Millipedes, centipedes, sowbugs, and pillbugs

Order: Polyxenida, Scolopendromorpha, Scutigeromorpha, Isopoda 

Description and life history

Millipedes

Millipedes (class: Diplopoda) are commonly dark brown and can reach lengths of 2.5- 3.8 cm (1-1 ½ inches) when fully grown. They have rounded and elongated bodies with two pairs of legs on most body segments. When disturbed, millipedes can protect themselves by tightly coiling up and secreting a compound that produces an unpleasant odor.  

These arthropods do not bite or sting and are scavengers that feed on decaying organic matter such as wood. Outdoors, millipedes reside under rocks, logs, or in lawns since they require high moisture, and usually die in homes within a couple of days. In the spring, a single millipede can lay between 20-300 eggs in soil, and newly hatched millipedes have only several pairs of legs. Immature millipedes add additional segments and legs with each molt until reaching sexual maturity, which can take two to five years depending on the species. Millipedes tend to migrate into homes during September-October and in the middle of spring shortly after periods of wet weather. The invasions usually end abruptly, and chronic millipede infestations tend to result in buildings with damp conditions. Millipedes can also be lawn pests. 

Duff millipede

Duff millipedes (Polyxenus spp.) inhabit pine forests and are also found in certain towns along the Front Range. These small millipedes only reach about 1 mm in length and are not as elongated as other millipedes in Colorado. Their bodies are not hardened like most millipedes, and duff millipedes have a tuft of hair protruding from the rear tip of the abdomen, as well as bands of protective hairs surrounding the body. These millipedes are often mistaken for the larvae of carpet beetles (family: Dermestidae) but are much smaller.  

The life history of duff millipedes is not well understood, but they feed on algae, fungi, and decaying organic matter. These millipedes are rarely observed outdoors where tend to reside in tree bark, fallen leaves, and even within ant nests. Eggs are fertilized indirectly by males, who deposit sperm on a silken mat which is then acquired by the female, and eggs are laid in a sticky mass covered with protective hairs. Immature stages develop through several molts and gradually increase in size with each molt until reaching maturity. Invasions tend to occur during hot, dry periods of summer and these millipedes tend to concentrate around areas of moisture such as sinks and bathrooms. 

Centipedes

Centipedes are closely related to millipedes and have similar morphology. However, centipedes have only one pair of legs per body segment and are more flattened than millipedes. All centipedes have a modified pair of legs, called forcipules, that contain venom and are located underneath the mouthparts. While centipedes can bite when physically confined, their bites are only mildly painful and do not pose significant health risks. Unlike millipedes, all centipedes are predators that feed on insects, small arthropods, or even small mammals and lizards in the case of larger species such as Scolopendra polymorpha, which is also known as the common desert centipede and can reach lengths of over 15.2 cm (6 inches). While individuals can vary in color, adults of S. polymorpha typically have red-yellow bodies and blue-black stripes running horizontally across each body segment. This species is native to the plains of eastern Colorado and the West Slope at lower elevations. Around the home, it can be found under potted plants, garden hoses, piles of trash or debris, or furniture. 

Some centipedes, like the house centipede (Scutigera coleoptrata), have very long antennae and legs, with the last pair of legs often exceeding the length of the body. When fully grown, house centipedes can reach lengths of 2.5-3.8 cm (1 – 1 ½ inches), or 7.6-10.1 cm (3-4 inches) with the legs and antennae fully extended. These centipedes are gray or yellow with banded legs and three stripes running along the length of the body. House centipedes were introduced to North America from the Mediterranean and are the only centipedes adapted to life indoors. They can run quickly and will often disperse when the lights are turned on in a room. These centipedes lay eggs in spring and upon hatching, immatures are smaller than adults and have only four pairs of legs. With each molt, the immature centipede gains additional pairs of legs. House centipedes can survive outdoors and tend to invade homes during cool periods such as late summer and early fall. They can live as long as seven years. 

Sowbugs and pillbugs

Sowbugs and pillbugs are terrestrial crustaceans that tend to reside under rocks or other cover outdoors. These arthropods have gills and therefore require high moisture levels for respiration. They are grey and measure about 12.7-19 mm (1/2-3/4 inch) long. Pillbugs roll into a ball when disturbed, while sowbugs do not. These arthropods feed on decaying organic matter and small plants and do not bite or sting. Sowbugs and pillbugs have similar life history, as both are dormant during winter and actively breed during warmer seasons, with females carrying up to 200 eggs in a pouch. After hatching, the immatures remain in the pouch for over a month before leaving to seek out food sources. Sowbugs and pillbugs can invade homes after periods of extended wet spring weather.