While hiking with my son on Snake Mountain in Addison County, Vermont, I noticed a rather impressive millipede on the trail. When I think “millipede,” I usually think about small, slow-moving, cylindrical invertebrates that frequent moist, leafy areas including my compost pile. The individual on the trail fit the bill in every way other than size. This millipede was nearly four inches long and as thick as a pencil. In eastern North America, there’s just one millipede of such proportions: Narceus americanus, the American giant millipede.
Millipedes can be distinguished from their cousins the centipedes by counting the legs on any segment on the creature’s fifth or later segment: centipedes have just one pair per segment, two pairs confirm you are looking at a millipede. If meticulous leg counting is not for you, there is an even easier way to separate these relatives: centipedes are faster.
Millipedes use their many legs to methodically move forward, backward, or side-to-side. The pairs of legs move in sync, like paired oars in a boat, which earned them an older name: “galley worms.” From front to back, each pair of legs moves just a little after the pair in front, creating a wave-like movement of the legs. Several such stadium-like waves moving at the same time produce a smooth glide that slowly gets the millipede where it needs to go.
Centipedes need speed because they make their living by chasing down and eating other invertebrates. Millipedes, on the other hand, eat decaying organic matter. They are important in breaking down leaf litter on forest floors, gleaning what nutrition they can from the leaves. In places lacking earthworms, millipedes can be the most important recyclers of leaves.
The giant millipede forgoes leaves, dining exclusively on rotting logs. Wood is essential for other parts of its life cycle, too. Females lay their eggs one at a time and wrap each in masticated wood to form a protective capsule that retains moisture. Reproduction starts in late spring and continues into August, by which time a female may have laid as many as 50 eggs, staying with each one until it hatches before moving on to repeat the process.
Like all arthropods, millipedes must shed their entire exoskeleton to grow. Molting leaves them vulnerable to predation because the new exoskeleton starts out soft and hardens up over time. For protection during the annual August molt, a millipede digs a chamber inside rotting wood, seals itself in, and molts in the privacy of its own wood-paneled, plush accommodations.
Because Narceus americanus depend so critically on snags and fallen logs, they tend not to show up in early successional woodlands dominated by smaller trees; you may have more luck finding them in well-established forests that have had time to accumulate an abundant supply of wood in various stages of decay.
Millipedes use an effective approach to tunneling that is more like bulldozing than digging: they synchronize their many pairs of legs to push in the same direction. The force exerted by all of these legs would telescope a lesser arthropod, but millipedes have an exquisite adaptation to prevent this crushing experience. Millipede segments plug each into the next like a series of ball-and-socket joints. Pressure from behind pushes the ball shape at the front of each segment into the socket of the one ahead of it. As this slow, patient pressure displaces some rotting wood, the millipede continually eats on the job, using strong mandibles to consume additional wood. In time, a tunnel results.
A millipede’s lack of speed could, of course, increase its chance of becoming a meal for some fellow traveler. But millipedes also have solutions to that particular problem: they use chemical warfare. The list of defensive chemicals used by millipedes varies by species and includes hydrochloric acid, quinones, alkaloids, and even cyanide. The bitter taste and irritating characteristics of these chemicals deter predators. In rare cases, prolonged exposure to millipedes (for example if one became entangled in clothing) can cause a painful skin condition called millipede burn.
Unless you intend to pocket a large millipede, or perhaps slip a few into your sock, you will be entirely safe from these gentle creatures. They don’t bite, and when threatened they curl into a flat spiral with their head in the center and the legs protected, leaving just the tougher dorsal surface of their segments facing outward. So if you happen upon one of these giants of the invertebrate world during your next hike, perhaps place it off trail in its direction of travel. And wash your hands; that’s always good advice.
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