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How Spiders Can Cruise Across Water

Spiders that make themselves "balloons" to fly can also fashion "sails" to cross water.
F Tetragnathid spider using silk as anchor. Image: Alex Hyde

Spiders are usually known for their gangly legs and super silk. But those two qualities can also make them pretty awesome sailors.

In a study published today in the journal BMC Evolutionary Biology, researchers say that some terrestrial spiders can cruise across water like ships, by raising a pair of legs as sails and using their silk as an anchor to slow down or stop their movement.

A Linyphiid spider raises its legs to "sail." Image: Alex Hyde

While similar studies have previously been carried out on spider species such as the Dolomedes raft spider—which can hunt by running across water—little has been done with spiders that aren't usually associated with water.

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"We discovered that spiders can basically float on water. They could move smoothly using wind power, using their legs and sometimes abdomen as sails," Morito Hayashi, lead author from London's Natural History Museum, told me over the phone. "A spider is almost like a tiny robot. We can see its exact movements, so it's a very nice model to observe animal behaviour."

For their study, the researchers used 325 spiders from 21 common species collected from small islands in a nature reserve in Nottingham, a city in the UK. They placed the spiders on both dry surfaces and in trays of water, while directing a fake wind—generated by an air pump—in their direction. "The moment they got wind, the spiders raised two legs and took advantage of the wind to move across the water," said Hayashi. "They didn't do this on dry surfaces."

A Tetragnathid spider sailing using legs. Image: Alex Hyde

But why do some spiders need to be able to cruise through waters? Some common spiders are known to float through the air using a method called "ballooning." They do this by using their silk to catch the wind, which propels them into the air. This ability lets them cover greater distances (they can move at speeds of 30 km per day under favourable wind conditions), and colonise new habitats.

Ballooning, however, isn't without risks: Spiders often don't know where the wind will end up carrying them, or what kind of surface they'll end up on. They could end up crash-landing on a puddle, marshland, or lake. So being able to cruise over water is basically a survival technique, suggested Hayashi.

A C Linyphiid spider using its abdomen instead of legs to "sail." Image: Alex Hyde

"Seventy percent of our planet is basically covered by water. When spiders balloon and fly through the sky, they might end up landing on water; that's why they need the skill to move on water," said Hayashi. "Imagine if you landed in the middle of water, and you had to move over large distances. You wouldn't be able to walk all the way, but using wind power to sail across water would be an energy efficient move."

While the researchers write that spiders who fly through the air turn out to be "the most eager 'sailors,'" Hayashi pointed out that non-flying spiders can be pretty apt on water too. "Most spiders who balloon do sail; but the sailors are not always ballooners," he said.