"Train millipedes" keep eight-year timetable
Sometimes trains are unreliable - but at least the trains where you live probably aren’t delayed by millipedes on the line...
Researchers from Japan and the USA have shown that so-called ‘train’ millipedes have an amazing - and disruptive - eight-year cycle of emergence, similar to the bewilderingly large 13- and 17-year swarms of periodical cicadas in North America.
Reporting in Royal Society Open Science, Keiko Niijima and colleagues confirm the existence of this eight-year life cycle, and provide an insight into the reason why these organisms emerge all at the same time to cause huge swarms.
The train millipede - scientifically known as Parafontaria laminata armigera - is a foul smelling inhabitant of the central mountainous regions of Japan, where they often live in forests and develop underground over a period of 7 years. On the eighth year they come to the surface, now fully mature, to engage in an orgy of mating and egg laying before dying a few weeks later. They emerge in such large numbers that since the 1920’s they have been known to actually disrupt train services in areas where they live - this is what gives them their name.
While this eight-year cycle had previously been suspected, the researchers set out to verify it by growing brooods of millipedes in the field, and by correlating historical records of their swarms.
Eleanor Drinkwater, independent arthropod researcher from the University of York, said, “they have really put together this huge jigsaw of so many different sources of data. Really, a Herculean amount of work has gone into this paper to show this periodicity.”
In other organisms that have this kind of life cycle, such as some species of cicadas that emerge only at prime numbered intervals of 13 or 17 years, the behaviour is thought to be a way to boost their survival. Their sheer numbers simplify overwhelm predators, and the irregular cycle is difficult for those same predators to biologically track.
Although the millipede behaviour appears similar, it may have evolved in a different way as a response to cooler conditions during the last ice age.
“There have been lots of different theories. One of which - which is a very popular theory - is the idea that it has to do with predator avoidance. Yoshimura, who is one of the top authors on this millipede study, has been doing lots of really interesting modelling work. He argues that it is the ice age, and cold conditions, that could have been one of the things that has triggered this switch,” said Drinkwater.
Periods of cooling allow the millipede to shed its outer coat and grow, and the hypothesis is that its longer life cycle may have evolved so that it could hibernate and survive harsh conditions.