A long black bug with lots of legs is almost certainly either a millipede or a centipede. Both are common household invaders, especially in damp weather, and both can look alarming when they show up on a bathroom floor or basement wall. They’re not insects (insects have six legs), but they’re easy to tell apart once you know what to look for.
Millipede or Centipede: How to Tell
The quickest way to identify what you’re looking at is leg placement. Millipedes have two pairs of tiny legs per body segment, tucked underneath a rounded, cylindrical body. They move slowly, almost like a wave rippling down their length. Centipedes have one pair of legs per segment, and those legs stick out visibly to the sides. Centipedes are noticeably faster, darting across surfaces in quick bursts.
Body shape is the other giveaway. Millipedes look like smooth, dark tubes. Their bodies are rounded when viewed head-on. Centipedes are flatter, with a more segmented appearance and longer, more prominent legs. If the bug you found curled into a tight spiral when disturbed, that’s a millipede. If it sprinted for the nearest crack, that’s a centipede.
Millipedes: The Slow, Harmless Ones
Most of the long, dark, many-legged bugs people find indoors are millipedes. Adults range from half an inch to about 6.5 inches long, and common species are brownish to black with an elongated, cylindrical body. Despite the nickname “thousand-leggers,” most species have between 30 and 400 legs depending on their size and number of segments.
Millipedes are not dangerous. They don’t bite, don’t sting, and don’t carry disease. Some species release a mildly irritating fluid when handled that can stain skin or cause minor irritation, but that’s the extent of it. Their normal diet is rotting leaves, decaying wood, and other decomposing plant material. They’re recyclers by nature, not predators.
When millipedes show up indoors, they usually don’t survive long. Homes are too dry for them, and there’s nothing to eat. They wander in from moist outdoor habitats and typically die within a day or two. Occasionally, heavy rains can trigger mass migrations where dozens or even hundreds of millipedes appear on sidewalks, foundations, and garage floors at the same time. The exact trigger for these events isn’t fully understood, but saturated soil seems to push them out of their hiding spots.
Centipedes: Fast, Flat, and Occasionally Bitey
Centipedes are the predatory counterpart. They hunt other small insects and arthropods, which is actually why they end up indoors: they’re following prey. The most common indoor species is the house centipede, which has distinctively long, striped legs and moves extremely fast. Other species are darker, flatter, and shorter-legged, fitting the “long black bug” description more closely.
Unlike millipedes, centipedes can bite. They use a pair of modified front legs (technically called forcipules) to inject venom into prey. A bite from a small centipede feels similar to a bee sting, with immediate pain, redness, and swelling that lasts several hours to a few days. Larger species can deliver more venom and cause more intense pain. Serious allergic reactions are possible but very rare. People with known allergies to bee or wasp stings face slightly higher risk.
If you do get bitten, wash the area with soap and water. A warm compress can help neutralize the venom, while ice reduces swelling. Over-the-counter pain relievers and antihistamines handle most symptoms. If pain or swelling worsens over several days, or you develop hives, dizziness, or swelling of the mouth or throat, seek medical attention.
Why They’re in Your Home
Both millipedes and centipedes are moisture-seekers. They live naturally under rocks, leaf litter, mulch, and in soil. When conditions near your home’s foundation replicate that damp environment, these creatures end up close enough to wander inside through gaps, cracks, or open doors. Bathrooms, basements, garages, and crawl spaces are the most common spots because they tend to hold more humidity.
Specific attractants include mulch or woodpiles stacked against the foundation, paving stones that trap moisture underneath, leaky hose connections, air conditioning units that drip near the house, and gutters or downspouts that deposit water too close to the structure. Heavy rain is the single biggest trigger for indoor sightings.
How to Keep Them Out
Since moisture is the root cause, the most effective long-term fix is drying out the perimeter of your home. Pull mulch, firewood, and leaf piles at least a foot or two away from the foundation. Check that downspouts and gutters direct water well away from the house. Inspect crawl spaces, hose bibs, and areas near air conditioning units for standing water or slow leaks. Even small, consistent moisture sources can sustain a population right next to your walls.
Seal entry points by repairing torn window screens, adding weather stripping to doors, and caulking gaps around pipes, vents, and foundation cracks. A dehumidifier in the basement or crawl space can make the indoor environment less hospitable if humidity is consistently high.
For active infestations, a perimeter insecticide applied around door frames, window casings, baseboards, and the outdoor foundation line creates a barrier. Indoor stragglers can simply be swept up or vacuumed. Since millipedes die quickly indoors on their own, a few sightings usually don’t require chemical treatment. Centipedes are harder to eliminate because they’re faster and better at hiding, but reducing their prey (other small bugs) by keeping the home clean and sealed will discourage them from sticking around.
One Bug or an Infestation?
Finding a single millipede or centipede indoors is normal and not a sign of a larger problem. These are solitary wanderers most of the time. If you’re finding several per week, especially in the same area, that points to a moisture issue nearby or an entry point worth sealing. The mass millipede migrations after heavy rain are temporary and resolve on their own once conditions dry out. Centipedes appearing regularly may indicate a healthy population of other pests they’re feeding on, which is worth investigating separately.