Hundreds of Curled-Up Bugs on the Basement Floor

No. Millipedes have no venom, no fangs and no ability to bite — their mouthparts scrape soft decaying plant matter. They cannot damage your home, your belongings or your food. The one caution is that many species release a defensive fluid when threatened or crushed, which smells unpleasant, can mildly irritate sensitive skin, and stains light flooring. That is a good reason to sweep or vacuum them rather than step on them, and to wash your hands if you handle one, but it is not a health hazard.

Millipedes migrate as a population, and your house was in the path. In autumn — and after prolonged heavy rain at any time — large numbers that built up over the summer in lawn thatch, leaf litter and mulch begin moving together across the surface at night, looking for somewhere to overwinter. A building standing in the way intercepts hundreds over a couple of nights. It is an event with a beginning and an end, not a resident infestation, and it usually runs a few days to a fortnight.

No. Millipedes need constantly damp conditions and decaying plant matter to eat, and a heated house offers neither. They dry out and die within a day or two indoors, which is exactly why you find them dead and coiled rather than thriving. They cannot reproduce inside. What looks like an infestation is a large number of arrivals from outside, all dying on schedule. The exception is a chronically wet crawlspace or basement, which can be damp enough to sustain them — and that is a moisture problem worth solving on its own merits.

Coiling is a millipede's entire defense — it has no speed, no bite and no sting, so when threatened, or when it is drying out and dying, it curls into a flat spiral to protect its soft underside and legs behind the armored plates of its back. Dead millipedes stay in that curl, which is why the floor ends up scattered with small dark coils. A live one that has been disturbed will usually uncurl and move off if you leave it alone.

Speed tells you instantly. Millipedes are slow, rounded and tube-like, dark, with two pairs of short legs per segment producing a rippling wave, and they coil when disturbed. Centipedes are fast — startlingly so — flattened, with one pair of long legs per segment splayed out sideways, and they run for cover rather than curling. Centipedes are also predators hunting other insects, and they turn up as individuals year-round wherever it is damp, not in autumn waves.

There is very little point. The millipedes indoors are already dying because the air is too dry for them, so killing them faster changes nothing about how many are arriving from outside. Interior treatment also puts product where you live for no real gain. All the useful work is outdoors: cutting the damp habitat against the foundation, sealing ground-level gaps, and on properties with a history, a perimeter treatment timed to land before the autumn migration starts.

Reduce the damp organic layer immediately around the house and seal the ways in. Dethatch the lawn where it runs to the foundation — that is often the single biggest reservoir and the one people overlook. Pull mulch back to leave a dry strip of bare soil or gravel against the wall and keep it shallow. Clear leaf litter before autumn rather than after. Lift boards, stones and landscape timbers off the soil. Extend downspouts and regrade. Then seal foundation cracks, sill plate gaps, garage door seals and basement window seals.

Window wells collect exactly what millipedes want and then trap them. Leaves and organic debris accumulate, moisture holds, the well is shaded, and an animal that drops in generally cannot climb the smooth sides back out. So during a migration a well fills up — and there is a basement window right there whose seal may not be perfect. Clearing the wells, improving their drainage and fitting covers is one of the more effective single steps on a house that gets these every autumn.

Not necessarily wrong, but it is telling you something. A mass arrival during a genuine migration can happen to a perfectly sound house that simply stands in the path. What it does indicate is that the ground immediately around your foundation is damp and organic enough to support a large population, and that there are ground-level gaps letting them in. Neither is an emergency, but both are worth correcting, because the same conditions support sowbugs, springtails, centipedes, and eventually rot.

Late summer, if you had a bad autumn last year — that is the call that actually prevents the problem, because a perimeter treatment applied before the movement starts means arriving millipedes contact it at the foundation rather than after they are inside. Applied in the middle of a migration it helps considerably less. Also call if you are finding them year-round rather than seasonally, which points at a damp crawlspace or basement sustaining them and is a different conversation.