Millipede Mystery: Scientists Discover Why The Arthropods Swarm Tracks Every 8 Years

A century-old mystery of millipedes' behavior has been solved, thanks to a team of researchers who spent years studying the unique creatures.

There have been reports of "train millipede" outbreaks in Japan since 1920. During the outbreaks, thousands of poisonous millipedes swarm the railway tracks in Honshu, Japan, obstructing train operations. What's strange about these arthropods is that they disappear for years after swarming the tracks, only to resurface after about eight years.

In 1977, Keiko Niijima, a researcher at the Forestry and Forest Products Research Institute proposed that the millipedes responsible for the swarms might have an eight-year periodicity, Live Science reported. Now, Niijima and her teammates, Momoka Nii and Jin Yoshimura, both from Shizuoka University, have finally confirmed that the millipedes' life cycle lasts eight years.

As part of their study, the researchers spent years going back to the locations of the outbreaks to collect specimens of the millipedes, which are endemic to Japan.

"We finally confirmed the 8-year periodicity of this millipede using detailed surveys of life histories over 8 years," the researchers wrote in their study, published in Royal Society Open Science. "This is the first confirmed case of periodical non-insect arthropods."

The study explains why millipedes just suddenly disappear and then appear again after some years. Starting from the egg, the train millipedes go through seven larval instars or phases of development, with the creatures molting in the summer every year, the researchers said. They then stay in the soil from the second to the seventh instars.

It's the adult train millipedes that swarm in autumn, although those in the seventh instar, or the last stage before adulthood, may also swarm. The adult millipedes feed, hibernate, mate and produce a new batch of eggs before dying. Then, the eight-year cycle will start all over again.

"In summary, this millipede needs 7 years from egg to adult and one more year for maturation," the researchers wrote, adding that they are not emerging in mass numbers to preventing attacks from predators as they have quite an impressive defense mechanism. They release cyanide when they are attacked.

Sometimes, the millipedes inadvertently disrupt train services just because their next destination is on the other side of the tracks, study co-author Yoshimura told Live Science.

Some other insects also exhibit similar behaviors. In the U.S., periodical cicadas spend most of their lives burrowing in the ground and then emerge from the soil every 13 or 17 years, depending on the species.

In May 2020, for instance, experts expected as many as 1.5 million cicadas to emerge after over a decade of burrowing, posing a problem for some but a unique spectacle for others.