Scientists have determined the swarms of thousands of millipedes that infest parts of Japan aren't random.
The arthropods, which gather in clusters so large they can delay train traffic, are on a unique eight-year life cycle.
This makes the 'train millipedes' the only other periodical animal besides cicadas, and the only non-insect.
Researchers uncovered the creepy crawler's strange stages by studying broods in two parts of central Japan for nearly 50 years.
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Massive swarms of millipedes have been known to disrupt traffic in Japan every few years for at least a century. Now researchers at Shizuoka University have determined the athropods operate on a unique eight-year life cycle
Ecologist Keiko Niijima has been gathering data on millipedes in the mountainous areas of Central Japan since the 1970s but reports of swarms blocking trains in the region date back at least to the 1920s.
They'd seem to surge once every handful of years and then recede, like a slithering tide.
Niijima found evidence of a brood surfacing every eight years — except 1944, when World War II meant no reliable records were kept.
She partnered with Shizuoka University biologist Jin Yoshimura who studies periodical cicadas, which hatch in vast numbers every 13 or 17 years.
The millipedes spend seven years in the soil, growing from egg to adult, and then another to mature, before bursting to the surface with thousands of their broodmates
Combining Niijima's data with the historical record, they determined the train millipede, known scientifically as Parafontaria laminata armigera, was on a rare eight-year life cycle.
Only a few living things have such long cycles, including cicadas, bamboo and a few species of plants.Insurance Loans Mortgage Attorney Credit Lawyer
The train millipede is the first record of a periodical non-insect arthropod.
Parafontaria laminata armigera are all on an eight year life cycle, but not every brood is on the same cycle. Outbreaks happen different years in different parts of Japan
Each one is under an inch-and-a-half, but clustered together they can stretch out more than 650 feet.
Niijima confirmed the eight-year periodicity by tracking the life cycle of millipede broods at Mount Yatsu and Yanagisawa, examining the two sites one to five times a year, from 1972 to 2016.
Team members would dig up to eight inches in the dirt to collect millipedes on a polyethylene sheet, 'using forceps or an aspirator.'
Niijima studied the millipedes from 1972 to 2016, digging into the dirt and sucking