If I asked you to name an animal that could stop a train, you might guess something big, like an elephant or a bison.

Or you might guess a group of smaller animals, like a herd of deer or a flock of geese. But I’m going to need you to think smaller.

Like, much smaller. Because this is a story about how millipedes stopped a train. [♪♪ INTRO ♪♪] First, I need you to know that there’s one really specific thing that separates these millipedes from your average millipede and it has to do with their pretty unique life cycle, but we’ll get to that in a little bit. Normal millipedes range in length from a few millimeters to about 35 centimeters, and our train millipedes are totally normal in that respect.

They’re about 3.5 centimeters long. Many millipedes are also armed with toxic compounds that they use as chemical defenses against predators. And, again, our train millipedes seem like any other millipede there, except their toxic secretions are cyanide.

And most millipedes eat dead and decaying plant matter, and burrow through soil, aerating it, including our train millipedes. So, again, they seem like pretty typical millipedes. It’s also worth pointing out that millipedes are not the same thing as centipedes, which are their own kind of Bizarre Beast.

Some of the main differences between the two groups of arthropods are body shape, number of legs per body segment, and antenna length. Millipedes tend to have rounded bodies, two pairs of legs per body segment, and short antennae, while centipedes tend to have flattened bodies, a single pair of legs on each segment, and long antennae. And, as far as I know, no centipede has ever stopped a train.

But the so-called ‘train millipedes’ have, and yes, that is how they got that name. Our story starts in the central mountains of the Japanese island of Honshu, the largest and most populous of the country’s five main islands. Since 1920, several train lines that run through the forest there have periodically had to stop running when their tracks have become covered in millipedes, numbering in the millions.

They would appear in the fall, their small, light-colored bodies piling up on the train tracks and stopping traffic as they slowly made their way to new feeding grounds. This strange thing didn’t happen every year. The millipedes would swarm and then disappear for almost a decade.

And this just seemed like a strange quirk of the landscape for decades. The long gaps between the outbreaks, and the lack of good data about how long these millipedes lived or whether or not they were all the same species meant that no one spent much time looking for patterns in their emergence. World War II also obviously got in the way of people recording such insignificant problems as millipede outbreaks.

But in 1972, the millipedes caught the attention of a Japanese ecologist who began a field study that ran through 2016. The project involved surveying two sites near the affected train lines for millipedes, digging up the soil and identifying the species found there along with their life stages. And what they found was that train millipedes go through seven ‘instars’ or juvenile stages on their way to adulthood.

The researchers also combed through the track maintenance records of the Japanese National Railway, going back to 1920. And they noticed that every eight years, they were finding adult millipedes in their ongoing surveys, and the historical train records noted outbreaks happening at that same time interval - every eight years. Together, these two pieces of evidence pointed to the strange thing that makes these millipedes different from all other millipedes: they’re periodical, like cicadas.

Most of their lives are spent underground, except for every eight years when they emerge en masse to feed, mate, lay their eggs, and mess up the train schedule. And the reason they have this long, synced-up life cycle doesn’t seem to be the same

as it is for periodical cicadas. In cicadas, it’s proposed to be an anti-predator defense, sort of a “haha, you can’t eat us all!” kind of thing.

But, like we mentioned before, these millipedes secrete cyanide, so predators don’t want to eat them anyway. For the millipedes, their periodical life cycles might have more to do with temperature and central Japan’s particular geologic history. This part of the country became mountainous between 30,000 and 10,000 years ago, during the most recent ice age.

So the ancestors of the train millipedes had to adapt to a double-whammy of cold, both from the overall climate at the time and from a relatively sudden change in elevation. And they did! Another researcher had previously studied the train millipedes and found that they need around 80 days of 5 degrees Celsius temperatures to trigger molting between two of their instars and more than 60 days of similar temperatures to lay eggs.

And this, like their periodical life cycle, makes them unique among millipedes. Needing these long periods of cold temperatures to trigger molting and egg laying, along with the fact that they hibernate during the winter months, means that their life cycle got stretched out and they ended up on an eight-year swarming schedule. But, the thing is, we actually haven’t seen the millipedes swarm in large enough numbers to stop trains since 1984.

They still emerge every eight years, but the massive outbreaks have stopped. And that might be because the climate is changing, potentially disrupting the long cold periods they need to complete their life cycle, but more research is needed to confirm that. One thing is for sure though: if the train millipedes were to swarm again, the next time to keep an eye out for these Bizarre Beasts is September and October of next year, 2024.

The Bizarre Beasts pin club subscription window is open from now until the end of July 17th! Joining the club gets you this great millipede pin and helps us keep making the show. And we’re trying something a little different this month: shirts!

If you missed out on the millipede pin or you just want more millipedes in your life, you can get this month’s pin art on a t-shirt for a limited time – it’s only available this month, July 2023. Thank you for watching and we'll see you next month for more Bizarre Beasts [♪♪ OUTRO ♪♪]