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Wednesday, June 8, 2011

Spiders Evolved Spare Legs

from National Geographic (image from 27bslash6)
Arachnids missing up to two limbs can build webs and hunt with ease.

Scientists may have uncovered why spiders are so creepy-crawly—they have more legs than they need, a new study says.

After collecting thousands of female spiders in the wild, scientists found that more than 10 percent of the arachnids were missing at least one of their eight legs.

"We wondered if this was handicapping them in any way," said study co-author Alain Pasquet at the University of Nancy 1 in France.

The research team placed 123 Zygiella x-notata spiders in individual plastic boxes, where the animals could build webs. Sixty of the spiders were eight-legged, while 63 were each missing one or more legs.

Pasquet and his colleagues found that webs built by spiders missing at least one leg did not differ much from the webs built by spiders that were intact.

Six-Legged Spiders Still Good Hunters
The scientists then placed flies in the enclosures and found that leg-lacking spiders were also perfectly capable of catching and eating the insects.

"We were really surprised—we expected missing a leg to harm the spiders' ability to catch food, and it didn't at all," Pasquet said.

Based on the findings, the authors propose that spiders have legs that they don't really need—an advantage when it comes to escaping a predator that's put the bite on a limb, for example.

Yet there does appear to be a limit to how many legs a spider can lose. In the wild, the team found few spiders missing more than two legs. And in the lab, these five-legged spiders built shoddy webs.

Spider-legs study published in a recent issue of the journal Naturwissenschaften.

Pasquet A, Anotaux M, Leborgne R (2011) Loss of legs: is it or not a handicap for an orb-weaving spider? Naturwissenschaften DOI: 10.1007/s00114-011-0799-7

Leg loss is a common phenomenon in spiders, and according to the species 5% to 40% of the adults can present at least one missing leg. There is no possibility of regeneration after adult moult and the animal must manage with its missing appendages until its death. With the loss of one or more legs, female orb-weaving spiders can be penalized twice: firstly, because the legs are necessary for web construction and secondly, the legs are essential for the control of the prey after its interception by the web. During development, spiders may be also penalized because regeneration has energetic costs that take away resources for survival, growth and reproduction. All these consequences should influence negatively the development of the spider and thus its fitness. We investigated the impact of leg loss in the orb-weaving spider, Zygiella x-notata by studying its frequency in a natural population and web building and prey capture behaviours in laboratory. In field populations, 9.5% to 13%, of the adult females presented the loss of one or more legs; the majority of individuals had lost only one leg (in 48% of cases, a first one). Leg loss seems to affect all the adult spiders, as there is no difference of mass between intact spiders and those with missing leg. Data obtained with laboratory-reared spiders, showed that the loss of legs due to the moult is rare (less than 1%). Considering changes in web design, spiders with missing legs decreased their silk investment, increased the distance between spiral turns but did not change the capture surface of the web. Under our laboratory experimental conditions, spiders with one or two lost legs did not present any difference in prey capture efficiency. In laboratory conditions, spiders with lost leg(s) did not show any difference in egg sac production or in longevity (adult lifespan) compared to intact spiders.

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