The spiders, says expert Brandi Fleshman, were trying to use their silk to catch air and migrate somewhere else.
ANCHORAGE, Alaska — Tom Underwood was walking his golden retriever near the Huffman Park Drive post office in Anchorage, Alaska, earlier this month when he came across an extraordinary sight: a field with silky, white webs strewn over the tops of a wide swath of yarrow, fireweed and other plants, in a strip maybe 20-feet deep and 40-feet long.
The gauze was filled with lots and lots of spiders — itself unusual since most spiders don't gather in groups.
Underwood's discovery was interesting enough to a Cooperative Extension Service pest technician that he collected some of the spiders, preserved them, and sent the specimens and photos to a spider expert in Fairbanks, Alaska.
The spiders, says expert Brandi Fleshman, were trying to use their silk to catch air and migrate somewhere else. It's called ballooning.
"I would say the event is somewhat unusual," Fleshman said in an email. She said it's not something you see every day, but it's not unheard of.
Fleshman is a University of Alaska at Fairbanks graduate student who is expanding scientists' knowledge of spiders in Alaska. Starting in 2007 when she was an undergraduate, she has been researching, collecting and adding to the list of kinds of spiders known to inhabit Alaska. It's now more than 500, from fewer than 300 that had been listed by others.
Ballooning, she said, "involves climbing up on some high surface like the tops of vegetation, raising their body up (basically standing on their tip-toes), and letting out strands of silk into the wind. If the updraft is enough to catch the silk and produce lift, the spider lets go and is carried off as far as the breeze will take it."
It may take many tries, and with a lot of spiders doing it, the silk may accumulate, she said.
Derek Sikes, curator of insects for the University of Alaska Museum in Fairbanks, said in the Aleutians spiders get from island to island this way.
The spiders next to the Huffman post office were pea-sized (including their legs), and a species that is widespread across much of the world. They are Centromerus sylvaticus, Fleshman said.
The species has not been reported as gregarious, she said. "However, there are numerous accounts in the literature of mass dispersal events of spiders" — a bunch of them attempting to move from a particular spot at once.
When she first saw photos of the Huffman spiders and their silk, she thought they might be leaving because of poor conditions for overwintering there, she said in an email to the Cooperative Extension technician in Anchorage, Michael Rasy. But on further study, she learned that there would likely be more species of spiders migrating with them if that were the case.
Another likely conclusion found in spider literature is that such migration is "a natural part of the life cycle of some spiders, and just something they do after maturing," said Fleshman.
Spiders from a single species would mature around the same time, which might be why so many tried to go at once, she said.
"I can only speculate."
Where the spiders from Huffman ended up is a mystery.
Underwood, the dog-walker who found them, said he went back a few days after he spotted the spiders and gauze — which was around Sept. 6 — and the spiders and most of the silk were gone.