Every eight years during the fall, a plague swarms with millipede train lines in mountainous Japan and earns them the nickname ‘train millipede’.
When we work together, these small beasties (about 3 cm or 1.18 inches long) – which play a major role in cycling nitrogen in Japan’s larch forests – have forced trains to stop.
Until now, scientists were not entirely sure what made them swarm with such a special regularity, but a 50-year research project has finally confirmed that the species – Parafontaria laminata armigera (P. la) – exists in a rare eight-year life cycle.
This confirmation is incredibly exciting as cicadas are the only other known periodic animals with longevity for so long.
“This millipede needs seven years from egg to adult and another year to maturation,”
“Thus the eight-year periodicity of P. la was confirmed by tracing the complete life history from eggs to adults in two different locations. “
We do not know why cicadas occur at 13- and 17-year intervals, but thanks to some incredible research, we now understand the eight-year life cycle of the train millipede.
Lead author and government ecologist Keiko Niijima first began making observations of these millipedes in 1972, and two capitals were surveyed between once and five times a year for many of the years between then and 2016.
It was a whole operation, and when they got to the two places at Mt. Yatsu and Yanagisawa, the job was not exactly easy and fast either.
“The soil to a depth of 0-5 cm was excavated, spread on a polyethylene sheet, and the millipedes on the sheet were collected using tweezers or an aspirator,” the researchers explain.
“Then the same procedure was repeated at 5-10, 10-15 and 15-20 cm depths.”
By collecting any millipedes they found, they discovered that the millipedes have seven phases (called instars) to grow up, all of which remain in the ground and hibernate in the winter and then melt in the summer.
“The millipede makes a melting in the summer every year and has seven larval installations,” the researchers write.
“They grow up by the eighth melting after eight years of egg deposition.”
Then the adults swarm on the surface in September and October, sometimes traveling up to 50 meters to become bubbly before going to sleep in the winter and copying again in late spring.
By August, the females have laid 400 to 1,000 eggs, and the adults are all dead – ready for another eight-year-old generation.
As with cicadas, the eight years of the millipede are not all synchronized everywhere.
In fact, the team suspects that there are seven fry over the mountainous region of central Japan that completed their life cycle each in different years. That said, they do not move much, so a particular train line will continue to have the same problem every 8 to 16 years from a juvenile.
Looking at historical records dating back to the 1910s, researchers were able to attribute almost every reported millipede swarming to one of the seven fry.
“We have shown the existence of a periodic millipede, a new addition to periodic organisms with long life cycles: periodic cicadas, bamboo and some plants in the genus Strobilanthes, “writes the team.
“Parafontaria laminata armigera is the first registration of periodic non-insect arthropods. “
With arthropods and insects making up a huge percentage of all animals on earth, and only a fifth having been identified or named, there are likely to be many more long periodic life cycles out there.
All we have to do is find them.
The research is published in Royal Society Open Science.