Last week, spiders descended in droves upon a town in southern Brazil – literally.

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.

Reis 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.

An early report suggested the swarming spiders were Anelosimus eximius, a social species of spider that weaves communal webs, lives together as adults, and shares childcare duties.

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."

Whether the spiders are setting up camp or dispersing is an open question. It's possible that Reis caught the conglomerate just as they had moved in to a new home – in which case he'll see spiders in the sky whenever he visits his friends. At least for as long as insects are plentiful and the neighborhood is safe from birds, or until it's time to reproduce. P. bistriata colonies dissolve before the spiders make more spiders, Aviles said. When they are clumped together, the groups tend to comprise single families.

"I suppose those can be quite large," Aviles said. "Or, in some cases, multiple families may remain aggregated, giving rise to a colony as huge as the one shown in the video."

It's also possible the spiders were caught in the act of dispersing, and that the massive web overhead is temporary, though that's more likely if the spiders are, in fact, Anelosimus eximius. An easy to make a determine which species they are is to look for the presence of an orb web, which would point toward Parawixia, Aviles said. Or better yet, snap a close-up photo of one of the spiders. Any volunteers?

Video: Acoisacoisada1/YouTube