At the beginning of the 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 railroad tracks, which ran through thick forest, were overrun with swarms of centipedes, each arthropod as white as a ghost. The creatures, which are not insects and emit cyanide when attacked by a predator, performed on a race that remained mysterious even after sinking into the fallen leaves and soil.
Trains have resumed service, and the centipedes have not been seen for a long time. But about a decade later, they reappeared as spirits rising from the earth, again engulfing railroads and mountain roads. They seemed to follow this pattern over and over again.
Centipedes fascinated Keiko Niijima, a government scientist who began working in the mountains in the 1970s. During her career, she gathered reports on their emergence 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 of Japan who studies periodic cicadas. These insects break out to mate and die in large numbers every 13 or 17 years. She wanted to work with Dr. Yoshimura on the idea that the train centipedes could do something similar.
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, especially the sub- species Parafontaria laminata armigera, are indeed periodic, the first time that 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 centipedes are no longer swarming in as many numbers as they once did.
When the centipedes get up, they head to new feeding grounds, Dr Yoshimura said. These are almost always adults spotted in motion; when the creatures arrive at a new bed of decaying leaves for food, they eat, mate, lay eggs, and die.
Dr Niijima and many of his colleagues who submitted reports of centipede emergence also carefully collected invertebrates from the soil near where the swarms had been observed. They hoped to confirm the timescale on which the centipedes grew – if there were new juveniles every year in the same location, the creatures probably wouldn’t be periodic. But if they grew slowly over the years, it would fit the picture better.
Over time, it became clear that not only did they develop over the course of eight years, but there were also several different sets, or broods, living their cycles in separate parts of the mountains. Researchers identified seven broods – the event of 1920 was the uprising of Brood VI, they write, which has been spotted again almost every eight years since. The only gap in Brood VI’s record is in 1944, when the unrest leading up to the end of World War II meant that no swarms were recorded.
The periodicity of cicadas may have evolved during a period of global cooling to maximize mating opportunities, reported Dr Yoshimura and colleagues in previous work, with all available adults mingling at the same time. It is not yet clear what circumstances led centipedes to adopt their own regularity, although it should be noted that all broods live at a relatively high altitude. Perhaps the extremes of a mountain lifestyle have pushed them to periodicity.
However, one of the broods has not been seen for many years. Others seem to be shrinking.
“We haven’t seen train obstructions for many years,” said Dr Yoshimura. “Something is changing.”
He suspects that climate change may affect the life cycle of centipedes, noting that they appear to emerge later in the year than before. He also wonders if their decreasing numbers may hinder mating success, hastening their decline.
“We are still wondering what the main reason for the drop in numbers is,” he said.