Photograph: Alex Hyde. Spider uses abdomen as “sail”

Charles Darwin, while at sea on the HMS Beagle in 1932, was flabbergasted upon observing thousands of small, red spiders floating through the sky. Carried on the wind by their silky spider webs, the spiders even landed on board the ship, becoming entangled in the ship’s rigging. Observations of this kind led Darwin to declare in On the Origin of the Species and The Voyage of the Beagle, that this behaviour “renders it probable that the habit of sailing through the air is as characteristic of this tribe, as that of diving is of the Argyroneta [the diving bell spider]”.

While Darwin expressed his amazement at the sight of spiders floating through the air and succinctly recorded his observations in his journals, little, until very recently, has been known about the ability of spiders to sail on water. Now, in a new research breakthrough, a team of international researchers has uncovered one of nature’s best kept secrets, shedding light on the dazzling ability of spiders to navigate on water.

The research findings show that the spiders employ their legs and abdomen to adopt different postures which allow them to catch the wind and glide smoothly along the surface of the water, travelling in the direction that they wish. Dr. Mohammed Bakkali points out that these abilities make the spiders seem like “genuine sailing boats”. He explains: “Their abilities in water mitigate the risks posed by flying in such an uncontrolled way”.

Scientists proved years ago that many spider species take advantage of the wind in order to take flight and, thanks to the silk they secrete, are able to ascend up into the sky and travel for dozens and even hundreds of kilometres. They do this in order to disperse, take over new territory, and search for new resources.

“We have all witnessed the astonishing downpour of shimmering spider webs that appear to fall from the sky, as Darwin himself witnessed when he was in the middle of the ocean during his voyage on the Beagle”, explains the UGR scientist.

However, Bakkali elucidates: “This peculiar mode of flight seemed particularly puzzling due to the great risks posed to the flying spiders. Wingless, they are forced to fly in the direction taken by the wind and their flight comes to an end only when the wind dies down. They are, in effect, drifting at the mercy of the winds. This loss of control over the flight course and the landing location entails huge risks.”

Spiders are land animals and over two-thirds of the earth’s surface is water. “Upon deciding to fly, they run the risk of ending up in oceans (as Darwin observed), seas, rivers, lakes, swamps, reservoirs, ponds, and so forth, which is why natural selection should not have allowed for such risky behaviour without the mitigating force that we have just discovered”, Bakkali reveals.

If Darwin were still with us, he would probably be pleased to learn that the enigma of the spiders falling onto his Beagle in the middle of the ocean is also, in fact, compatible with one of his great scientific discoveries - natural selection. The spiders, it seems, were not exhibiting such risky behaviour after all. On the contrary, the “risky” flying behaviour, together with their sailing abilities, are precisely what explain their ability to colonize new territories and further the survival of their species.

Photos: Alex Hyde

Bibliography

The findings, published in the prestigious journal BMC Evolutionary Biology, are the fruits of research collaborations between Prof. Morito Hayashi from the Zoology Department at the Natural History Museum of London, Prof. Mohammed Bakkali from the University of Granada's Genetics Department, the professional photographer Alexander Hyde, and Prof. Sara Goodacre from the University of Nottingham.

Sail or sink: novel behavioural adaptations on water in aerially dispersing species Hayashi et al. BMC Evolutionary Biology (2015) 15:118 DOI 10.1186/s12862-015-0402-5

Read the full paper here:

• Sail or sink: novel behavioural adaptations on water in aerially dispersing species (external link)

Contact the researcher:

Dr. Mohammed Bakkali

Genetics Department,

University of Granada

Phone: +34 958 248 926

Email: mbakkali@ugr.es