How Spiders Win the Lottery

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On a cloudy spring day, a little spider scales a tall blade of grass. At the peak, the spider arches up, points its abdomen up to the sky and begins releasing strands of silk from its silk glands. Tens of thousands of strands fill the air, fanning out and then coming back together to form a triangular sheet. A passing breezes catches the silk and suddenly the spider is airborne, riding its homemade parachute into the wild blue yonder.

Spider use these “ballooning” flights to escape from danger and to colonize new habitats. Most times, they only travel a few feet, but the right conditions can carry a spider over vast distances. Sailors have found them landing on ships thousands of miles from shore, and scientists have discovered eight-legged travelers in air samples collected by atmospheric data balloons.

All sorts of tiny arthropods travel this way, and some plants and fungi also use the wind to spread seeds, spores and pollen. Scientists call it passive airborne dispersal and from our perspective, passive is the key word. The tiny flyers seem to be left at the mercy of the elements and there seems to be little opportunity for them to strategize or make the most of their trip. The direction and distance they travel—or whether they travel at all or get stuck waiting to take off—are decided by the movement, direction and speed of the air.

Some researchers dub it the “aerial lottery.” The flyer buys their ticket, catches a breeze and crosses their metaphorical fingers that they land safely in a place they want to be. Whether they’ve won (new habitat, safe from danger, yay) or lost (atmospheric data balloon, boo) isn’t revealed until they’ve landed, and by then their play is over.

The journey appears completely out of their hands, yet many passive dispersers wind up exactly where they should want to be: still sort of close to where they started (where there are reliable, if shrinking, resources), but away on their own with untapped resources and no competition from their fellow spiders/seeds/whatever-they-ares. This winning play is the “shortest unique flight,” similar to the “lowest unique bid” needed to win some auctions and games. Despite all appearances, then, there may be a way to improve one’s odds of winning.

The trick to winning the aerial lottery, scientists think, is all in the take off, the last stage of the game where the “player” still has some control. Plants and fungi have been known to launch their wind-dispersed pollen, spores and seed only in certain conditions. Spiders and other wind-sailing critters, meanwhile, can choose the time and location of their launch.

New research by Andy M. Reynolds from the UK’s Rothamsted agricultural research station suggests that a winning strategy is based on taking flight in specific weather conditions. Warm, gentle breezes on days with some cloud cover are ideal for making the shortest unique flight. In more stable conditions the flight might be unique, but will last longer. In less stable conditions, the flights are shorter but less likely to be unique. The ideal launch seasons, Reynolds suggests, are spring and autumn, exactly when spiders tend to ramp up their “ballooning behavior.”

Whether these creatures win or lose at their lottery is more relevant to us than you might think at first. Spiders are a great help in controlling pests, and knowing where and when they take flight can benefit farmers. “Each day of the growing season around 1,800 spiders land in each hectare of arable farmland after ballooning,” Reynolds said in a statement about the study. “If the farmers can predict the influx of spiders, they can reduce the amount of pesticides accordingly," saving money and hassle. Similarly, being able to predict the spread of problematic fungi can help control them and the diseases they cause, giving us a leg up in this strange game of chance.

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July 8, 2013 - 1:00pm
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