杏吧原创

Sailors and travellers, including Charles Darwin aboard HMS Beagle, have often reported seeing 鈥渂allooning鈥� spiders flutter from the air into the sails of their ships, far away from any shore.

Dispersing spiders are known to use strands of silk to remain airborne in gusts of wind, but what happens if they are swept offshore and land in water?

We thought they would drown, but it turns out they are as adept at sailing as they are aeronautics.

鈥淚t was like an illusion,鈥� says of London鈥檚 Natural History Museum, who first noticed common UK spider species sailing in the lab. He was studying their flight, trying to figure out how they take off when he spotted the sailing behaviour. 鈥淚 was amazed that these common spiders, found in everyone鈥檚 gardens, had such skilful sailing behaviour that no one had noticed before.鈥�

鈥淥ne of the most amazing things is that no one had noticed this behaviour before,鈥� says his colleague of the University of Nottingham, UK.

Some species of spider form diving bells out of silk to enable them to breathe under water, while others are known to catch and eat fish.

But until now, no one realised that common spiders can sail, probably because the species that do it are small, typically just a couple of millimetres long.

鈥淲ater was always thought to be the ultimate barrier to dispersion,鈥� says Goodacre. 鈥淣ow, we know they can survive in water, so with this get-out-of-jail card, they can move far greater distances than we thought.鈥�

To find out how they do it, Goodacre, Hayashi and their colleagues observed the sailing skills of 325 spiders of 21 species caught at random on islands in ponds and lakes in various nature reserves around Nottingham in the UK.

Back in the lab, they placed individual spiders on small water trays and then used small air pumps to expose the spiders to breezes of between 3 and 80 centimetres per second.

All the spiders were able to stand on water thanks to their water-repellent legs. And 201 of them, covering most species, showed off sailing skills.

Most attempted to catch the wind and cruise forward by making 鈥渟ails鈥� from parts of their bodies. Some pointed two forelegs up in a V-shape (see picture above), while others thrust their abdomen skyward 鈥� the equivalent of a handstand on the water.

When exposed to a breeze on solid ground, they showed none of the behaviours, which suggests these are used specifically for sailing.

The spiders sailed just as well on salt and fresh water, and were able to manoeuvre even in turbulent water.

Some also created the equivalent of an anchor by throwing out strands of silk for attachment to surfaces, such as the side of the water tray. They may use these to haul themselves onto objects from water, or onto a suitable landing spot.

Goodacre thinks that size is a limiting factor 鈥� only those not too heavy can skim across the water 鈥� which means this behaviour is not common. 鈥淚鈥檇 say the limit is probably around 5 millimetres long,鈥� she says.

Her team now hopes to show that spiders sail in natural conditions, too.

Also, they want to examine what this means for evolution and geographic dispersion, given that spiders may be able to travel much further than thought.

鈥淭his may help explain why spiders are among the first species to colonise new habitats like islands,鈥� says of Humboldt University in Berlin. 鈥淪piders were thought to colonise exclusively by air; maybe they are good sailors too.鈥�

Journal reference: BMC Evolutionary Biology, DOI: 10.1186/s12862-015-0402-5