Every eight years during the fall, a plague of millipedes spreads across railway lines in mountainous Japan, earning the nickname ‘millipede train’.
Working together, these small animals (about 3 cm or 1.18 inches long) – which play a large role in nitrogen cycling in Japan’s larch forests – forced trains to skid to a stop.
Until now, scientists were not sure what was causing them to cluster with such peculiar 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 periodic animals known to have such a long life expectancy.
“This millipede needs seven years from egg to adult and another year to maturation,” writes the team in their new article.
“Thus, the eight-year periodicity of the P. la was confirmed by tracing the complete life history of eggs to adults in two different locations. “
We don’t know why cicadas appear at intervals of 13 and 17 years, but thanks to some incredible research, we now understand the eight-year life cycle of train millipedes.
The centipede train seethed. (Keiko Niijima)
Lead author and government ecologist Keiko Niijima began conducting observations on these millipedes in 1972, and two main sites were surveyed between one and five times a year for many of the years between then and 2016.
It was quite an operation, and when they arrived at the two locations in Monte. Yatsu and Yanagisawa, the work was also not exactly easy and fast.
“The soil to a depth of 0–5 cm was dug, spread on a sheet of polyethylene and the millipedes on the sheet were collected with tweezers or a vacuum cleaner”, explain the researchers.
“Then, the same procedure was repeated for depths of 5–10, 10–15 and 15–20 cm.”
Collecting whatever centipedes they found, they found that millipedes have seven stages (called instars) of growth, all of which remain in the ground and hibernate during winter and change in summer.
“Millipedes on the train change each summer in the summer and have seven larval instars,” wrote the researchers.
“They become adults in the eighth molt, eight years after the eggs are laid.”
(K. Niijima)
Then adults swarm on the surface in September and October, sometimes traveling up to 50 meters for fun before hibernating during the winter, and copulating again in late spring.
In August, the females laid 400 to 1,000 eggs and the adults died – ready for another eight-year generation.
As with cicadas, the millipede’s eight years are not all in sync everywhere.
In fact, the team suspects that there are seven litters in the mountainous region of Central Japan that have completed their life cycle in different years. That said, they don’t move much, so a particular train line will continue to have the same problem every eight to 16 years of a litter.
Looking at historical records dating back to the 1910s, researchers were able to assign almost all of the millipedes who swarmed to one of the seven litters.
“We show the existence of a periodic millipede, a new addition to periodic organisms with long life cycles: periodic cicadas, bamboo and some plants of the genus Strobilanthes“writes the team.
“Parafontaria laminata armigera it is the first record of periodic non-insect arthropods. “
With arthropods and insects making up a large 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 need to do is find them.
The research was published in Royal Society Open Science.