Males and females usually have to mate to produce offspring, with males usually depositing sperm directly into the reproductive organs of the female. There may or may not be any courtship behaviour. Bristly millipede males must first spin a web on which they deposit their sperm. The female then approaches the web and puts the sperm into her own reproductive organs. In some pill millipedes a male attracts a female to mate with squeaking noises made by rubbing the bases of his legs against his body. He then grasps the females body with his legs. A sperm packet is released behind his head and passed back from one pair of legs to the next, until it reaches the reproductive organs of the female. In other pill millipedes the male covers the sperm packet in dirt before passing it back with his legs to his mates reproductive organs.
Millipedes lay their eggs in the soil. Some species make individual cases for their eggs out of chewed-up leaves. In some species, the female, and occasionally the male, guard the eggs until they hatch. Although young millipedes resemble small adults, they are usually have no legs when they first hatch from the egg.
After they molt, or shed their exoskeleton for the first time, they have six body segments and three pairs of legs. They add additional body segments and pairs of legs with each molt until they reach the maximum adult number. Millipedes molt in sheltered places underground or in cracks in the soil. This is a very delicate stage of their lives. Millipedes reach adulthood in one or two years, sometimes longer. Adults live for one to eleven years, although some individuals may live longer.