In the early 20th century, a train line opened for service in the mountains west of Tokyo. But in 1920, train crews found themselves stopping traffic for an unusual reason. The train tracks running through thick forest were overwhelmed by millipedes, each arthropod being white as a ghost. The creatures, which are not insects and emit cyanide when attacked by a predator, were on a mission that remained mysterious even after they fell into the dead leaves and soil.
The trains resumed operations and the millipedes were not seen again for a long time. But about a decade later, they reappeared as spirits rising from the ground and engulfing train tracks and mountain roads again. They seemed to follow this pattern over and over again.
Now, in a paper published Wednesday in the journal Royal Society Open Science, Dr. Niijima, Dr. Yoshimura and Momoka Nii, also from Shizuoka University, a detailed case that these millipedes, specifically the subspecies Parafontaria laminata armigera, are in fact periodic, the first time this behavior has been observed in a non-insect animal with a life cycle from birth to death that lasts eight years. However, they also report that the millipede is no longer swarming in numbers as large as before.
When the millipedes rise, they are on their way to new feeding areas, said Dr. Yoshimura. It is almost always adult adults who are seen on the go; when the creatures arrive at a fresh bed of decayed leaves to eat on, they eat, mate, lay eggs, and die.
Dr. Niijima and many of her colleagues who submitted reports of millipede emergence also carefully collected invertebrates from the ground near where swarms were seen. They hoped to confirm the time scale over which the millipedes evolved — if there were new young in the same place each year, it was likely that creatures were not periodic. But if they grew slowly over the years, it would fit better into the picture.
Over time, it became clear that they not only evolved over the course of eight years, but there were also several different sets or fry that lived their cycles in separate parts of the mountains. Researchers identified seven loaves – the event in 1920 was the rise in Bread VI, they write, which has been seen again almost every eight years ago. 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 swarm was recorded.
Periodicity in cicadas may have evolved during a period of global cooling to maximize mating opportunities, Drs. Yoshimura and collaborators reported in previous work with all available adults mixed at once. What circumstances caused the millipedes to adopt their own peculiar regularity is not yet clear, although it is noteworthy that all fry live at relatively high altitudes. Perhaps the extremes of a mountain lifestyle pushed them to periodicity.
However, one of the loaves has not been seen for many years. Others appear to shrink.
“We have not seen train obstacles for many years,” said Dr. Yoshimura. “Something is changing.”
He suspects that climate change may affect the life cycle of the millipede and notes that they appear to be emerging later in the year than they used to do. He also wonders if their declining numbers may be a deterioration of successful mating, accelerating their decline.
“We are still wondering what the main reason is for declining numbers,” he said.