Spiders!Science does have its creepy side. Then again, why would it not? Still, perhaps the phrase raining spiders ought to serve as a warning about what comes next.

And while the idea of spiders falling from the sky like rain is not entirely accurate, well, you get the picture. Arachnophobes might want to skip Nadia Drake’s report for Wired Science:

When 20-year-old web designer Erick Reis left a friend’s house on Sunday, he saw what looked like thousands of spiders overhead, reported G1, a Brazilian news site, on Feb. 8. The large, sturdy spiders were hanging from power lines and poles, and crawling around on a vast network of silk strands spun over the town of Santo Antonio da Platina.

Social spidersReis did what many of us might do: He pulled out his camera and shot a video of spiders seemingly falling from the sky.

As creeptastic is it may be, “The phenomenon observed is not really surprising,” said Leticia Aviles, who studies social spiders at the University of British Columbia. “Either social or colonial spiders may occur in large aggregations, as the one shown in the video.” The reason, she and others say, is simple: This is how they hunt.

Er … yeah. I’m not a fan of spiders. Ironically, I had a dream the night before I saw this article that had something to do with a staggering number of spiders. I have no idea why, or what it means. Forgive me, please, if I haven’t paused to dwell on the Freudian or Jungian interpretive values involved.

Initial speculation suggested a very social species called Anelosimus eximius, which, according to Drake, “weaves communal webs, lives together as adults, and shares childcare duties”.

And isn’t that lovely?

However, it appears that initial assessment may be wrong. The spiders in the video are more likely a species of colonial spider that aggregates individual webs and lives in groups only temporarily, dispersing before reproducing, Aviles said.

“The spiders I saw in the video are not Anelosimus eximius,” said Deborah Smith, an entomologist at the University of Kansas who specializes in social spiders. She notes that A. eximius is a bit smaller than the arachnids Reis filmed, and may not live that far south. “The spiders in the video are very large and robust,” she said. “It might be worth looking at Parawixia bistriata, a large, group-living orb weaver, to see if that one fits the bill.”

Arachnologist George Uetz agrees. “This is definitely not Anelosimus eximius,” said Uetz, who studies spiders at the University of Cincinnati. He notes that the spiders appear to be spread out on a colonial network of individual orb webs (rather than building a communal nest) and resemble big, orb-weaving spiders — perhaps Parawixia bistriata. “This colony is quite large,” he said, noting that the spiders aren’t actually raining down. “The web is fixed, although it is very fine and mostly invisible,” he said.

Cornell University arachnologist Linda Rayor and Aviles also agree that what’s probably being filmed is a massive P. bistriata colony. That species lives in South American savannas and spins colonial webs. A bit of good news is that their venom is not believed to be harmful to humans, Uetz said.

If this is Parawixia, or a similar species, there’s a reason the spiders may have appeared to come out of nowhere. “At night, they all collect in a colonial retreat, probably out of sight in a tree,” Uetz said. ”Then they build the colonial framework early in the day, and build individual webs upon it. They sit on these webs and capture prey.”

Parawixia bistriataTerra da Gente has a reasonable page describing these spiders, but it’s in Portuguese, and online translators still require some interpretation:

A tangle of spiders, to draw attention. So is the routine Parawixia bistriata that immature, thereby join itself to increase the probability of survival. But an alliance so we can say, is unusual among spiders. Normally tolerance closeness of brothers ends sooner in most species.

P. bistriata gatheredI’m not entirely certain the idea that P. bistriata venom isn’t harmful to humans gives much comfort in the face of irrational fears. Still, though, a 2003 paper by Fontana et al., originally published in the British Journal of Pharmacology, explored the stuff in a pharmaceutical context:

The venom also contains a component capable of regulating GABA uptake. Figure 1b shows that increasing concentrations of spider venom dramatically inhibited GABA uptake. As shown in Figure 2b, the venom inhibits the maximal velocity of GABA uptake, while the KM remains unchanged indicating a noncompetitive inhibition by the venom. This ability of a toxin to mediate the inhibition of GABA uptake in synaptosomes was also observed with extracts from venom of the wasp Agelaia vicina (Pizzo et al., 2000). Drugs that inhibit GABA uptake may provide an effective means for protecting the brain against neuronal injury (Fisher & Bogousslavsky, 1998) and for treating epilepsy (Meldrum, 1997), as illustrated by the clinically used drug tiagabine.

Not that it will help arachnophobia, of course.


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