Scientists Now Know Why Thousands Of Millipedes Mysteriously Swarmed Japanese Train Lines

The millipedes live on Honshu, which is the largest of Japan’s four main islands. As it turns out, they regularly cross the Hokuriku, Koumi and Chuo lines, which between them link some of Japan’s most important cities. These lines also pass through the mountains at the island’s center, where they’ve been reporting millipede-related delays on and off for a century.

Millipede invasions tend to happen in the fall, and for the brief period when they arrive it becomes impossible for the trains to move. Although they’re barely more than an inch long, when the millipedes rise from the earth in their thousands the swarm can span more than 650 feet. There’s nothing you can do but wait for them to pass.

It’s not just the numbers that are off-putting for train drivers. These millipedes are Japanese natives known as Parafontaria laminata armigera and they have their own special defense. You might associate cyanide with Agatha Christie novels, but this potentially lethal poison is also how these millipedes repel predators.

So there you have it: every so often thousands of ghostly white millipedes with bodies containing lethal toxins cover every inch of train tracks, nearby roads and the surrounding forest. Then they just disappear into the soil and under dead leaves. As strangely as they appear they vanish again and trains can finally resume their journeys. But why? What’s the reason?

For several years after the first reported outbreak in 1920 the trains continued to run and it was as though the millipede invasion had never happened. When one did reoccur, enough time had passed that it wasn’t obviously linked to the first. There may have been regular millipede swarms throughout the 20th century, but it took a long time for experts to realize they were following a pattern.

There were other things to distract scientists from spotting any regular habits in these millipedes’ appearances. Other swarms that weren’t part of the sequence may have been recorded, including those of different kinds of the bug. Confusion about the lifecycle of these ghost-like arthropods may also have made understanding difficult.

It was in the 1970s that a scientist by the name of Keiko Niijima took an interest in the so-called train millipedes. She worked at the Forestry and Forest Products Research Institute and was the first person to suspect their appearances might not be random. She started to collect information about the swarms, but it would still be decades before her theories could be confirmed.

In particular, Niijima thought that the millipedes might be following an eight-year life cycle. She contacted other researchers and asked for help in collecting these specific arthropods and studying their births, lives and deaths. This could then be combined with the historical data and previously conducted surveys to establish a more thorough picture of the creatures’ habits.

Eventually Niijima spoke to a biologist called Jin Yoshimura, who was based at Shizuoka University. Yoshimura wasn’t an expert in millipedes but he did know a lot about cicadas. The phases Niijima was proposing for the millipedes were similar to the ones cicadas experience. In fact, at that point cicadas were the only creature known to have that kind of life cycle.

Some cicadas can remain dormant underground for 13 or even 17 years before they emerge. Then they go through a swift cycle of birth and mating before they vanish. The world goes from no cicadas to countless numbers to none again in a very short span of time. Niijima saw similarities here with what she had observed in her millipede research.

Niijima and Yoshimura joined forces with another professor from Shizuoka University by the name of Momoka Nii, who is an expert in mathematical and systems engineering. They went out into the mountains of Honshu to collect millipedes and then started counting their body segments, as well as their legs.

It’s legs and body segments that allow you to tell the age of a millipede. In this particular analysis the scientists looked at two different broods, with one set coming from Mt. Yatsu and the other from Yanagisawa. Niijima had been examining these two sites regularly since the 1970s, so she had pretty good records of their history.

Between 1974 and 1980 Niijima had sorted dozens of millipedes by hand at Mt. Yatsu alone. This meant digging shallow squares of dirt, then sifting through the soil with aspirators and forceps to find the arthropods in question. She did this as many as five times a year, which can’t have been the most glamorous job. Still, it did give her lots of valuable information.

Similar studies were carried out at Mt. Yatsu in the 1980s and 1990s; at this stage the analysis was also extended to cover Yanagisawa and other sites. Special devices known as Tullgren apparatus and Berlese apparatus were used for extracting the millipedes from the soil once it had been laid on a polyethylene sheet. This involved a mesh sieve and metal plate that was lit by a lightbulb.

Next the tiny immature forms of the millipedes, which are known as nymphs, were placed under a microscope for identification. The researchers needed to make sure these were Parafontaria and not some other species. This meant comparing sizes and body parts to ensure they were the same kind of millipedes as those that had regularly been swarming the train lines and to see how they changed as they grew.

Think how butterflies go from larvae to caterpillars to cocoons before they reach their full-grown forms. Millipedes also have different stages, which are known as instars, in their life cycle. Eggs are laid in summer between the litter and soil on the forest floor, then hatch after about a month. The new millipedes will go through seven instars before they become adults.

Each larval phase before the millipedes reach adulthood will last about a year. Every summer on roughly the anniversary of their birth they will molt, but during the time in between they hibernate in the soil until it’s time for the next stage. Most of the lives of the young millipedes are spent hidden away.

Once they reach adulthood the millipedes become a little more active, though they still hibernate during winter. They emerge from their final molt at the end of summer and spend the fall getting ready to produce the next generation. When they wake up in spring they may mate a bit more before laying their eggs in the summer so the cycle can begin again.

Once they understood the train millipede’s life cycle, our intrepid trio of scientists needed to check exactly when and where swarms had occurred. The Prefectural Forest Research Center carries out regular patrols in the mountains, and it shared some of its observations. There’s even a website where scholars, teachers and members of the public can report their own experiences and research.

Also of great importance were the official records kept by the Japan National Railway. Daily maintenance reports included information on millipede obstructions in 1936, 1938, 1976 and 1984, some of which were also described in contemporary newspapers. The oldest data came from Niekawa Station in 1920, then Kiyosato Station in 1936. As with many of the outbreaks, they came in intervals of eight or 16 years.

It’s not only on the train lines and in the forests that the millipedes have been observed. Bodies of the creatures have been found piled on roads and in gutters. At one point in 1984 records showed the critters’ corpses were in densities yielding 768 millipedes in just 10 square feet. The arthropods keep coming back to the Koumi Line every eight years, but luckily they haven’t actually blocked a train since 1984.

Admittedly there were a few gaps and discrepancies in the records of millipede obstructions. There’s no official report of a swarm in 1944, but that may have been because of WWII. Meanwhile, some apparent appearances outside of the regular cycle may have been different instars, or even different species. There could also have been overlap between different millipede territories.

It was time to compare the known outbreaks of the millipedes with what the scientists knew about the Parafontaria life cycle. This would hopefully reveal why the swarms only appeared at certain times, and did so in such a predictable pattern. It would soon become clear just how closely linked the emergence of these arthropods was to their life cycles.

The scientists had tracked every known appearance of the arthropods. They had also observed millipede growth until they knew it took eight years for the creatures to reach full adulthood. It couldn’t be a coincidence that the gap between swarms and the length of the life cycle of the train millipede seemed to last the same amount of time.

In fact, it seems that the life cycle of the train millipede maps almost perfectly to the swarms that stopped the trains. The obstructions occur when the millipedes come of age and finally burst from the soil to feed and mate. Then they disappear again because they burrow back into the earth for protection in the winter cold. Once their eggs are laid their life cycle is at an end and they can die.

Train millipede life cycles follow this pattern because when they molt and lay eggs is so closely linked to surrounding temperatures. Without that time hibernating in the cold they can’t grow and develop. It’s a stark contrast to other species of millipede, some of which can molt multiple times in a single year.

The montane millipede, Ochogona caroli, is just one example of a similar arthropod which actually has a very different life cycle. It lives in Japan’s Central Alps, where it spends two years at 2,200 feet above sea level and three at 6,500 feet. By contrast, the eight years it takes the train millipede to reach full maturity is almost unheard of in nature.

Well, it’s unheard of in millipedes, but that doesn’t mean there aren’t other living things with similar habits. The most famous is the cicada, but there are also plant examples, such as bamboo. Train millipedes are part of a very elite club, and they still have some traits that are completely their own. Cicadas don’t rely on winter hibernation to dictate when they emerge. They swarm together so individuals are less likely to be caught by predators.

There are other species that swarm, such as the common millipede, Pleuroloma flavipes. They’re not periodical like our particular arthropods, though. There’s also some evidence that these seven-year-old train millipede instars may also swarm before they’re adults, which may be part of what confuses records. They’re the ones just a year away from maturity.

Once the train millipedes have reached adulthood they don’t travel far. It can be a journey of less than 200 feet at a crawl before the female mates and lays her eggs. There will be dozens of new millipedes waiting to be born by the time she dies soon afterwards. And it’s less that they invade train lines, than the lines have impinged on their territory.

Many of the routes built in the last century cross the millipedes’ feeding grounds. All they want to do when they swarm across human constructions is reach the dead leaves on the other side. These make a delicious banquet for a hungry arthropod and if the train tracks get in the way, then so be it.

They need to move quickly to find more food because they all emerge at the same time. New-born millipedes crawl from the earth together and immediately consume all the surrounding edibles. That means they need to find new sources of sustenance rapidly before they can start producing the next generation.

There’s also some variation because the millipedes come in different broods at different sites. These broods might be synchronized within their groups, but the Mt. Yatsu millipedes aren’t necessarily following the same schedule as the ones at Yanagisawa. That’s part of the reason there are discrepancies in human records of the swarms.

It also seems the regular cycle of emerging millipedes may not be as reliable as it used to be. Train obstructions are becoming rarer and rarer, with fewer arthropods swarming and at later times of year. One theory is that the temperature variations of climate change are stopping the creatures following their usual seasonal schedule.

And with millipedes emerging later in the year, that may affect the rest of their life cycle. Fewer such creatures each year means reduced mating opportunities, which may cause numbers to drop even more. The researchers aren’t sure if this is definitely the reason, but they are curious.

Fewer arthropods might be good news for the train staff and travelers who aren’t having their journeys interrupted, but it could have longer-term repercussions for the forest. Train millipedes have a direct impact on nitrogen levels, which in turn can adversely affect trees and other animals. The whole local ecosystem could be disrupted if they’re not there.

So far, one brood seems to have outright disappeared, whilst others are becoming smaller and smaller. There are other places in Asia where different species of arthropod may cause obstructions, but they’re not the train millipedes with their unique eight-year life cycle. For instance, tiny black millipedes were once seen swarming a subway in China.

Most millipedes don’t even live in the city. They prefer rural areas where they can burrow in the warm and damp soil. Many of the Chinese creatures died at that subway because they baked when exposed to the burning sun. The habits of the train millipede are special and unique, which makes their increasing rarity more worrying.

We may not know for sure what the future holds for the train millipedes, but at least we finally understand their behavior a little better. The mystery of the arthropod swarms has been solved at last and with it we’ve gained insight into a unique species. It’s changed our perspective on this part of the natural world.