In the early 20th century, a railway line opened up mountains in western Tokyo. But in 1920, train crews stopped traffic for an unusual reason. The train tracks, running through thick bushes, were overwhelmed by millipedes that each arthropod was white as a ghost. The creatures, which are not insects and emit cyanide when attacked by a predator, were on some message that remained mysterious even after they sank into the dead leaves and soil.
The trains are back in service, and the centipedes have not been seen for a long time. But about a decade later, they reappear as spirits rising from the earth, engulfing train tracks and mountain roads. They seem to be following this pattern over and over again.
The millipedes fascinated Keiko Niijima, a government scientist who started working in the mountains in the 1970s. Over the course of her career, she has collected reports on their origins and coordinated other researchers to gather millipedes during their life cycle. A few years ago, she contacted Jin Yoshimura, a mathematical biologist at Shizuoka University in Japan who studies periodic cockroaches. Those insects erupted to mate in large numbers every 13 or 17 years and die. She wanted to work with Dr Yoshimura on the idea that the centipede train would possibly do something similar.
In an article published on Wednesday in the journal Royal Society Open Science, dr. Niijima, dr. Yoshimura and Momoka Nii, also from Shizuoka University, point out in detail that these millipedes, specifically the subspecies Parafontaria laminata armigera, are indeed periodic. the first time this behavior is observed in an animal that does not have insects, with a life cycle from birth to death lasting eight years. However, they also report that the centipedes are no longer as large as before.
When the millipedes rise, they are on their way to new feeding areas, said dr. Yoshimura said. It is almost always mature adults who are traveling; when the creatures arrive at a fresh bed of decayed leaves to eat, they eat, mate, lay eggs and die.
Dr. Niijima and many of her colleagues who submitted reports of centipede emergence also carefully collected invertebrates from the ground near where swarms were seen. They hoped to confirm the time scale within which the millipedes evolved – if there were new juveniles in the same place every year, the creatures would probably not be temporary. But if they had grown slowly over the years, it would have better suited the picture.
Over time, it became clear that not only did they develop over the course of eight years, but that there were also different sets or broods that lived out their cycles in different parts of the mountains. The researchers identified seven hatcheries – the event in 1920 was the rise of Bread VI, they write, which has since been re-observed almost every eight years. The only gap in Bread VI’s record is in 1944, when the disorder after the defeat of Japan in World War II meant that no swarm was recorded.
Temporality in cicadas may have evolved during a period of global cooling to maximize mating opportunities, Drs. Yoshimura and co-workers reported in earlier work, while all available adults mixed at the same time. What the circumstances led to the millipedes adopting their own peculiar regularity is not yet clear, although it is noteworthy that all the broods are relatively high. Perhaps the extremes of a mountain style forced them to periodicity.
However, one of the hatchlings has not been seen for many years. It seems like others are shrinking.
“We have not seen any train barriers for many years,” said Dr. Yoshimura said. “Something changed.”
He suspects that climate change may affect the life cycle of the millipedes, noting that it appears to appear later in the year than earlier. He also wonders if their declining numbers could harm the successful mating, which will accelerate their decline.
“We’re still wondering what the main reason is for the decline in numbers,” he said.