Video: Science Explains Why Thousands of Daddy Longlegs Swarmed This House

A daddy longlegs in the Leiobunum genus. Fritz Geller-Grimm via Wikimedia Commons CC3.0

It's among the worst of nightmares for an arachnophobe: a house full of thousands of spider-like creatures, congregating on the ceilings and walls and dropping to the floor in clumps.

And that's exactly what's captured on video in the YouTube video below (simply titled "NOPE"). But the video leaves a lot of questions unanswered. What are these creatures, and why are they gathered in such large numbers?

These animals are arachnids but not spiders, explains Prashant Sharma, a scientist and postdoctoral fellow at the American Museum of Natural History. They are known as harvestmen, or daddy longlegs. Spiders are in the order Aranea, while harvestmen are in the order Opiliones. There is a popular urban legend that harvestmen are venomous, but this is completely false, as no known species possesses venom glands — unlike spiders, which often do. Their mouthparts also are not hollowed fangs but are instead optimized for grasping and chewing insects and detritus, upon which they feed. They are actually completely harmless to people, Sharma says.

They are known to gather together in large numbers for several reasons. For one, they excrete chemicals from special glands in their bodies that give them a "distinct, unpalatable smell," Sharma says. "With a large group, the effect of the repugnatorial glands is much more pronounced and also helps to repel larger predators."

Secondly, when they are massed together, they appear bigger and are more likely to scare away predators, he says. "When some species are disturbed, like the ones shown in the video, they rock up and down in a rhythmic, undulating motion," Sharma notes. "While a large group of them does this together, the effect is disturbing" to other animals.

"The whole 'swarm' can move simultaneously, looking like a wobbling mass, which must be scary for a predator, and many humans," says Jonas Wolff, a doctoral student who studies daddy longlegs at the University of Kiel in Germany.

Many daddy longlegs are active at night and form clumps during the daytime while they are resting, since they do not burrow and lack the silk-making ability required to make webs, Sharma adds. The people who filmed this video happened upon them in the day and likely disturbed their slumber.

Both scientists think the daddy longlegs in this video are in the genus Leiobunum, a long-legged and mostly nocturnal "species-rich group common in North America and Europe," Sharma says. These arachnids excrete a sticky glue-like substance that helps them ensnare prey, research by Wolff has shown.