Every eight years, in the fall, a plague of millipedes swarms train lines in mountainous Japan, earning them the nickname “millipede train”.
Working together, these small animals (about 3 cm or 1.18 inches long) – which play an important role in the nitrogen cycle in Japanese larch forests – forced the trains to stop.
Until now, scientists weren’t quite sure what made them swarm with such regularity, but a 50-year research project has finally confirmed that the species – Armored laminate fittings (P. la) – exists in a rare life cycle of eight years.
This confirmation is incredibly interesting, because cicadas are the only other periodic animals known to have such a long lifespan.
“This millipede needs seven years from egg to adult and another year to mature,” the team writes in their new paper.
“Thus, the periodicity of eight years a P. la was confirmed by tracking the complete history of life from eggs to adults in two different locations. “
We don’t know why cicadas appear at 13 and 17 years old, but thanks to incredible research, we now understand the eight-year life cycle of train millipedes.
The millipede train is swarming. (Keiko Niijima)
Leading author and government ecologist Keiko Niijima began commenting on these millipedes in 1972, and two major sites were researched one to five times a year for many of the years from then until 2016.
It was a pretty big operation, and when they got to the two sites at Mt. Yatsu and Yanagisawa, the job was not easy and fast either.
“The soil at a depth of 0-5 cm was dug, spread on a polyethylene sheet and the millipedes on the sheet were collected using tweezers or a vacuum cleaner,” the researchers explain.
“Then the same procedure was repeated for depths of 5-10, 10-15 and 15-20 cm.”
Collecting any millipedes they found, they discovered that millipedes have seven stages (called instars) of growth, which remain in the soil and hibernate in winter and then move in summer.
“The millipedes of the train make an outflow every summer and have seven larval plants,” the researchers write.
“They become adults until the eighth move after eight years of laying eggs.”
(K. Niijima)
Then, adults swarm to the surface in September and October, sometimes traveling up to 50 meters to become stiff before hibernating in winter and copulating again in late spring.
By August, the females had laid 400 to 1,000 eggs, and the adults had all died – ready for another eight-year generation.
As with cicadas, the eight years of the millipede are not synchronized everywhere.
In fact, the team suspects that there are seven seedlings in the mountainous region of central Japan that have completed their life cycle each in different years. That being said, however, it doesn’t move much, so a certain train line will continue to have the same problem every eight to 16 years from a brood.
Looking at historical records dating back to the 1910s, researchers have been able to assign almost every millipede reported to swarm to one of the seven young.
“We have shown the existence of a periodic millipede, a new addition to periodic organisms with long life cycles: periodic cicadas, bamboos and some plants of the genus Strobilanthes“, writes the team.
“Armored laminate fittings is the first record of non-insect periodic arthropods. “
Given that arthropods and insects make up a huge percentage of all animals on Earth and only a fifth are identified or named, there may be many longer periodic life cycles.
All we have to do is find them.
The research was published in Royal Society Open Science.