In the early 20th century, a train line was opened for service in the mountains west of Tokyo. But in 1920, train crews stopped traffic for an unusual reason. The train tracks, which ran through the dense forest, were dominated by swarms of centipedes, each arthropod as white as a ghost. The creatures, which are not insects and emit cyanide when attacked by a predator, were on some mission that remained mysterious even after falling on dead leaves and soil.
Trains resumed service and the millipedes were not seen again for a long time. But about a decade later, they reappeared like spirits rising from the earth, engulfing train tracks and mountain roads once again. They seemed to follow this pattern over and over.
Millipedes fascinated Keiko Niijima, a government scientist who started working in the mountains in the 1970s. Throughout her career, she collected reports of its emergence and coordinated other researchers to collect centipedes throughout its life cycle. A few years ago, she contacted Jin Yoshimura, a mathematical biologist at Japan’s Shizuoka University who studies periodic cicadas. These insects erupt to mate and die in enormous numbers every 13 or 17 years. She wanted to work with Dr. Yoshimura on the idea that millipedes on the train could be doing something similar.
Now, in an article published Wednesday in the Royal Society Open Science magazine, Dr. Niijima, Dr. Yoshimura and Momoka Nii, also from Shizuoka University, present a detailed case that these millipedes, specifically the subspecies Parafontaria laminata armigera, are in fact periodicals, the first time this behavior was observed in a non-insect animal, with a life cycle from birth to death that lasts eight years. However, they also report that millipedes are no longer swarming in numbers as large as before.
When millipedes rise, they are on their way to new eating areas, said Yoshimura. Adult adults are almost always seen in motion; when the creatures reach a new bed of decaying leaves to feed, they eat, mate, lay eggs and die.
Dr. Niijima and many of her colleagues who sent emergency reports from millipedes also carefully collected invertebrates from the soil near where the swarms were seen. They hoped to confirm the time scale on which millipedes were developing – if there were new juveniles each year in the same place, the creatures would probably not be periodic. But if they were growing slowly over the years, that would fit the picture better.
Over time, it became clear that not only were they developing over eight years, but there were also several different sets, or litters, living their cycles in separate parts of the mountains. The researchers identified seven broods – the 1920 event was the birth of brood VI, they write, which has been seen again almost every eight years since then. The only gap in Brood VI’s record is in 1944, when the disorder after Japan’s defeat in World War II meant that no swarms were recorded.
The periodicity in cicadas may have evolved during a period of global cooling to maximize mating opportunities, Dr. Yoshimura and colleagues reported in previous work, with all available adults mixing at the same time. It is not yet clear what circumstances led the millipedes to adopt their own peculiar regularity, although it is remarkable that all litters live at relatively high altitudes. Perhaps the extremes of a lifestyle in the mountains would put pressure on them for periodicity.
However, one of the litters has not been seen for many years. Others seem to be shrinking.
“We haven’t seen any obstructions on trains for many years,” said Dr. Yoshimura. “Something is changing.”
He suspects that climate change may be affecting the life cycle of millipedes, noting that they appear to be emerging later in the year than before. He also wonders whether decreasing their numbers can hamper successful mating, accelerating their decline.
“We are still wondering what the main reason for the decrease in numbers is,” he said.