the “snake millipedes” of the old field guides — and Ommatoiulus is a mostly Iberian genus of around 60 described species. This one is the overachiever that left home.

What it eats: the quiet recycling job

At heart, the Portuguese millipede is a detritivore — a creature that eats dead things so the rest of the world does not have to wade through them. Its natural diet is dead plant litter: fallen oak and pine leaves, mosses, liverworts, the damp brown layer at the bottom of the world. Gut-content studies of Portuguese millipedes collected in Portugal found them full of fragments of oak (Quercus) and pine (Pinus) litter, plus a surprising amount of fresh moss. It is not a predator. It does not hunt. It is the cleanup crew, and like most cleanup crews it is invisible until it stops showing up — or until it shows up ten thousand at a time.

That is the honest ecological story, and it is worth saying out loud before the pest section: in its home range on the western Iberian Peninsula, where it lives in pine and oak forests, this animal is a normal, useful part of the soil food web. It shreds dead leaves into smaller pieces, its droppings feed microbes, and the nutrients cycle back into the soil. Every handful of healthy soil is full of animals doing exactly this job. The Portuguese millipede only becomes a problem when it leaves home, loses its predators, and discovers that a suburban garden is an all-you-can-eat buffet with no closing time.

What it does not like is just as useful to know, because the dislikes are the levers you pull to get rid of it. It does not like dry conditions: in its home range it survives long, dry summers only by going quiet — a dormant, quiescent state, waiting out the drought in the soil — and indoors, in a dry house, it dies within a few days. It does not like bright, exposed, open ground either: it is a creature of damp, dark places, active at night and drawn to low light rather than daylight, which is why you find it under leaf litter, under pots, and along the damp shaded side of the house, and almost never out in the sunny middle of a dry lawn. Dry, bright, and exposed is the opposite of everything it wants — and that is the whole strategy in the “how to get rid of them” section, in one sentence.

The curl and the chemistry

Disturb a Portuguese millipede and it has two moves. The first is the famous one: it curls into a tight spiral, head tucked in the center, armored rings facing outward, and plays dead with total commitment. The second, if you keep bothering it, is thrashing — violent, snake-like wriggling that is less dignified but surprisingly effective at getting dropped. Both are defenses of an animal that cannot bite, cannot sting, and cannot run away fast. It has exactly one other trick, and it is a chemical one.

Along the sides of its body, the millipede carries rows of tiny defensive glands, one pair per segment. Squeeze or crush the animal and those glands discharge a pungent yellowish fluid. The active ingredients are quinones — specifically, the p-benzoquinones that are the signature defensive chemistry of the whole Julida order, including the well-studied 2-methyl-1,4-benzoquinone. These are the same class of compounds that show up in photographic developer and some dyes, which tells you most of what you need to know about their personality: they are reactive, they stain, and they do not wash out politely.

Here is what that means in practice. The fluid smells bad — sharp, chemical, and memorable. It stains skin, clothes, and carpet, and the stain can be effectively permanent, because benzoquinones react with keratin, the protein in your skin and hair, and bind to it. It is extremely irritating if rubbed into the eyes. It is not venom — there is no bite, no injection, no systemic poison — but it is a genuine chemical irritant, and the correct response to getting it on you is to wash it off with soap and water and not to rub your eyes while you do it. The correct response to finding a millipede is to sweep it up with a soft broom, not to step on it. Crushing is how the fluid gets out, the stain gets made, and the smell gets released. The broom is not just the polite option; it is the chemically literate one.

Life cycle: two years to turn black

Portuguese millipedes are slow in every sense that matters. They grow by molting through a series of stages, adding segments and legs as they go, and they take about two years to reach adulthood — the tenth or eleventh stage, when they finally turn black. The one-year-olds are light brown with a darker stripe down the back, small enough and pale enough to be mistaken for an entirely different animal. Adults are the smooth black cylinders everyone recognizes.

They lay most of their eggs in autumn, in the soil, and the young hatch into the leaf litter to begin their long, slow climb to blackness. In southern Australia, grassland densities of older juveniles and adults have been measured at more than 40 animals per square meter — and in a single backyard, populations have been estimated at 1,000 to 5,000 individuals. That number is the whole story of this animal in one statistic: one millipede is a curiosity; five thousand is a weather event.

The swarm: rain, light, and walls

Most of the year, Portuguese millipedes are invisible — underground, under leaf litter, doing the recycling job. Then the weather changes and they erupt. Rainy weather, especially in spring and autumn, stimulates mass activity: thousands of millipedes come to the surface at once, and the swarm is on. In Australia, mass appearances are a normal autumn event in cities like Hobart, Launceston, and Adelaide, and the numbers are the kind that make homeowners reach for the phone.

What the swarm does next is what makes this animal a household pest. Portuguese millipedes are attracted to low light at night — a positive phototactic response, in the jargon — so they march toward porch lights and lit windows, pile up in large numbers at the bottom of walls, and climb up walls and eaves. They come indoors through any small opening, and once inside they do not breed and eventually die: a dry house is a desert to an animal built for damp leaf litter, and indoor millipedes survive only a few days. The result is the classic scene — a drift of small black curls along the skirting board, a cluster of them dead under the furniture, a homeowner wondering what they did wrong. The answer is usually nothing. It rained, the lights were on, and five thousand millipedes were looking for somewhere damp.

And then there are the trains. Masses of Portuguese millipedes on railway lines during their activity periods have caused locomotives to slip on crushed millipedes, disturbed signaling equipment, and caused train cancellations in South Australia, Victoria, and Western Australia. Crushed millipedes were implicated in a minor train crash at Clarkson, Western Australia, in 2013, when they appear to have affected the train’s braking. In Adelaide, suburban trains are fitted with brushes ahead of the front wheels specifically to sweep millipedes off the line. Read that sentence again: a city fitted its trains with brooms because of a one-and-a-half-inch millipede. That is not a pest. That is a legend.

The Florida angle: the little black worms on your patio

Here is the part Florida readers actually came for. After a heavy rain, small dark millipedes appear on Florida sidewalks, driveways, and patios in numbers that look like a mistake — dozens of them, curled into tight spirals or crawling slowly across the concrete. The University of Florida’s IFAS Extension has answered this exact question from puzzled homeowners: you are looking at small millipedes, most commonly the garden millipede, brownish-black and about an inch long. They do not bite, sting, or transmit diseases. They do not infest food, clothing, or dry, structurally sound wood. They are scavengers, feeding mostly on decomposing vegetation, and they come out in force when waterlogged soil forces them to the surface and higher ground — or when hot, dry conditions drive them to seek moisture. IFAS’s advice for the ones outside is simple: no chemical treatment is recommended. They are outside, doing their job, and they will go back to it.

One honest clarification, because this site does not blur its animals: no verified record puts the Portuguese millipede itself on a Florida patio. Its confirmed introduced range is Australia, New Zealand, South Africa, and the Atlantic islands — not North America. The small black millipedes on a Florida patio are its relatives and ecological look-alikes, mostly the garden millipede and the other introduced species in the look-alikes section below, behaving in exactly the same way for exactly the same reasons. The Portuguese millipede is the species that took that behavior — the rain, the swarm, the march toward the lights, the invasion of the house — and scaled it up until it stopped trains. Your patio version is the same story at a smaller font size, and everything in the “what to do” section applies to it word for word.

Is it a pest?

Plain answer: yes. The Portuguese millipede is a nuisance pest — not dangerous, but a pest in the garden and home sense, in the full meaning of the word. It does not bite, sting, or spread disease, and it does not eat your house. But it invades homes in the thousands, stains carpets and clothes with its defensive fluid, damages seedlings and soft fruit, and, in Australia, has stopped trains. If that is not a pest, the word has no meaning.

The qualifier is that none of this comes from malice, or even from appetite for living plants. This is a detritivore that found itself in a landscape with no natural enemies — its Portuguese predators, the European hedgehog and the devil’s coach horse beetle (Ocypus olens), never made the trip — and with endless damp gardens full of dead leaves. It did what any animal does with unlimited food and no predators: it multiplied until the numbers themselves became the problem. The only thing that has reliably knocked Australian populations back is a parasitic nematode, Rhabditis necromena, which spread from native millipedes and has been linked to marked population declines since the 1970s peak. Even the millipedes, it turns out, have someone smaller than them to worry about.

Effects on the garden

Both sides of the ledger, because this animal genuinely belongs on both sides. On the helping side, it is a decomposer: it shreds dead leaves into smaller pieces, its droppings feed the microbes, and the nutrients in that leaf litter cycle back into the soil faster than they would without it. A garden with millipedes in the leaf litter has a faster, healthier decomposition cycle than a garden without them, and in normal numbers you would never notice them doing it — which is the point. The best soil workers are invisible.

On the harming side, the trouble starts when the numbers explode. In plague numbers the Portuguese millipede stops being a quiet recycler and starts eating the living garden too: it damages seedlings, nibbles soft fruit, and in vineyards feeds on grape berries, breaking the skins and opening the bunches to rot — and its defensive secretions in the ferment can taint the wine. The line between “helpful decomposer” and “garden pest” is not which species it is. It is how many of them there are.

Effects on other garden animals

The Portuguese millipede sits in the middle of the food web, and it is mostly defended, not dangerous. Its defensive secretion deters most predators: birds that try one tend not to try a second, and the fluid makes the animal unpalatable to most predatory arthropods. The predators that do eat it are the specialists and the stubborn. In its home range in Portugal, the European hedgehog (Erinaceus europaeus) and the devil’s coach horse beetle (Ocypus olens) both prey on it. In Australia, meat ants (Iridomyrmex species) are one of the few predators that will consume it. And its most effective enemy is not a predator at all but a parasite: the nematode Rhabditis necromena, which spread from native millipedes and has been linked to the marked decline of Portuguese millipede populations in South Australia since their 1970s peak.

What about the native millipedes it lives alongside? The evidence is reassuringly dull: as an invader in the southern Australian detritivore community, it does not appear to have negatively affected native millipedes sharing a similar range, and it seems to have occupied vacant niches — the damp, disturbed, garden-and-grassland habitats the natives had not filled. It does not hunt other animals, it does not compete for live prey, and its food is dead leaves, which are rarely in short supply. The harm it does to other garden animals is indirect and chemical: a mouthful of benzoquinones teaches a bird or a beetle to leave millipedes alone, and a garden full of Portuguese millipedes is a garden where the predators have learned to look elsewhere for dinner.

Look-alikes: the concrete checks

Small dark millipedes all look alike until you know what to check, and the checks are wonderfully simple — color, texture, shape, speed, and a leg count. Work through them in order and you will rarely go wrong. Each of the look-alikes below has its own full guide in the library, and each one links back here — and to each other.

Rusty Millipede (Trigoniulus corallinus) — the easy one. If it is rusty, it is not Portuguese. The rusty millipede is pinkish-brown to brick red, noticeably larger than the Portuguese millipede at up to three inches long, and so wormlike — with faint black stripes down its sides that look like shadows — that it is routinely mistaken for an earthworm until its legs start moving. Check: red or rusty color, bigger than your thumb, wormlike. It is common in Florida, it is harmless, and it was the first millipede ever to have its genome sequenced, back in 2015, which is a fun fact to deploy at parties. Read the full guide: Rusty Millipede.

Greenhouse Millipede (Oxidus gracilis) — the small, bumpy one. At about 23 millimeters (just under an inch) it is shorter than an adult Portuguese millipede, and its body is dark brown to black on top with pale, creamy-white legs — the pale legs are the giveaway. The second giveaway is texture: where the Portuguese millipede is smooth as polished plastic, the greenhouse millipede is visibly bumpy and ridged along its back. Check: under an inch, pale legs, bumpy back. It is one of the most common millipedes in Florida gardens and greenhouses, and it is harmless. Read the full guide: Greenhouse Millipede.

Flat-backed millipedes (including Asiomorpha coarctata and Apheloria virginiensis) — the ones that look like centipedes. Flat-backed millipedes belong to the order Polydesmida, and they are flattened top-to-bottom with wide lateral keels on each segment called paranota — little flanges that make the animal look wide and flat instead of round. Adults usually have around 20 segments, and many are brown or black, sometimes with bright yellow, orange, or red markings. Check: flat, not round; flared edges on every segment; often brightly marked. They are the most commonly confused with centipedes, but count the legs — two pairs per segment, same as every millipede — and the confusion ends. Some flat-backed species defend themselves with cyanide compounds rather than benzoquinones, which is a different chemistry set and a good reason to admire them with your eyes rather than your hands. Read the full guides: Cyanide Millipede and Black-and-Yellow Flat Millipede.

Bumblebee Millipede (Anadenobolus monilicornis) — the one dressed like a bumblebee. Black with bright yellow bands, about 1¾ inches long, with red legs and red antennae — the red legs are the giveaway. A Caribbean immigrant, first found in Florida in Monroe County in 2001, now one of the most common millipedes on South Florida patios, showing up in numbers after rain. Check: yellow bands plus red legs, South Florida, after rain. A harmless detritivore, same as the rest of the round-backed crew. Read the full guide: Bumblebee Millipede.

Florida Ivory Millipede (Chicobolus spinigerus) — the big native one. Black with pale ivory bands, up to about 3½ inches long — one of the largest millipedes in Florida, and the only native among the three most common patio millipedes (the other two, rusty and bumblebee, are immigrants). Found through the Florida Peninsula and Panhandle and up into Georgia, Alabama, and South Carolina, and popular enough in the pet trade that you may meet one in a terrarium before you meet one in the garden. Check: big, black-and-ivory bands, Florida native. Read the full guide: Florida Ivory Millipede.

American Giant Millipede (Narceus americanus) — the big grey one, with an asterisk. Up to about 4 inches long, grey with reddish bands, and the largest millipede most eastern-US gardeners will ever see — but a Florida photo ID cannot be confidently called this species. The Narceus americanus/annularis complex is taxonomically unresolved, and Florida’s big Narceus is most likely the smokey oak below. Check: big, grey with red bands — then check the smokey oak entry before you name it. Read the full guide: American Giant Millipede.

Smokey Oak Millipede (Narceus gordanus) — the big sandy one, and the one Florida gardeners actually meet. Sixty to 120 millimeters long, sandy tan in color, living in Florida scrub and oak woods — this is the large Narceus under a Florida log, where the American giant’s name gets used by mistake. Check: big, sandy-tan, scrub or oak habitat, Florida. Read the full guide: Smokey Oak Millipede.

Florida Scrub Millipede (Floridobolus penneri) — the rare one, and the one to leave alone. Uniformly slate grey, 60 to 92 millimeters long, and found nowhere on Earth except the Lake Wales Ridge scrub in Polk and Highlands counties. It is critically imperiled (G1G2/S1S2) and not a patio animal at all — if you ever meet one, you are standing in rare scrub, and the right move is to admire it and leave it exactly where it is. Check: grey, ridge scrub only, and rarer than everything else on this page. Read the full guide: Florida Scrub Millipede.

Centipedes — the one that actually matters, because this is the split with consequences. Centipedes have one pair of legs per segment, not two, and they move fast — darting, flickering, gone under the baseboard before you have finished the sentence. Millipedes move slowly, in a steady wave, and curl up when disturbed; centipedes run. Centipedes are also flattened, with long legs that stick out sideways and a pair of venomous front fangs, and they can bite. Millipedes have short legs tucked under a round body and no bite at all. Check: count the legs per segment (one pair means centipede, two pairs means millipede), then watch the speed (fast means centipede, slow means millipede), then watch what it does when poked (runs means centipede, curls means millipede). Three checks, three answers, and you will never mix them up again. Read the full guide: Garden Centipedes.

How to get rid of them

First: do not panic. They do not bite, they do not sting, they do not eat your house, and they do not transmit disease. They are a nuisance, not a danger, and they are a temporary one — the swarm passes, the indoor ones die within days, and the outdoor ones go back to the leaf litter.

Second: sweep, do not squash. A soft broom into a dustpan, tipped outside into the garden, is the whole technique. Squashing releases the defensive fluid, which stains and stinks and is the only part of this animal that can actually hurt you. Sweeping keeps the chemistry inside the millipede, where it belongs.

Third: make the house less inviting. The long-term fixes are all habitat fixes. Pull damp leaf litter and mulch back from the foundation — that wet layer against the wall is both food and shelter, and it is the staging ground for the march indoors. Seal the gaps: door sweeps, window frames, the little cracks around pipes and vents. And turn off the outdoor lights at night during swarm season, or switch them to motion sensors, because the lights are the beacon the millipedes are marching toward. A dark house is an invisible house.

Fourth: do not spray the patio. UF/IFAS Extension is explicit on this one — for millipedes outdoors, no chemical treatment is recommended. They are outside, in their habitat, doing the recycling job, and a sprayed patio is a poisoned patio that still has millipedes on it tomorrow, because the leaf litter is still there and the rain is still coming. Fix the habitat and the lights; leave the chemistry in the bottle.

Frequently asked questions

Does the Portuguese millipede bite or sting?
No. It has no venom, no fangs, and no stinger. Its only defenses are curling into a spiral and releasing a smelly, staining defensive fluid. It cannot hurt you by biting, because it does not have the equipment.

Is it poisonous or dangerous to people or pets?
Not poisonous in the venom sense, but the defensive secretion is a real chemical irritant. It stains skin and clothes, sometimes permanently, and it is extremely irritating if rubbed into the eyes. Wash skin contact off with soap and water, keep it out of eyes, and do not let pets eat millipedes — the fluid is designed to make predators spit them out, and it works.

Why are there hundreds of them on my patio after rain?
Rain floods their underground and leaf-litter habitat, so they come to the surface in large numbers and head for higher, drier ground — which is your patio, your walls, and your porch light. In Australia this happens mostly in spring and autumn; in Florida the same pattern follows heavy rain at any time of year.

Will they live in my house?
No. They cannot breed indoors and they die within a few days in a dry house — they are built for damp leaf litter, not living rooms. What you find indoors is a temporary invasion, not a colony. Sweep them out and seal the gaps they came through.

How do I get rid of them?
Sweep them up with a soft broom and tip them outside. Then fix the causes: pull leaf litter and mulch back from the foundation, seal gaps around doors and windows, and turn off outdoor lights at night during swarm season. Do not spray — UF/IFAS recommends no chemical treatment for millipedes outdoors.

Does the yellow stain wash off?
Often not completely. The benzoquinones in the defensive fluid react with keratin in skin and bind to fabric, and the stain can be permanent on clothes and carpet. On skin it fades as the skin sheds, over days. Wash immediately with soap and water for the best chance, and handle millipedes with the broom, not your hands.

Is it a worm?
No, and the legs give it away. It is an arthropod — a relative of lobsters, crabs, and insects — with a hard segmented exoskeleton and up to around 200 legs. Worms have no legs at all. If it has legs, it is not a worm, no matter how wormlike it looks.

How is it different from a centipede?
Count the legs: millipedes have two pairs per body segment, centipedes have one. Then watch the speed: millipedes are slow, centipedes are fast. Then watch the reaction: millipedes curl up, centipedes run — and centipedes can bite. Two pairs, slow, curls: millipede. One pair, fast, runs: centipede.

Are they good for the garden?
Yes, mostly. As detritivores they shred dead leaves, speed up decomposition, and return nutrients to the soil — the same job earthworms do, from a different branch of the family tree. They only become a garden problem in huge numbers, when they start nibbling seedlings and soft fruit alongside the dead stuff.

Did they really stop a train?
Yes — more than once. Masses of Portuguese millipedes on the rails have caused locomotives to slip, disrupted signaling equipment, and caused cancellations in South Australia, Victoria, and Western Australia. A minor crash at Clarkson in 2013 was blamed on crushed millipedes affecting braking, and Adelaide’s suburban trains carry brushes ahead of their front wheels to sweep millipedes off the track. A one-and-a-half-inch animal, versus a train, and the trains needed special equipment. Never underestimate a detritivore with a million friends.

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🌱 Keep learning

Three related guides worth reading next:

• Compost Accelerators — Twelve things that actually wake up a lazy pile, and the microbes behind all of it, Florida edition.
• Harvesting Worm Castings — How to collect the finished product from your worm bin, from the other great detritivores in the garden.
• Soil Food Web Explorer — Meet the whole underground crew, from bacteria to beetles, and see where the millipedes fit in the chain.