Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world
Species: Illacme plenipes
Habitat: a tiny area of oak forest near Silicon Valley in California
There's a problem with the names of things: they're often wrong. Peanuts are not nuts, catgut is generally made from sheep, shooting stars are actually rocks, and any country that calls itself a Democratic Republic is almost certainly a totalitarian dictatorship.
The same is true of those notorious little scuttlers, the millipedes and centipedes. Some centipedes have well over 100 legs ? Gonibregmatus plurimipes has 382 ? so we should really call them multicentipedes. And no known millipede has 1000 legs: it's rare for them to have more than a few hundred.
One species, however, comes close. Illacme plenipes can have up to 750 legs, more than any other animal. We know little about it, but as we find out more, it seems its overabundance of lower limbs is the least of its peculiarities.
She got legs
I. plenipes was thought to be extinct, as it had not been seen since 1928. But then it was rediscovered in 2005, by graduate student Paul Marek, now at the University of Arizona in Tucson. Marek found it lurking underground in a single ravine in San Benito County, California (Nature, doi.org/cxh24p).
Its name means "the acme of plentiful legs", which is about right. Males have a mere 300 to 400 legs, but females have up to 750 despite being just 3.2 centimetres long. Their leg count edges out their closest competitor, a Puerto Rican millipede called Siphonophora millepeda, which has 742.
Animals don't evolve a multitude of legs just to amuse us: there's presumably some advantage. Marek says I. plenipes's legs may help it to burrow underground, where it spends all its time.
Or they could be a sort of accident, a consequence of I. plenipes evolving another anatomical advantage: its digestive tract is spiral-shaped and thus has a high surface area, allowing it to absorb more water and nutrients from its food before excreting it. Marek says I. plenipes may have evolved a very long gut to cope with a sparse diet ? with the legs as an incidental result.
Silky smooth
Marek and colleagues have now taken a closer look at I. plenipes to find out how it lives. It seems to have a rather dangerous existence and has evolved accordingly. While most burrowing animals are streamlined, so they can move smoothly through soil, I. plenipes is anything but. "It has a significant diversity of spines, projections, setae [bristles] and other sharp pointy objects that seem they would get snagged," says Marek.
That suggests it has many predators to defend against. Each segment of its body has defensive pores facing out to the side, which secrete a defensive chemical. Marek says I. plenipes may use the spines to deter predators, and then squirt them with chemicals if they don't back down.
It also has hairs on its back, which produce a sticky substance similar to silk. This could be another defence, used to gum up predators. Or the silk could help keep the millipede clean by seeing off junk that would otherwise stick to its spines.
There's a third possibility. I. plenipes spends a lot of its time on buried sandstone boulders, and the sticky silk may help it cling on. If that's true, the silk is effectively acting as another set of legs.
Right now we don't know what the silk is for, and we may not have long to find out. I. plenipes is confined to an area of just 4.5 square kilometres. "The species is exceedingly rare," says Marek. "Species in the US that are more secure than I. plenipes are [categorised as] federally threatened and endangered. Folks should consider formal protection for this species."
Journal reference: ZooKeys, DOI: 10.3897/zookeys.241.3831
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