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Illacme plenipes, the leggiest animal on Earth, gets another look

The views expressed are those of the author and are not necessarily those of Scientific American.


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Illacme plenipes

Illacme plenipes, the animal with the most legs in the world. Credit: Paul Marek et. al.

*Insert Angelina Oscars Leg joke here*, but the leggiest animal in the world is actually the millipede Illacme plenipes, and researchers have published the most comprehensive analysis of the species’ anatomy, including what it does with its record 750 legs.

Illacme plenipes means ‘in highest fulfillment of feet’, because the females of the species have the most legs of any animal in the world, including the other 9,999 known species of millipede. The species was first described in 1928 by O. F. Cook and H. F. Loomis from the U. S. Department of Agriculture, based on seven specimens found in Central California. No photograph, illustration or clear location description was provided in the original report, so I. plenipes was not seen again in the wild till almost 80 years later, when millipede expert Paul Marek from the University of Arizona and his brother set out to find one for themselves.

Using a number of mapping technologies including Google Maps, the pair managed to rediscover the extremely rare I. plenipes on 29 November 2005 under a large, moss-covered sandstone boulder in San Benito County, California. “To find this species in particular, one believed extinct over the past 80 years and this relict animal with 750 legs, was wonderful,” says Marek. “To tell you the truth, and this is the experience every time I find a species I’ve never seen before, it was an exhilarating experience. Even when reading about other entomological discoveries (whether it be the Lord Howe Island stick insect or bioluminescent cockroaches) it’s exciting to think about all the fantastic and diverse life forms that live with us on the planet.”

Marek announced the species’ rediscovery in a 2006 issue of Nature, and released the first-ever video footage and scanning electron micrograph images of the cream-coloured arthropod. He discovered three more I. plenipes millipedes in December 2005, January 2006 and December 2007, all under sandstone boulders, and has described every inch of the species in ZooKeys today.

habitat

Oak forest understory habitat of I. plenipes. Top, base of sandstone pinnacle, where specimens were found. Bottom, mossy oak forest—close-up of habitat where I. plenipes individuals were encountered. Credit: Paul Marek et. al

I. plenipes belongs to the pan-tropical order Siphonophorida, which spans Central and South America, South-East Asia and Australasia. It is the only representative of the family Siphonorhinidae in the Western Hemisphere and its habitat spans just 4.5 km in diameter in the northwestern foothills of the Gabilan Range in San Benito County, just south of San Francisco.

Marek and his team distinguished their specimens from other millipede species based on an anatomical analysis, taking particular interest in the genitals. “For this relict species in particular, and especially since there’s nothing like it in the Western Hemisphere, [distinguishing the species] is pretty straightforward. It’s so completely different, anatomically, from anything else in the area,” he says. “For questions about species that are more closely related to one another, millipede taxonomists use the anatomical differences in the genitalia under the lock-and-key hypothesis – in a crude way, ‘the idea that a lock from one species cannot be opened with a key from another’.”

I. plenipes has no eyes, and shuns the light to live in subterranean environments 10 to 15 cm beneath the soil. Not only does it have more legs than any other known organism, it has huge antennae relative to its body size – just 3 to 4 cm long and about 0.5 mm wide – and is covered in little hairs that secrete a viscous silk-like substance. The lower third of its gut, called the metenteron, spirals fully visible through its translucent exoskeleton, which can be built of up to 192 body segments, and unlike the beak-like mouthparts of other millipedes in its family, I. plenipes‘ mouthparts are rudimentary and fused to leave just a small opening for sucking up fluids. “A spiraled metenteron coupled with the extreme number of segments lengthens the digestive tract and hence the body. This lengthening might function to increase the absorptive surface area in order to extract maximum benefit from a water or nutrient-deficient diet,” the researchers suggest, but just what this diet consists of remains unclear.

Illacme plenipes

Illacme plenipes female with 170 segments and 662 legs. Magnified view of posterior segments with corkscrew-shaped metenteron visible through cuticle. Credit: Paul Mark et. al.

The females of the I. plenipes species can have up to 750 legs, whereas the males sport a maximum of 562. Millipedes in general use their many legs to burrow around obstacles in their environment, each pair of legs acting to push and propel their little bodies through in the earth. Marek and his team suspect that the I. plenipes millipede has an exceptional number of legs to move around its subterranean habitat, and also to help it cling tightly to the undersides of the sandstone boulders found exclusively in the area. Its sticky, silk-like secretions could also help to adhere them to the boulders, but as the secretions increased with handling, the researchers suggest they could also act as a defence mechanism.

Illacme plenipes

Scanning electron micrograph image of Illacme plenipes' head and antennae showing the unusual covering of hairs. Credit: Paul Marek et. al.

The future of this very rare species is uncertain, due to its limited habitat and what appears to be low population numbers. “Exploitation as a result of over-collecting, [is] something that we were cognisant of and tried to avoid while gathering information about these millipedes, [and the] loss of this species’ habitat and extinction is a real possibility,” says Marek. “The idea with conservation is to preserve as much biological diversity as possible, and not only is this species so different than anything else in the area, the habitat that it lives in is filled with so many different and unique species – things that we know so little about. There are wonderful creatures in this area. Animals like trapdoor spiders, slugs, beetles, salamanders and scorpions. Plants like giant draping oak trees, ferns, mosses, and a number of different fungi and flowers. Many of these species are found here in this limited area, and no place else on earth.”

Order my new book, Zombie Tits, Astronaut Fish and Other Weird Animals, here.

Bec Crew About the Author: Bec Crew is a Sydney-based science writer and award-winning blogger. She is the author of 'Zombie Tits, Astronaut Fish and Other Weird Animals' (NewSouth Press). Follow on Twitter @BecCrew.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. bergamotley 7:50 am 11/16/2012

    Great article! I notice that ScienceDump on Facebook may have copied this material without attribution.

    Link to this
  2. 2. Becky Crew in reply to Becky Crew 10:31 pm 11/16/2012

    Wow that’s annoying. Thanks for letting me know, Dorothy, I’ll check it out.

    Link to this

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