Home | Newsletter | Mostly Millipedes

MOSTLY MILLIPEDES
by Ari Kornfeld

Myriapods (many limbs) -- millipedes and centipedes -- occupy the leaf litter and topsoil of both forest and backyard. These close relatives of insects look like segmented worms with jointed legs on each segment. As you might guess from their looks, myriapods are evolutionarily between segmented worms (see Nature of the Northcoast, Winter 2001) and insects. Like worms, myriapods have many body segments, most of which look alike. Like insects, myriapods have rigid exoskeletons that must be shed periodically in order to grow. Unlike insects, however, the exoskeleton is not covered with a waxy, waterproof, cuticle thus forcing myriapods to live in a damp environment in order to avoid desiccation.

Millipedes (Diplopoda, i.e., two limbs) and centipedes (Chilopoda, i.e., limbs on the margin) seem very similar at first glance. Both can be found under rocks, forest litter, or garden clippings and both live several years from egg through maturity. Beyond this superficial resemblance, however, they are quite different.

Millipedes have two pairs of legs on most body segments, are detritovores (feeding on dead plant material), move slowly, and are shy -- curling up to hide when disturbed. Centipedes (left), on the other hand, are quick-moving predators with only one pair of legs per segment and with two straight "tails" sticking out of the last segment. They will bite you with venomous fangs if you pick them up. While the bite of our local centipedes feels like a bee-sting but some centipede bites can be extremely damaging or even fatal.

Millipedes are not entirely defenseless either but instead of overt aggression, they resort to chemical warfare. A threatened millipede secretes toxic chemicals such as cyanide, benzoquinones and alkaloids. Smell your hands after you've handled a millipede and you may detect the almondy odor of cyanide. This defense is definitely effective. I once watched a robin happily prospecting for worms in our compost heap until he came upon a millipede. After several attempts to carry it away, the robin spit the millipede out and flew off. The millipede crawled back into the ground not much worse for the wear. Nevertheless, millipedes do have many predators, from microscopic parasites to beetles and vertebrates.

Millipedes come in three basic shapes: cylindrical, somewhat flattened, and bristly. Our most notable local millipedes, all of the order Polydesmida, appear to have flat backs because of plates projecting on both sides of each segment.

Of these, the yellow-spotted millipede (Harpaphe haydeniana and H. telodonta, right) is most often seen. Its dark body, 2-3 inches long, with yellow spots at the tip of each plate make it quite distinctive. While digging through the soil in my garden I have found dozens of pale immature Harpaphe, their yellow spots barely visible without the dark background. During the summer months, as the top layers of soil dry out, millipedes migrate deeper below the surface, their many legs giving them extra power to burrow through the soil. During the wet winter they move back toward the surface. In spring swarms of mating Harpaphe may be seen in places such as Prairie Creek Redwoods State Park. Although the exact cause of these great aggregations is not known, Nancy Baumeister -- a PhD candidate studying millipedes at Oregon State University -- suggests that they are pheromone-induced mating swarms.

Another local polydesmid, (probably Kepolydesmus pungo) is colored burnt-orange. Its color, together with its flattened appearance, makes it easily mistaken for a centipede.

The Sierra Nevada is home to bioluminescent millipedes of the polydesmid genus Motyxia (aka Luminodesmus). M. sequoiae looks very much like a drab version of the previous two polydesmids until you see it at night. It glows blue-green in the dark!

The bristly millipede Polyxenus sp. (left) of the order Polyxenida is the most unusual of our local millipedes. Though not uncommon, it is practically unheard of because of its habitat and tiny size. Instead of living in the ground, these tiny animals live in the bark of trees such as redwoods where they feed on algae or decayed bark. Although no more than 4 mm long (1/6^th inch) the bristly millipede is intricately ornamented. Each segment sports two rows of serrated bristles. Bundles of detachable barbed bristles protrude from the tail end. When attacked, the millipede will turn and brush the predator, trapping the attacker in a tangle of interlinked bristles and allowing the millipede to escape.

For more information and photos of myriapods (and their look-alike kin isopods) check out our photo gallery.