Every eight years during the fall, a plague of centipedes swarms train lines in mountainous Japan, earning them the nickname ‘train millipedes’.
Working together, these tiny bugs (about 3cm or 1.18 inches long) – who play a huge role in cycling nitrogen in Japan’s larch forests – have forced trains to a stop.
Until now, scientists weren’t quite sure what caused them to swarm with such peculiar regularity, but a 50-year research project has finally confirmed that the species – Parafontaria laminata armigera (P. la) – exists on a rare eight-year life cycle.
This confirmation is incredibly exciting, given that crickets are the only other periodic animals known to have such a long lifespan.
“This centipede takes seven years from egg to adult and another year to mature,” the team wrote in their new paper.
“So is the eight-year periodicity of P. la was confirmed by tracing the complete life history from eggs to adults at two different locations. “
We don’t know why crickets appear at intervals of 13 and 17 years, but thanks to incredible research, we now understand the eight-year life cycle of the train millipedes.
The train is crawling with centipedes. (Keiko Niijima)
Lead author and government ecologist Keiko Niijima first began conducting observations of these millipedes in 1972, and two main sites were surveyed between one and five times a year between then and 2016 for much of the years between then and 2016.
It was quite an operation, and when they got to the two locations on Mt. Yatsu and Yanagisawa, the job was not exactly easy and fast.
“The soil was dug to a depth of 0–5 cm, spread on a polyethylene sheet and the centipedes on the sheet were collected with forceps or an aspirator,” the researchers explain.
“Then the same procedure was repeated for 5–10, 10–15 and 6–20 cm depths.”
They collected all the millipedes they found and found that the centipedes have seven stages (so-called stages) of growing up, all of which remain in the ground and hibernate during the winter and then molt in the summer.
“The centipedes of the train molt every year in the summer and have seven larval stages,” the researchers write.
“They reach maturity by the eighth moult at eight years after the egg deposit.”
(K. Niijima)
Then the adults swarm to the surface in September and October, sometimes traveling up to 50 meters to become frisky before hibernating in winter, then mate again in late spring.
In August, the females laid 400 to 1,000 eggs and the adults have all died – ready for a new eight-year-old generation.
As with crickets, the eight years of the centipede are not synchronized everywhere.
In fact, the team suspects that there are seven brothers in the mountainous region of central Japan who each completed their life cycle in different years. That said, they don’t move much, so any given train line will continue to have the same problem from one brood every eight to 16 years.
Looking at historical records dating back to the 1910s, the researchers were able to attribute nearly every reported millipede swarm to one of the seven broods.
“We have demonstrated the existence of a periodic centipede, a new addition to periodic organisms with a long life cycle: periodic crickets, bamboo and some plants in the genus Strobilanthes‘the team writes.
“Parafontaria laminata armigera is the first record of periodic arthropods that are not insects. “
With arthropods and insects making up a huge percentage of all animals on Earth, and only one-fifth identified or named, there are likely to be many more long periodic life cycles.
We just have to find them.
The research is published in Royal Society Open Science.