Centipedes & millipedes
One of these can bite and the other cannot, and telling them apart takes one look at a single body segment. Neither is an insect, and only one species on this site is genuinely dangerous.
3 species, each checked against a reference source. Dangerous ones first — that is the question people arrive with.
Medically significant
Everything else
Start with a photograph
Identify this bug from a photo. The result includes confidence, common lookalikes and a plain safety assessment; read its accuracy and limits before treating a low-evidence photo as definitive.
Found a mark but not the animal? See why a bite cannot identify the bug.
Count the legs on one segment
This is the rare identification that is completely reliable and takes no expertise at all. Everything else follows from it.
- One pair of legs per segment — centipede. A hunter, with venom, and capable of biting.
- Two pairs per segment — millipede. A detritivore with no fangs and no venom at all.
- Fast and darting — centipede. Slow and rippling — millipede.
- Flattened body — centipede. Cylindrical, tube-like body — millipede.
- Coils into a flat spiral when alarmed — millipede. Runs — centipede.
The frightening one indoors is the harmless one
The house centipede is, by a distance, the animal in this group that generates the most alarm — fifteen pairs of enormously long legs, sudden bursts of speed, and a habit of appearing in bathrooms. It is also effectively harmless, struggles to break human skin, and spends its time eating cockroaches, silverfish and spiders. Meanwhile the genuinely serious animal here, Scolopendra gigantea, does not occur in the United States at all and is only ever met in captivity. Alarm and risk point in opposite directions in this group.
Common questions
Do millipedes bite?
None of them. Millipedes have no fangs and no venom. Some secrete a defensive fluid that stains skin and irritates eyes, which is worth washing off but is not a bite.
Is the fast thing in my bathroom dangerous?
No. That is a house centipede, and it is hunting the insects you would rather not have. Bites are very rare and comparable to a bee sting.
Do centipedes really have a hundred legs?
Never exactly. The number varies by species and is always an odd number of leg pairs — a house centipede has fifteen pairs, a giant Scolopendra around twenty-one.
How do I keep them out?
Reduce damp and reduce their prey. Both groups need moisture, and centipedes follow the insects. Drying out basements and bathrooms does more than anything sprayed.