At the beginning of the 20th century, a railway line was opened in the mountains west of Tokyo. In 1920, for unusual reasons, the train crews found they were stopping traffic. The railroad tracks that ran through dense forest were overwhelmed by swarms of millipedes, each arthropod as white as a ghost. The non-insect creatures that give off cyanide when attacked by a predator were on an assignment that remained puzzling even after sinking into the dead leaves and the ground.
The trains were put back into service and the millipedes were not seen for a long time. But about a decade later they reappeared like ghosts rising from the earth and devouring the railroad tracks and mountain roads again. They seemed to follow this pattern over and over again.
The centipedes fascinated Keiko Niijima, a government scientist who started working in the mountains in the 1970s. Throughout her career, she has collected reports of their formation and coordinated other researchers to collect centipedes throughout their life cycle. A few years ago, she contacted Jin Yoshimura, a mathematical biologist at Shizuoka University in Japan who studies periodic cicadas. These insects broke out every 13 or 17 years to mate and die in enormous numbers. She wanted to talk to Dr. Yoshimura are working on the idea that the millipedes could do something similar.
Now in An article published in the Royal Society Open Science on Wednesday, Dr. Niijima, Dr. Yoshimura and Momoka Nii, also from Shizuoka University, present a detailed case that these millipedes, particularly the subspecies Parafontaria laminata armigera, are indeed periodic, the first time this behavior has been observed in a non-insect animal with a life cycle from birth to death, which lasts eight years. However, they also report that the millipedes are no longer as numerous as they were before.
When the millipedes get up, they’re on their way to new feeding grounds, said Dr. Yoshimura. It is almost always adult adults who are seen on the go. When the creatures arrive to a fresh bed of decaying leaves to feed, the creatures will eat, mate, lay eggs, and die.
Dr. Niijima and many of her colleagues who provided reports of the occurrence of millipedes also carefully collected invertebrates from the ground near the flocks. They hoped to confirm the timescale over which the millipedes evolved – if there were new juveniles in the same place every year, the creatures probably weren’t periodic. But if they grew slowly over the years, that would fit the picture better.
As time went on, it became clear that not only did they evolve over the course of eight years, but that there were also several different groups or breeds that lived their cycles in separate parts of the mountains. The researchers identified seven broods – the event of 1920 was the rise of Brood VI, they write, which has been rediscovered almost every eight years since. The only loophole in the record of Brood VI is in 1944, when the disruption resulted in no swarm being registered after Japan’s defeat in World War II.
Dr. Yoshimura and coworkers have reported that the periodicity of cicadas evolved during a period of global cooling to maximize mating opportunities in previous work, mix with all available adults at once. What circumstances caused the centipedes to adopt their own regularity is not yet clear, although it is noteworthy that all broods live at relatively high altitudes. Perhaps they pushed the extremes of a mountain lifestyle towards periodicity.
However, one of the broods has not been seen for many years. Others seem to be shrinking.
“We haven’t seen a train obstruction in many years,” said Dr. Yoshimura. “Something is changing.”
Suspecting that climate change could affect the centipede’s life cycle, he notes that they appear to develop later in the year than earlier. He also wonders if their decreasing numbers can detract from the successful mating and accelerate their decline.
“We’re still wondering what is the main reason the numbers are falling,” he said.