Long-distance spiders can sail across the high seas

Many spiders are known to ‘balloon’, a behaviour in which they use strings of spun silk to catch wind currents and move between locations. This is a risky strategy as the wind could set them down in the sea.

But new research has uncovered that these spiders can slide across the surface of the water, using their legs or abdomen as sails and their silk as anchors to latch on to floating objects and hitch a ride to land.

Strike a pose

When subjected to currents of air on dry land, the spiders in the study hunkered down against the wind, but when on water they struck elaborate poses to catch the breeze.

Sailing behaviours ranged from the simple raising of a couple of legs, the refined walking on water and the bizarre lifting of the abdomen while dipping the head to create an ‘upside-down’ sail. Some spiders even dropped silk in the water to slow down or attach to floating objects, which they then hitched a ride on.

These sophisticated behaviours were found in spiders that balloon, suggesting they were well prepared for encounters with water in their risky airborne adventures. However, even those spiders that don’t have the tendency to balloon were able to sail, suggesting that they could use the strategy if necessary, such as when encountering puddles or floods.

Researchers tested 21 species of spiders collected from small islands in the Attenborough Nature Reserve, near Nottingham. Most of the species belonged to the Linyphiid family, commonly known as money spiders. All of the spiders had water-repellent legs, allowing them to stand on water.

Early colonisers

Spiders are often early colonisers of fresh terrain, such as volcanic islands and cleared land, causing a big change in new ecosystems as they are also top invertebrate predators. They are important crop pest hunters, and their spread into farmland has been well studied.

How terrestrial spiders fared across long distances was less well understood, even though they have been known to travel up to 30 kilometres in a day.

The finding was published today in the journal BMC Evolutionary Biology by Museum researcher Dr Morito Hayashi, Dr Mohammed Bakkali from the Universidad de Granada, Spain, and Dr Sara Goodacre from the University of Nottingham. Derbyshire-based wildlife photographer Alexander Hyde was also on hand to capture some of the spiders’ sailing poses.

Lead author Dr Hayashi said: ‘Even Darwin took note of flying spiders that kept dropping on the Beagle miles away from the sea shore. But given that spiders are terrestrial, and that they do not have control over where they will travel when ballooning, how could evolution allow such risky behaviour to be maintained?’

Dr Goodacre added: ‘Being able to cope with water effectively “joins the dots” as far as the spider is concerned. It can move from one land mass to another, and potentially across huge spatial scales through the air. If landing on water poses no problem then in a week or two they could be a long way away from where they started.’

Dr Bakkali concluded that: ‘Had Darwin been among us today, he would probably be pleased to know that natural selection is the rule, even when it comes to the maintenance of the risky flight behaviour of those little spiders that were falling down on the Beagle ship from apparently nowhere.’