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Pelican spiders: slow, safe assassins

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8:30am, March 9, 2014
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Spiders, thank goodness, haven’t evolved assassin drones. But the specialized hunters of the family Archaeidae can kill at a distance.

It’s a distance of only a few millimeters. But that’s substantial for these teensy dramas, and enough space to let a group called pelican spiders bring down their wary and dangerous prey: other spiders.

The pelican name comes from their profiles. “They look like little birds,” says Hannah Wood of the University of California, Davis. The spider’s body is about the size of a grain of rice, with a front segment that has evolved into a stretched “neck” with a little round “head” on top. (The mouth is actually at the bottom of the “neck”). And a pair of jawlike fanged projections called chelicerae folds down against the neck, where a pelican would tuck its beak.

Pelican spiders don’t build webs. Instead they creep through foliage, tiptoeing upside down under leaves to hunt. A female will carry her eggs with her, in a silk bag she attaches to one leg in the third of her four pairs. The spiders’ back six legs do the walking while the front two sweep circles in the air feeling for prey. A pelican spider that picks up the silk trail of another spider will spend hours at the edge of that spider’s web, plucking now and then and waiting. Unlike the quick spiders you might see skittering up a garden shed wall, Wood says, stalking pelicans are “slow and deliberate.”

But when they strike, it’s fast. The jawlike chelicerae rise 90 degrees and then slam fanged tips into the prey. “Then they pull out one chelicera and leave the other one hanging out there with the spider prey impaled on it,” Wood says.

Next it’s just a matter of waiting for the venom to work. Thanks to the pelican spider’s long neck and chelicerae, its prey struggles at a harmless distance.

Attacking at jaw’s length is an ancient trick. Biologists first discovered extinct pelican spiders in fossils before realizing the family still lives (in Madagascar, South Africa and Australia). Today’s species split off on their own trajectory as the supercontinent Pangaea was breaking up some 180 million years ago, Wood and her colleagues reported last year in Systematic Biology.

Now Wood studies a related family, the trap-jaw spiders (Mecysmaucheniidae), that has evolved the opposite approach to hunting. The spiders have shorter, thicker “necks,” and their superpower is speed. They strike so fast that it’s difficult to see more than a blur even in video recorded at 30,000 frames per second.

Ninjalike as pelican spiders are, they’re not the stuff of nightmares. “I’ve never had one try to bite me,” Wood says. When she reaches to catch them, they just drop to the ground. “They’re very shy.”

AT ARM'S LENGTH  Pelican spiders hunt other spiders, plucking at their webs to lure the prey closer and then using long necks and jaws to hold them at a distance.
Credit: Hannah Wood

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