Smell almonds in the leaf litter, and you have probably just met the cyanide millipede. Asiomorpha coarctata — also called the long-flange millipede, and filed by some scientists under the name Orthomorpha coarctata — is a small, flat-backed millipede with yellow spots on its flanges and a chemical defense that smells like marzipan and means business. It is one of the most widely traveled millipedes on Earth, an introduced Southeast Asian that now lives all around the tropics, including the Gulf Coast of North America — which is to say, including Florida. It is harmless to people, poisonous to predators, and almost impossible to tell from its cousin the greenhouse millipede unless you know the one check that separates them. This guide is about that check, that smell, and that millipede.

And if you are reading this in Florida because you just found a small black flat-backed millipede with yellow spots under a log — stay with us. It is not dangerous, it cannot bite, and everything in the “what to do” section works for yours too.

What it looks like

The cyanide millipede is small, flat, and dark, with a field mark you can see from a foot away: yellow spots. An adult is about 2 centimeters long — under an inch, with females slightly larger, up to about 2.75 centimeters — and 1.5 to 3 millimeters wide, with around 20 body segments. The body is dark brown to black on top, flattened top-to-bottom rather than round, and each segment carries a pair of wide lateral keels called paranota. On many individuals each paranotum carries a bright yellow patch, so the animal looks black with a row of yellow spots down each side, like a tiny warning sign repeated twenty times. That is exactly what it is: the yellow is aposematic coloration, advertising the poison inside.

The paranota are the whole identification story, and they are also the reason for the “long-flange” name. On the cyanide millipede the paranota of the mid-body segments are long and pointy, and their sharp rear corners stick out past the back edge of each segment — if you look at the animal from above, the flanges look like little arrowheads pointing backwards. That single feature is the check that separates it from the greenhouse millipede, whose flanges are short and do not project past the segment edge. Short flanges, greenhouse; long pointy flanges, cyanide millipede. Hold that thought — the look-alikes section will make you use it.

Like other flat-backed millipedes, it has no eyes worth mentioning — the Polydesmida are generally blind or nearly so — and it navigates by touch and smell, waving its antennae ahead of it. It moves slowly, in a steady wave, and when disturbed it curls up and waits for the trouble to go away. Young ones are paler than the adults and darken as they grow, adding segments with every molt.

The name game (Asiomorpha, Orthomorpha, and a shared nickname)

This animal has a name problem, and it is worth sorting out before anything else, because you will meet both names. The name used in this guide, Asiomorpha coarctata (Saussure, 1860), places it in its own single-species genus, Asiomorpha — a genus created for it because its male reproductive organs have a single simple tip where its relatives have two or three lobes, and because genetic work has shown it sits on its own distant branch of the family tree. The other name, Orthomorpha coarctata, keeps it in the big Southeast Asian genus Orthomorpha, which is where the most recent revision of the genus puts it. Both names refer to the same animal; the checklists disagree and the millipede does not care. We use Asiomorpha because it is the name under which most of the English-language pest and biodiversity literature lists it, and because it flags the animal’s odd-one-out status honestly.

The common names are worse. “Long-flange millipede” is the descriptive one and the safest. “Cyanide millipede” is the dramatic one — and it is shared. The western yellow-spotted millipede, Harpaphe haydeniana, is also called the cyanide millipede, and so are several other flat-backed species, because the cyanide defense is common to the whole order, not unique to this species. This one earns the name on the Gulf Coast, where it is the cyanide millipede a Florida gardener is most likely to meet. The taxonomy, for the nerds in the back: kingdom Animalia, phylum Arthropoda, class Diplopoda (the millipedes), order Polydesmida (the flat-backed millipedes), family Paradoxosomatidae, genus Asiomorpha. The Polydesmida are the largest order of millipedes, with around 3,500 species worldwide, and this one is the overachiever that left home and never came back.

Home, originally, is Southeast Asia — probably the region from Indonesia to Australia — and from there it has been carried by people, in soil and potted plants, to tropical and subtropical places all over the world: Hawaii, the West Indies, the Galápagos, and the Gulf Coast of North America, including Florida, Louisiana, and Texas. It arrived the way most Florida millipedes arrive: quietly, damp, and in a pot.

What it eats: the quiet recycling job

At heart, the cyanide millipede is a detritivore — a creature that eats dead things so the rest of the world does not have to wade through them. Its diet is dead plant material: fallen leaves, rotting wood, the damp brown layer at the bottom of the world, and the rich organic matter of a mulched bed. It breaks dead plant material into smaller pieces so bacteria and fungi can finish the job, which makes it an important animal in the soil food web despite its size. 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 in numbers after rain.

That is the honest ecological story, and it is worth saying out loud before the pest section: this animal shreds dead leaves, 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. In the Philippines, where it is a familiar garden animal, it is considered a common garden pest — not because it destroys crops, but because it turns up in numbers in dark, moist places and comes out in abundance during the rainy season. The Florida version behaves the same way: damp, dark, and full of dead leaves is home, and a dry sunny lawn is a desert it wants nothing to do with.

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: it breathes through its skin and dries out fast in open air, 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 mostly at night, which is why you find it under logs, 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 cyanide

Disturb a cyanide 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 is chemical, and it is the one the animal is named for. Along the sides of its body it carries defensive glands, and when molested it discharges a fluid containing hydrogen cyanide — the same compound that gives bitter almonds their smell — along with benzaldehyde, which smells like cherries or almonds. That is why a disturbed flat-backed millipede smells like a bakery and a chemistry lab at the same time, and why predators learn to leave it alone after one try.

Here is what that means in practice, because the word “cyanide” does a lot of heavy lifting in a headline. The amount one millipede produces is tiny — far too small to be hazardous to a person — but it is a real poison, and it is enough to kill or sicken a small predator that tries to eat the millipede. For you, the rules are simple: do not eat it, do not rub it in your eyes, and wash your hands after handling one. The fluid can irritate skin and eyes, and it can stain. It is not venom — there is no bite, no injection, nothing that gets it into your bloodstream — and the millipede cannot bite or sting at all. But it is a genuine chemical weapon, and it is the reason most predators leave the black-and-yellow ones alone after one try. Admire it with your eyes rather than your hands. The millipede would prefer that too.

One more honest note: the cyanide defense is not unique to this species. It is shared by most flat-backed millipedes — the whole order Polydesmida uses some version of it — which is why the “cyanide millipede” nickname belongs to several species at once. What makes this one special is not the chemistry but the combination: the chemistry, the yellow spots, the long flanges, and the Gulf Coast address.

Life cycle: born small, earning segments

Like all millipedes, the cyanide millipede grows by molting: it sheds its exoskeleton, adds new segments and new pairs of legs, and hardens up again, bigger than before. It hatches from an egg laid in soil or leaf litter with only a few segments and a handful of legs, and earns the rest over successive molts. Adults reach their full size — about 2 centimeters, females a little larger — with around 20 segments, which sounds like a lot until you remember the animal is named for having a thousand legs and delivers a small fraction of that. False advertising, but make an exception: it is still a lot of legs for an animal under an inch long.

The details of this species’ life cycle — exactly how long it lives, how many eggs a female lays — are thinly studied. What is certain is the pattern: damp seasons bring activity and breeding, dry spells send it deep into the soil and leaf litter to wait, and in Florida the same rhythm follows heavy rain at any time of year. If your garden suddenly has cyanide millipedes, it rained recently. It almost always rained recently.

The Florida angle: the Gulf Coast’s cyanide millipede

On the Gulf Coast — Florida, Louisiana, Texas — the cyanide millipede is the flat-backed millipede a gardener is most likely to meet in leaf litter, under logs, and around the damp edges of the garden. It is not the famous one; the famous patio swarmers are the Portuguese and rusty millipedes. It is the quiet one, smaller and flatter, found one or two at a time under a log rather than a thousand at a time on the patio. But it is here, it is established, and in rainy weather it comes out in numbers like the rest of them.

It comes indoors the same way the others do, through gaps at ground level, and it dies the same way indoors: within a few days in a dry house, because it cannot breed inside and cannot survive the dry air. What you find indoors is a temporary invasion, not a colony. Sweep them out and seal the gaps they came through. And because it is small, dark, and flat, it is the millipede most often mistaken for a centipede — which is the one mix-up in this whole library that actually matters, because centipedes can bite and millipedes cannot. Count the legs: two pairs per segment means millipede, one pair means centipede. The look-alikes section will walk you through it.

Is it a pest?

Plainly: it is a minor nuisance pest, not a dangerous one — and most of the time it is a helper you never notice. It does not bite, sting, or spread disease. It does not eat your house, your clothes, or your stored food. It does not damage structures. What it does is show up in numbers after rain, get into houses and greenhouses through gaps at ground level, and — in very large numbers — nibble tender seedlings and soft fallen fruit alongside the dead stuff it prefers. That last point is worth the qualifier: dead plant matter is its food, and living plants are a fallback it reaches for only when the crowd is huge and the dead leaves run short.

Against that, weigh the day job: it shreds leaf litter, speeds up decomposition, and returns nutrients to the soil — the same work earthworms do, from a different branch of the family tree. A few cyanide millipedes in the garden are a sign of healthy, organic-rich soil. A few hundred on the patio are a sign that it rained and the leaf litter is too close to the house. Both things can be true. The goal is not to exterminate them — it is to move the party from the patio back to the leaf litter where it belongs.

Effects on the garden

On the plus side, the cyanide millipede is a genuine soil helper. It eats dead leaves and rotting wood, breaks them into smaller pieces, and its droppings feed the microbes that finish the job — fungi and bacteria that turn leaf litter into plant food. In a compost pile it is a volunteer worker, and in a mulched bed it is part of the crew that keeps the mulch from becoming a matted, airless blanket. Gardens with millipedes have faster litter turnover and richer surface soil than gardens without them, all else being equal.

On the minus side, numbers change the story. In very large populations, cyanide millipedes can nibble seedlings, soft stems, and fallen fruit, and they can be a nuisance in greenhouses and around potted plants, where they hide in the drainage holes and under the pots. They are not in the same league as the insects that actually eat your crops — they are not a serious garden pest — but a greenhouse full of them is a greenhouse that needs its debris cleared, its benches dried, and its pots lifted. And unlike the round-backed millipedes, whose defense stains, this one’s defense smells — a disturbed cyanide millipede can leave a faint almond smell in a closed room, which is startling the first time and diagnostic forever after.

Effects on other garden animals

The cyanide millipede sits in the middle of the garden food web, and it pulls in both directions. As a detritivore it eats only dead plant matter — it does not hunt other animals, does not eat living insects, and does not compete with predators for prey. Its competition is with the other recyclers: earthworms, woodlice, and springtails all eat the same dead leaves, and in a rich leaf-litter bed there is usually enough dead matter for all of them. It helps the soil microbes by shredding litter into bite-size pieces, and its droppings are food for fungi and bacteria, which in turn feed the whole underground web.

As prey, it is a defended meal — more defended than most. Its hydrogen cyanide defense puts off nearly every predator that tries it once, and the yellow spots advertise the fact in advance, which is the whole point of aposematic coloration: the predator learns without the millipede having to die. Some animals eat flat-backed millipedes anyway — certain birds, toads, and ground beetles — and some ant species specialize in overpowering them. In Florida, the millipedes that survive are the ones predators learned to leave alone. For pets, the risk is small but real: a dog or cat that crunches a cyanide millipede will get a mouthful of foul, almond-tasting fluid and may drool, paw at its mouth, or vomit, then learn not to do it again. It is unpleasant, not an emergency — but keep an eye on any pet that eats a lot of them, and call a vet if the drooling does not stop.

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.

Greenhouse Millipede (Oxidus gracilis) — the twin, and the one check that matters. These two are the hardest pair in the whole library to separate, and the separator is the paranota. On the cyanide millipede the mid-body flanges are long and pointy, with sharp rear corners that stick out past the back edge of each segment. On the greenhouse millipede the flanges are short and do not project past the segment edge — that is the whole point of the name “short-flange.” The greenhouse millipede is also usually browner where the cyanide millipede is blacker, and it has a transverse groove across each segment that the cyanide millipede lacks. Check: flanges sticking out past the segment edge means cyanide millipede; flanges ending flush with the segment means greenhouse millipede. If you cannot see the flanges, you cannot separate them — and that is honest, too: even the experts check the male reproductive organs for certainty. Read the full guide: Greenhouse Millipede.

Portuguese Millipede (Ommatoiulus moreleti) — the smooth black one. The Portuguese millipede is slate-grey to black, smooth and cylindrical, and larger than the cyanide millipede at up to about 1.8 inches. Where the cyanide millipede is flat with flared flanges on every segment, the Portuguese millipede is round as a cable, with no flanges at all. Check: round and smooth means Portuguese; flat with flared edges means flat-backed, and then the flanges decide which one. Both are harmless detritivores, both swarm after rain, and the fix for both is the same. Read the full guide: Portuguese Millipede.

Rusty Millipede (Trigoniulus corallinus) — the red one. If it is rusty red, it is not a cyanide millipede. The rusty millipede is pinkish-brown to brick red, noticeably larger at up to three inches long, smooth and wormlike, and so often mistaken for an earthworm that people step on it before they see the legs. 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. Read the full guide: Rusty Millipede.

Black-and-Yellow Flat Millipede (Apheloria virginiensis) — the bigger, brighter cousin. The black-and-yellow flat millipede is also black with yellow markings and also defends itself with cyanide, which is why the two are confused — but it is much larger, up to two inches long, with a broad, heavy body and yellow legs, where the cyanide millipede is under an inch with dark legs. Check: big, broad, yellow legs means black-and-yellow flat millipede; small, narrow, dark legs means cyanide millipede. Both smell of almonds when disturbed, and both deserve the same respect: admire with your eyes, not your hands. Read the full guide: 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

Do not spray. UF/IFAS recommends no chemical treatment for millipedes outdoors, and it would not work anyway — the millipedes live in the leaf litter, not on a surface you can treat, and killing the ones on the patio does nothing about the thousands still in the mulch. The fix is boring, cheap, and effective, and it works for every millipede on your patio:

Sweep them up. A soft broom and a dustpan, tipped outside into the leaf litter away from the house. Do not crush them on the patio if you can avoid it — the almond smell is the point of the defense, and it works on concrete too.

Pull the buffet back from the house. Rake leaf litter and mulch back from the foundation by a foot or two, clear the gutters, and pick up fallen fruit. The millipedes live where the food is; move the food and they follow it.

Dry the edges out. Millipedes need damp. Fix dripping taps and sprinklers that soak the foundation beds, let the soil surface dry between waterings, and keep potted plants off the patio or on saucers that drain. Dry, bright, and exposed is the opposite of everything they want.

Seal the gaps. Door sweeps, weatherstripping, and caulk around windows and pipes close the routes indoors. They come in through gaps you can barely see, so look low — at ground level, where the wall meets the patio.

Turn the lights off. Outdoor lights attract millipedes on swarm nights, especially after rain. Switch them off during swarm season, or swap to motion sensors and yellow bulbs that draw fewer night-flying insects — and fewer of the animals that follow them.

Leave the compost pile alone. If you have a compost pile or a leaf-mold corner away from the house, let the millipedes work there. They are doing you a favor in the pile, and a busy pile keeps them out of your living room. The goal is not a millipede-free garden — it is a millipede-free house, with the garden crew still on the payroll.

Frequently asked questions

Is the cyanide millipede dangerous?
No — not to people. It produces a tiny amount of hydrogen cyanide as a defense, far too little to harm a person, but enough to sicken a small predator. Do not eat it, do not rub it in your eyes, and wash your hands after handling one.

Does it bite?
No. Millipedes have no venom, no fangs, and no bite at all. Their defenses are curling up and a bad-tasting chemical fluid. If it has fangs and it bit you, it was a centipede.

Why does it smell like almonds?
Because its defensive fluid contains hydrogen cyanide and benzaldehyde, and both smell like almonds — benzaldehyde is the compound that gives almond extract its scent. The smell is the warning: it means the millipede is defended, and predators learn to associate the smell with a bad taste.

How big does it get?
Adults are about 2 centimeters long, under an inch, with females slightly larger at up to about 2.75 centimeters. It is one of the smallest flat-backed millipedes you will meet in a Florida garden — much smaller than the black-and-yellow flat millipede, and smaller even than the Portuguese millipede.

How is it different from the greenhouse millipede?
Look at the flanges — the little keels sticking out from each segment. On the cyanide millipede they are long and pointy, with sharp rear corners that stick out past the back edge of each segment. On the greenhouse millipede they are short and end flush with the segment. Long pointy flanges mean cyanide millipede; short flanges mean greenhouse millipede.

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 dozens of legs. Worms have no legs at all. If it has legs, it is not a worm, no matter how wormlike it looks.

What does it eat?
Dead plant matter: fallen leaves, rotting wood, compost, and mulch. It does not eat living plants as a rule, though in very large numbers it may nibble seedlings and soft fallen fruit. It does not eat your house, your clothes, or your food.

Will it hurt my plants?
Almost never. A few millipedes in a bed are helpers, shredding leaf litter and feeding the soil. Only in huge numbers do they nibble seedlings and soft fruit — and at that point the fix is the same as for the patio: pull the mulch back, dry the edges, and let the population settle.

Will it hurt my dog or cat?
A pet that crunches one will get a mouthful of foul, almond-tasting fluid and may drool, paw at its mouth, or vomit once, then learn not to do it again. It is unpleasant, not an emergency — but call a vet if the drooling does not stop or the pet eats a lot of them.

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, dry out damp foundation beds, and turn off outdoor lights at night during swarm season. Do not spray — UF/IFAS recommends no chemical treatment for millipedes outdoors.

Is it the same as the western cyanide millipede?
No, but they share the nickname. The western yellow-spotted millipede, Harpaphe haydeniana, is also called the cyanide millipede, and it lives in the moist forests of the Pacific coast, from Alaska to California. This one is the Gulf Coast’s cyanide millipede — smaller, with yellow spots on black, and introduced from Southeast Asia. Same chemistry, different animal, different coast.

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🌱 Keep learning

Three related guides worth reading next:

• Greenhouse Millipede — The twin: tiny, blind, and everywhere, separated from the cyanide millipede by one flange check.
• Rusty Millipede — The other patio-swarm millipede: rusty red, up to three inches, and the first millipede ever to have its genome sequenced.
• Soil Food Web Explorer — Meet the whole underground crew, from bacteria to beetles, and see where the millipedes fit in the chain.