Millipedes vary in size, but household invaders are usually 1 inch long, dark brown varieties. In moist, wooded areas in the Northeast, large species measuring up to 4 1/2 inches may be seen. Millipedes have 2 pairs of legs per segment, except the first and last segments. There are total of 30 to 90 pairs of legs.

Millipedes need moisture and live, feed, and breed in moist soil and decaying leaf litter. Females can lay 20 to 300 eggs during summer, and young may not reach maturity until the second year. Adults and young overwinter. As a defense mechanism, millipedes give off a repelling smoky, acrid smell with a chemical combination that renders their bodies toxic to arthropods and other small animals. Ants that feed on millipedes are known to leave behind the exoskeleton and the distinct row of scent glands (one on either side per segment). Millipedes should not be confused with centipedes, which have only one pair of legs per segment and longer antennae. Centipedes are fast moving and venomous.

High populations of millipedes can develop in the landscape when moisture and temperature conditions are ideal. In some cases, changes in weather (drought or flooding rains) can force huge numbers of millipedes out of their harborages and into buildings. Millipedes have a tendency to crawl up things and may crawl up the side of a building and under siding. As alarming as this is, millipedes are harmless and do not survive indoors for more than a few days.