A rare phenomenon has left locals in a tiny South American town shocked.

Those living in São Thomé das Letras in Brazil got to see a “rain of spiders” when the Parawixia bistriata creates an intricate silk web during its reproductive and hunting phase.

The social spiders collectively build a massive web suspended in the air, with hundreds of spiders moving across it, creating the illusion that they are falling from the sky.

This phenomenon typically occurs after periods of high temperatures and fog. Colonies of up to 500 spiders cooperate to build their webs in tree canopies or elevated structures.

Though it can be unnerving to see, experts are assured do not pose a threat to humans. They are not aggressive and play a crucial role in regulating insect populations and promoting biodiversity.

A video of the huge web went viral on social media after it was shared by environmental activist Bruna Naomí at the end of December, eliciting reactions of fear and surprise.

Bruna told g1: “I was driving and we stopped the car to record it. It wasn’t the first time. Before this, they’ve stayed hidden in trees and their webs go unnoticed.

“Every year at this time they appear, and for us it’s always a joy to see them. What impressed us most this year was how far they cast their webs from one tree to another. It’s very far and we were puzzled about how they managed to do it.”

Some suggested to exterminate them while others made religious associations considering it an apocalyptic sign. Another person joked about mosquitoes not standing a chance: “There won’t be any mosquitoes left to bite us with so many spiders.”

Cláudio Maurício Vieira, coordinator of the Arthropod Division at the Vital Brazil Institute, said: “Some species of spiders build these giant webs, which can be used collectively throughout the year or during specific periods. This behaviour is a strategy to capture large amounts of food or facilitate reproduction.”

Professor Rodrigo Lopes Ferreira, head of the Department of Ecology and Conservation at the Federal University of Lavras, explained these spiders are semi-social and group mainly during the reproductive period, which usually occurs from September to March. 

“During the day, they form a ball of spiders. At night, they come out and make this web, so it looks like there’s a ‘spider rain’ in the sky,” said the professor in an interview with EPTV.

“It doesn’t have potent venom. Of course, if someone handles it carelessly and stresses the creature, they might get bitten, but the venom isn’t a major problem; it’s not among those we consider medically important.”