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The answer is blowing in the wind
Spiders are important predators of insect pests that often "parachute" into new areas on single strands of silk. Elspeth Bartlet reports on a new, more realistic, model of this behaviour by scientists at Rothamsted Research, which explains, for the first time, how these aerial hitchhikers can be transported for tens of metres to hundreds of kilometres.
On a fine summer's day, a young spider climbs to the top of a blade of grass, arches its body and lets out a strand of silk. The breeze catches the silk and the spider is dragged into the air and out of sight. This behaviour is called ballooning and it's the main dispersal method for many spiders, moth larvae and mites, enabling them to colonise new habitats, or escape from trouble. Most ballooners probably only travel a few metres but some intrepid aeronauts have been found on boats hundreds of kilometres out to sea and caught from balloons high in the air.
These epic journeys are difficult to explain, according to the existing mathematical model of ballooning, which was created in the late eighties. The model works well at describing ballooning behaviour in still air or in uniform air flows, but is less successful when conditions are turbulent. It also makes some questionable assumptions. "In Humphrey's model, ballooning spiders are represented as blobs on the end of rigid sticks, a bit like upside-down lollipops," explains Rothamsted researcher Dr Andy Reynolds. "In reality, the silk dragline is not rigid, but is both elastic and flexible, so our model allows the silken strands to stretch and twist".
The researchers have shown mathematically that, under turbulent conditions, the silk dragline becomes extremely contorted. Because the silk moulds to follow the eddies and twists of the airflow, spiders end up travelling further. "Our model easily explains the occasional long distance journey" says Dr Reynolds. "It also quashes the notion that spiders control the distance they travel by producing silk lines of a particular length. This is because the aerodynamics of the dragline depend on its shape, which is uncontrollable." The next step is to test predictions from the model with some observations of the spiders in action. "We've already observed what happens to ballooning spiders falling through still air," says Dr James Bell. "Now we want to put them into a wind tunnel under varying or turbulent wind conditions to see if our model is correct".
This research operates at the interface between the mathematical, physical and ecological disciplines, providing new opportunities for predictive biology. Dr Dave Bohan explains, "spiders are important predators that can control pests and alleviate the need for chemical pesticides- but only if they arrive in the right place at the time. Our model enables us to investigate how dispersal impacts on the population dynamics of these species in man-managed landscapes like farmland".
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