Centipedes, Millipedes and Sow Bugs

Centipedes and millipedes. They have a tendency to show up when you least expect them. Perhaps you have had the experience of walking into a room at night and turning on the lights just in time to see a centipede on the wall quickly scuttle away behind a picture frame; or maybe you have been in a basement and moved a piece of furniture to be greeted by the sight of millipedes wriggling around beneath it. What are these strange, many-legged creatures and why are they in your house?


Centipedes and millipedes are not insects but Myriapoda (literally “myriads of legs”), a subphylum of the anthropods, the broader group of invertebrates to which insects and arachnids also belong. Myriapods share many things in common: all have a single pair of antennae, simple eyes and a mouth on the underside of the head. All prefer to live in moist areas and feed on decomposing organic material (though centipedes are also predatory), and, as their name suggests, all Myriapods have multiple pairs of legs, ranging from as few as 5 up to 350 pairs.


centipedeCentipedes (“hundred legs”) are the most common Myriapod humans are likely to encounter in Colorado. The household centipede has a flattened body with 15 pairs of long, jointed legs attached along the sides. Each body segment has one pair of legs. It is the legs of the household centipede that make it most identifiable, for they are quite long and protrude from the sides of the body (as opposed to millipedes and sowbugs, whose legs are short, not jointed, and are underneath the body). The body is very light, usually brownish or grayish-yellow ; it may even appear translucent at times. It may be marked with three dark stripes visible from above. A pair of long, slender antennae extend forward from the head.

Centipedes are predatory, feeding on small insects, spiders and other anthropods (like sow bugs). Because they are hunters, they are very quick and agile. As centipedes are nocturnal, they are usually spotted running across a ceiling or wall attempting to hide in a dark location, like a Colorado basement. Like other Myriapods, centipedes prefer moist areas. They do bite humans, although the Colorado variety are not poisonous.


common-millipedeMillipedes (“thousand legs”) are dark creatures that have the appearance of a worm, usually about an inch to an inch and a half in length. Their body is more cyllindrical, unlike centipedes, who have flattened bodies. Millipedes have many more legs than the centipede, sometimes up to 400. Despite its abundance of legs, the millipede moves much more slowly than the centipede. Millipedes prefer to live underneath objects in damp locations where they can feed on decaying organic material. When touched, they will often curl up tightly. In Colorado, as well as other midwestern states, it is not uncommon to experience an influx of millipedes in a basement shortly after a rainstorm.

Sow bugs

sow-bugThe sow bug (also known as the woodlouse, roly-poly, or pillbug), though often lumped in with centipedes and millipedes, is not a Myriapod but is actually a type of land-dwelling crustacean, related to lobsters, crayfish and crabs. The sow bug is brownish-gray in color and usually reaches no more than 3/4 of an inch. It has an oval-shaped body with a series of overlapping plates on its back, resembling a small armadillo or turtle. A sow bug typically has fourteen legs. Like centipedes and millipedes, sow bugs prefer a moist environment. They feed on decaying organic matter.

The most notable characteristic of the sow bug is its ability to curl up into almost a perfect circle when threatened, giving rise to the common name “roly-poly.”

Household Invaders

Centipedes, millipedes and sow bugs may all occasionally get into the home. If you see centipedes in the home, or other anthropods, don’t panic; they do not reproduce in the home and most residences do not have an adequate degree of dampness to serve as permanent habitats for centipedes, millipedes and sow bugs. So why these creatures come into the home?

There are two reasons why these pests get into the home: overwintering and excessive rainfall. In the first case, centipedes, millipedes and sow bugs can leave the soil and leaf litter and crawl into homes, sometimes in very large numbers. This usually occurs in the late summer or early fall, as they are searching for protected places to overwinter. In the case of excessive rainfall, their normal environment may become flooded and force them to seek shelter in less moist areas.

They enter the home through cracks in foundations, under doors and through window wells. They are usually found in basements although they may also be found in ground-level rooms. It is common for centipedes to actively move indoors from outdoor harborage areas during spring and summer. They are most commonly seen in homes during warm weather. They can be found indoors during winter but are less common. Neither the millipede nor the sow bug are dangerous, although their presence in the home may be signs of a dampness problem. Centipedes may bite, though this is extremely rare since they typically try to avoid contact with humans. Their bites can be painful and lead to swelling, but they are generally no more dangerous or uncomfortable than a bee sting. Persons allergic to bee stings should also be wary of centipede bites.


In many cases it is not necessary to take any active steps for management of these pests. Some will inevitably get into your home throughout the year, and as they do not “nest” or reproduce in the home, you may be able to simply tolerate them until they go away (or else vacuum them up or smack them with a rolled up magazine). However, in certain situations they may come into the house in such numbers as to warrant action. Perimeter treatments around the home’s foundation are very helpful in keeping centipedes, millipedes and sowbugs from getting into the home to begin with; applications in the late summer and early fall are most effective. Spot treatment of potential centipede harborage areas indoors is also recommended (behind base boards, in cracks and crevices, along the walls in basements). Glue-traps are also effective for catching centipedes.

Environmental modifications can go a long way towards keeping these pests out of the home. The following steps are recommended for prevention in Colorado homes: Caulk or seal cracks and other openings in exterior foundation walls and around doors and ground-level windows by late summer.

  • Caulk or seal cracks and other openings in exterior foundation walls and around doors and ground-level windows by late summer.
  • Remove leaf litter and decaying vegetation from around the foundation which provide food and shelter for sowbugs and millipedes.
  • A border of bare soil around the building next to the foundation also helps to make the area a less favorable habitat.
  • Trim and thin foundation planting so that ventilation permits the soil to dry more quickly near the foundation.
  • Allow the soil near the house to dry between waterings. Roughening the soil surface will speed drying and will work plant materials into the soil where it is unavailable to sowbugs or millipedes.Remove unnecessary boxes, bags, and other clutter that gives centipedes favorable places to hide.
  • Caulk or seal behind baseboards and in cracks and crevices where centipedes like to hide.
  • Place a dehumidifier in damp areas to sufficiently dry the air. Structural repairs may be necessary to the home and/or yard if a dehumidifier does not keep a room sufficiently dry.

If you have repeated issues with centipedes getting into your house, it is probable that there is a high density of available insect prey in the home as well (such as ants, spiders). Getting rid of spiders or ants in the home will remove the food source for the centipedes and make your house a lot less attractive to them!