Number of families: About 121 families
Millipedes usually have long wormlike bodies that measure 0.08 to 11.8 inches (2 to 300 millimeters) in length. However, bristly millipedes resemble small (0.16 inches; 4 millimeters) caterpillars that are covered with tufts of stiff hairlike structures. Pill millipedes have short wide bodies that roll up into a ball just like pillbugs. Most millipedes are brownish, blackish, or dark greenish, but many are pale or pinkish. Others are brightly marked with yellow or red. The head has two pairs of jaws. The eyes, if they have any at all, are simple and have only one lens each. The antennae (an-TEH-nee), or sense organs, are short and seven-segmented.
The usually stiff bodies of millipedes are either flattened, rounded, or dome-shaped in cross-section and divided into eleven to 192 segments, depending on age and species. Each segment is formed while the millipede is still in the egg by the joining of two body segments. This is why most of the body segments have two pairs of legs. The first and last body segments are always legless. The first legless segment is a heavily armored collarlike segment that separates the head from the rest of the body. Segments two through four have one or two pair of legs each, except in the males of one group of millipedes where these legs are specialized and used for reproduction. Adults have anywhere from eleven pairs (twenty-two legs) to 375 pairs (750 legs) of legs. Males and females look very similar to one another, but males usually have longer legs so they can grasp the female while mating. The legs of millipedes are phylum class subclass order monotypic order suborder family attached directly underneath the body and are only slightly visible on the sides, if at all. This arrangement gives them the power they need for burrowing and allows them to get into narrow spaces without breaking off legs. The common name millipede, meaning "thousand-legger," refers to the fact that millipedes often have a lot of legs and not to a specific number.
Millipedes are found on all continents except Antarctica. Because they move so slowly on their own, most millipedes have small distributions. However, because many species are burrow-ers, they have been transported by humans throughout the world in soil and with plants. In fact, half of the species that are native to Britain have been introduced to North America this way. There are about seven thousand species of millipedes worldwide, with about fourteen hundred species in the United States and Canada.
Millipedes usually live in dark damp habitats, but Archispirostreptus syriacus and Orthoporus ornatus prefer dry habitats and live in deserts. Most species are under leaf litter, woodpiles, and stones. Soil dwellers are usually found in the top inch or two of soil. A few species climb trees. For example, some species of bristly millipedes live in the small cracks in tree bark. Although many millipedes are active at night, pill millipedes, such as Glomeris marginata, are usually active during the day.
Most millipedes eat decaying leaves and other vegetation, but some will eat shoots and roots of living plants. A few species are known to feed on animal remains or funguses. Many species will also eat their own waste pellets. It is believed that they obtain nutrition from funguses growing inside the pellets rather than from the waste itself.
Most millipedes lack a waxy layer on the outside of their ex-oskeletons, 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. Nar-
WHY DO BLIND MILLIPEDES HAVE THEIR OWN NIGHT LIGHTS?
Scattered in the mountains of central and southern California are millipedes that glow in the dark. These eight species of Motyxia represent the world's only bioluminescent (BI-o-LU-mih-NEH-sent) millipedes. Bioluminscent organisms produce their own light. All but their undersides glow bright white, causing them to resemble small glow sticks. They do not have eyes to see approaching predators, but their obvious glow might warn nighttime predators of their bad taste.
ceus 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.
Millipedes are an important, yet seldom appreciated, group of animals that break down dead plants and recycle them into food for other organisms. It has been estimated that they add two tons of manure per acre (0.40 hectares) of forest floor each year.
Sometimes millipedes damage gardens and crops by eating shoots and roots. In Japan thousands of Parafontaria laminata crushed by trains have resulted in the tracks becoming slick, causing railroad cars to lose traction.
Some people have strong allergic reactions to the defensive chemicals of millipedes. The defensive fluids of Spirobolus will stain and irritate human skin, whether or not the person is allergic to the chemicals.
No millipedes are considered endangered or threatened.
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