At the beginning of the 20th century, a train line was opened in the mountains west of Tokyo. But in 1920, train crews stopped running for an unusual reason. The tracks running through the dense forest were littered with swarms of centipedes, each arthropod white as a ghost. The non-insect creatures, which emit cyanide when attacked by a predator, were on some order that remained mysterious even after they had subsided in the dead leaves and soil.
The trains resumed, and the long-standing millipedes could not be seen again. But about a decade later, they reappeared as ghosts rising from the ground, engulfing train tracks and mountain roads once again. They seemed to follow this pattern over and over again.
Now, in an article published Wednesday in the journal Royal Society Open Science, Dr. Niijima, Dr. Yoshimura and Momoka Nii, also from Shizuoka University, present a detailed case that these centipedes, in particular the subspecies Parafontaria laminata armigera, are indeed periodic, this behavior is first 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.
When the millipedes rise, they are on their way to new places to eat, Dr. Yoshimura said. Adults are almost always seen on the move; when the creatures arrive in a fresh bed of decaying leaves to feed, they feed, mate, lay eggs, and die.
Dr. Niijima and many of her colleagues who reported the appearance of a centipede also carefully collected invertebrates from the soil near places where swarms were observed. They hoped to confirm the time scale in which millipedes evolve – if there are new young in the same place every year, the creatures are unlikely to be periodic. But if they grew slowly over the years, it would fit the picture better.
Over time, it became clear that they not only evolved over eight years, but there were also many different sets or chickens that lived in their cycles in different parts of the mountains. Researchers have identified seven chickens – they write, the event of 1920 is the sunrise of Brud VI, which has since been seen again almost every eight years. The only gap in Brood VI’s record was in 1944, when the disorder after the defeat of Japan in World War II meant that no swarm was registered.
The frequency of cicadas may have developed during global cooling to maximize mating opportunities, Dr. Yoshimura and colleagues reported in earlier work, with all available adults mixed at once. It is not yet clear what circumstances led the millipedes to adopt their particular regularity, although it is remarkable that all chickens live at relatively high altitudes. Perhaps the extremes of the mountain way of life pressed them to periodicity.
However, one of the chickens has not been seen for many years. Others seem to be declining.
“We haven’t seen any obstacles for trains in many years,” said Dr. Yoshimura. “Something is changing.”
He suspects that climate change could affect the life cycle of millipedes, noting that they appear to appear later in the year than before. He also wonders if their declining numbers could be a deterioration in successful mating, accelerating their decline.
“We are still wondering what is the main reason for the decrease,” he said.