Cloudy, with a chance of spider?
Apparently that was the forecast in Memphis, Tennessee, where millions of spiders have taken to the air and coated fields with their silk, according to local news reports.
In one area, spiders formed a nearly continuous, silken sheet that stretched for nearly half a mile. Some local homeowners lamented the arachnid invasion: “They’re just in the air, they’re flying everywhere,” local resident Debra Lewis told WMC Action News. “It’s like a horror movie.” (Also see "Millions of Spiders Rain Down on Australia—Why?")
But the spiders’ sudden appearance isn't something to be afraid of, says Todd Blackledge, a biologist and expert in spider silk at the University of Akron in Ohio.
“If you had taken the time to get down on your hands and knees, you would’ve seen all these spiders living their lives,” he says. Blackledge couldn’t identify the spider species from the photos available, but he says they could be orb-weaving and linyphiid (“sheet-weaving”) spiders.
In the spring and fall, millions of juvenile spiders crawl to the highest points of their habitat—say a fence pole, or a tall plant—and send out silk strands that allow them to be lifted on air currents. (Also see "Photos: World's Biggest, Strongest Spider Webs Found.")
The strands act “a little bit like a hot-air balloon," says Blackledge—hence the name for the behavior, ballooning. This means the spiders “go wherever the wind takes them,” allowing their populations to spread.
The vast majority of these high-flying arachnids will die during the journey, eaten by predators or killed by harsh weather conditions. But only a small fraction need to survive to set up shop in their new home.
Spiders, however, need the wind to cooperate in order to pull off their remarkable migrations, precisely what didn’t happen in Tennessee.
In fact, Blackledge suspects that the sheet-like “web” visible in photos from Memphis isn’t a web at all. Instead, the mass of silken threads are evidence of the spiders’ failed attempts at ballooning.
Cued by favorable conditions—perhaps warm ground temperatures, which could generate updrafts—the young spiders would have tried to send out their ballooning threads at more or less the same time.
However, unexpectedly strong breezes could have then blown these threads back down onto the grass over and over again, eventually creating a tangled mat. (Also see "Flirty Female Spiders Use Silk to Capture a Male's Interest.")
The grounded juveniles “would have a hard time biting you, and aren’t venomous to humans, so it’s not worth worrying about,” says Blackledge. “It’s a really amazing example of natural history.”
Such mass ballooning events aren't unique to the United States. They also occur elsewhere in the Northern Hemisphere—ballooning spiders have been spotted in Britain, for instance—and have happened periodically in Australia, as well. (Also see "Pictures: Trees Cocooned in Webs After Flood.")
Spiders not only balloon as a way of striking off on their own. They also do it to avoid natural disasters. In 2012, record rains in Australia spurred a mass ballooning event.
In that case, ballooning allowed the spiders "to move out of places where they'd surely be drowned," Robert Matthews, a professor emeritus of entomology at the University of Georgia, said of that event.
Although acres of spiderwebs may gross out some arachnophobes, the impressive feat shows "the versatility of things [spiders] can do with silk," Matthews noted.
Silk has been a "huge evolutionary breakthrough," he said, and "this is one more example of why spiders have been a successful group."
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Christine Dell'Amore contributed reporting.