Leap forward for 'flying' spiders [BBC 2006-07-12]
[Photo] Two male Erigone spiders on a grass seed head. The lower one is in a pre-ballooning posture ready to disperse, known as the "tip-toe" position. (Image: Rothamsted/BBSRC)
Scientists have shed new light on how spiders can travel epic distances by dangling on strands of floating silk.
A team from Rothamsted Research, UK, has updated a mathematical model describing this remarkable arachnid phenomenon, known as "ballooning".
It claims the revision gives a more realistic explanation of what actually happens to spiders in flight.
The new model shows how turbulent air can propel the creatures much further - even hundreds of kilometres out to sea.
"By making some modifications we've solved something that's flummoxed scientists for 200 years," said Rothamsted's Dr Dave Bohan.
"The previous model could explain spiders going up to 200m; what it couldn't do was explain why, for example, Charles Darwin on his ship, The Beagle, more than 100km off the coast of South America, could watch ballooning spiders," he told BBC News.
Most small spiders seeking new territory - or, indeed, a mate - will cast silk into the air and "parachute" to a new location. Peak periods for this activity tend to be in autumn and spring.
"The dragline can contort and twist with turbulence, affecting its aerodynamic properties and carrying its rider unpredictable distances" - Andy Reynolds
Their journeys may only take them to the other side of a field; but just occasionally, they can cover vast distances.
For 20 years, the best mathematical description of ballooning has been the so-called Humphrey model. But the Rothamsted team felt this did not adequately explain some of the features of spider flight, particularly in moving air flows.
The scientists say it assumed that spider silk was solid and rod-like, and treated the spiders as just blobs hanging on the bottom - a "kind of upside-down lollipop".
"Factoring in the flexibility of the dragline that the spiders cast into the breeze has shown how it can contort and twist with turbulence, affecting its aerodynamic properties and carrying its rider unpredictable distances," explained co-worker Dr Andy Reynolds.
The team plans to take spiders into a wind tunnel to watch how they handle turbulent flows. The scientists believe this will help them refine the model still further.
"Spiders are interesting from an applied point of view because they are major predators in agricultural situations," said Dr Bohan. "If we can understand how and why they move around, we may be better able to use them as biological control agents, to tackle farm pests."