Shortly before Halloween in 2018, an administration building in Glacier Bay National Park and Preserve in Alaska began growing a beard.
But the locks that made up the hairy bangs were not fine brown hairs. They were the slender legs of hundreds of daddy’s long legs tightly tucked together, hanging down their glorious gams.
Park officials took photos of the spooky growth at just the right time and posted them on Facebook and Twitter, and shared them again this grim season, terrifying viewers again.
The ability of Opiliones species, also known as reapers, to form these woolly knots has fascinated arachnid enthusiasts for years. But “we still don’t really know what triggers these aggregations,” said Mercedes Burns, an evolutionary biologist who studies eight-legged creatures at the University of Maryland in Baltimore County.
One of the main assumptions is that the Opiliones come together to avoid drying out when humidity drops in summer or fall, Dr Burns said. As their nickname suggests, most daddy’s long legs are built like grains of rice propped up by super skinny stilts, confronting them with “a big area-to-volume ratio problem,” she says.
With few places to store water and plenty of places to waste it, arachnids dry out quickly. Crouching together creates a microclimate for arachnids, much like a sweaty locker room, which can block the desiccation process.
Another possibility is that melees provide some protection against predators like birds, lizards, and insects. Unlike most of their spider cousins, Daddy’s long legs (which aren’t spiders) don’t produce venom, leaving them somewhat defenseless when taken solo. Grouped together, they might be able to alert each other more effectively in the event of danger.
Arachnids also secrete a potent – and sometimes harmful – substance that is believed to ward off hungry hunters and may be more potent when produced by many harvesters.
Kasey Fowler-Finn, an arachnid expert at Saint Louis University, recognizes the scent, but isn’t particularly disturbed by it. “It’s like sweet chocolate but bitter, maybe a little buttery,” she says. “I tried to train my dog on it.”
If the reapers are forced to disperse, there could be strength in numbers, if only by the bewildering power of centipedes scurrying in different directions. “They do this body-moving behavior that tends to scare people,” Dr. Fowler-Finn said. “It is assumed to be a distraction mechanism.”
Tight knit messes could also come in handy when it comes time to mate some leggy lotharios.
“Really, this is all guesswork,” Dr. Fowler-Finn said. “We don’t have a good answer.”
Congregations also come in many different forms. They can range from tens to hundreds of creepy crawling robots. Some could bring together several species of Opiliones. Others will dissolve and reform over time over time.
And when researchers meet the municipalities, they can also benefit.
“I put them together by literally picking up hundreds of individuals,” said Dr. Fowler-Finn. “And they’re just dripping from your hand.”
The aggregations may appear dense, but in reality, they are just legs – twisting appendages that are dry and brittle and can be removed by some species at will.
Dr. Burns loves his arachnids. But in all fairness, she said, “It’s pretty disgusting.”