Millipedes: Diplopoda

Most millipedes lack a waxy layer on the outside of their exoskeletons, or hard outer coverings, that helps to prevent the loss of body moisture. Like centipedes, millipedes spend most of their time in cool wet places and become active only at night or after rains.

Many millipedes defend themselves by rolling their bodies up into a ball or spiral. This behavior protects the legs and delicate underside of the animal, leaving only the hard plates of the body segments exposed. Some species also protect themselves by producing toxic or bad-smelling chemicals through a series of openings on the sides of their bodies. Some larger tropical species can actually squirt their attackers with a defensive spray. Bristly millipedes do not produce these defensive chemicals. Other species behave strangely when threatened. For example, Diopsiulus regressus alternates between flipping its body into the air and running short distances.

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 behavior. 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 coaxes a female to mate with squeaking noises made by rubbing the bases of his legs against his body. He then grasps the female's body with his legs. A sperm packet is released behind his head and passed back from one pair of legs to the next like a conveyor belt, 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 mate's 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, guards the eggs until they hatch. Although young millipedes resemble small adults, they are usually legless 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. Narceus americanus and Orthoporus ornatus seal themselves off in special chambers dug for this 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.