Millipedes are incredibly successful and resilient. They’ve been in the business of vegetable waste management for the past 420 million years. Millipedes (members of the arthropod class Diplopoda) were the first animals to move onto land when land itself showed up on this planet in the Silurian Period. Thus far, millipedes are the oldest fossils of terrestrial animals we have found. But how do we know that these animals lived on land? The fossils clearly show stigmata: openings at the base of every one of their many legs that lead into internal air-filled tubes (called tracheae), bringing oxygen directly to the vital organs. The first food available on land was smelly, rotting vegetable-like matter. So, millipedes recognized an opportunity when they showed up on land and made a living of it.
Millipedes use their powerful jaws to eat their veggies. After chewing the veggies to bits, this material then passes through the millipede’s digestive system, which is equipped with fungi and nematode worms to assist in digestion. Millipedes are essential for soil health: by releasing processed organic matter at the hind end, they act as decomposers and help return nutrients back into soil.
Today, we know of just over 13,000 described millipede species (you can dig into the list in the online catalog MilliBase). They occur on every continent except today’s Antarctica. Over their long history, millipedes have traveled with the continents, wherever the continents happened to be. Millipedes perform important ecosystem services, especially in today’s deciduous forests, which could drown in their own leaf litter were it not for millipedes to chew the discarded leaves down. Their activities are part of a multi-step decomposition process that helps create a rich and thick humus layer in North American forests. This humus layer acts like a sponge that holds moisture for a long time, to water the trees in time of drought.
So, how have millipedes managed to survive for so long? Over time, they’ve evolved a lot of special features that helped their success. One suite of complex adaptations has led them to be, well, unpleasant—as in, unpleasant to eat. Most millipedes give off foul-smelling and bad-tasting secretions, produced by glands along each side of their bodies. Some of these secretions are cyanide; others are benzoquinones. South American monkeys and Malagasy lemurs pick up millipedes and squeeze them a bit so that the millipedes excrete their nasty chemicals. Rubbing these secretions all over their bodies, the monkeys and lemurs outsmart the mosquitoes, using a natural repellent. But the lemurs and monkeys are also smart enough not to eat these bad-tasting millipedes. In fact, it takes special courage (and special abilities) to eat millipedes. A few animals have succeeded in doing that, but we know very little about who preys on millipedes and how millipede populations are regulated and balanced.
We do know that the millipedes’ adaptations have helped them continue to perform important work over hundreds of millions of years, and we depend on them for their ability to recycle and renew the environment.