Every eight years during the fall, a plague of millipedes invades train lines in mountainous Japan, giving them the nickname “millipede of trains.”
Working together, these small beasts (about 3 cm or 1.18 inches long) – which play an important role in nitrogen cycling in Japan’s larch forests – have forced trains to stop.
Until now, scientists weren’t entirely sure what made them swarm with such peculiar peculiarity, but a 50-year research project has finally confirmed that the species … Armored laminate fittings (P. la) – exists in a rare eight-year life cycle.
This confirmation is incredibly exciting, as cicadas are the only other periodic animals known to have such a long shelf life.
“This millipede takes seven years from egg to adult and another year to mature,” the team writes in its new paper.
“Therefore, the periodicity of eight years of P. la it was confirmed by tracing the entire life history from eggs to adults in two different places. “
We don’t know why cicadas appear at 13- and 17-year-old intervals, but thanks to incredible research, we now understand the eight-year life cycle of train millipedes.
The millipede train swarming. (Keiko Niijima)
Government lead author and ecologist Keiko Niijima began making observations on these millipedes in 1972 and examined two major sites between one and five times a year for many of the years from then until 2016.
It was quite an operation, and when they reached the two places on Mt. Yatsu and Yanagisawa, the work was not easy or quick either.
“The soil to a depth of 0-5 cm was excavated, spread on a polyethylene sheet and the millipedes of the sheet were collected with tweezers or a vacuum cleaner,” the researchers explain.
“Then, the same procedure was repeated for 5-10, 10-15 and 15-20 cm depth.”
Collecting the millipedes they found, they discovered that millipedes have seven stages (called instars) of growth, which remain in the ground and hibernate during the winter and then move in the summer.
“Train millipedes make a moult every summer and have seven larval instars,” the researchers write.
“They become adults by the eighth moult after eight years after laying eggs.”
(K. Niijima)
The adults then swarm to the surface in September and October, sometimes traveling up to 50 meters to reach winter before hibernating during the winter and copulating again in late spring.
In August, females have laid 400 to 1,000 eggs and all adults have died, ready for another eight-year-old generation.
As with the cicadas, the eight years of the millipede are not synchronized everywhere.
In fact, the team suspects that there are seven cubs in the mountainous region of central Japan that have completed their life cycle each in different years. That said, however, they don’t move much, so a particular train line will continue to have the same problem every eight to 16 years of a brood.
Looking at historical records dating back to the 1910s, researchers were able to attribute almost every informed millipede swarming to one of the seven pups.
“We have demonstrated 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 periodic arthropods that are not insects. “
With arthropods and insects making up a huge percentage of all animals on Earth and only one-fifth have been identified or named, there are likely to be many longer periodic life cycles.
All we have to do is find them.
The research has been published in Royal Society Open Science.