Another possibility is that the scrums provide some protection against predators like birds, lizards and insects. Unlike most of their spider cousins, daddy longlegs (which are not spiders) don’t produce venom, leaving them somewhat defenseless when caught solo. Bunched together, they might be able to more efficiently alert each other to danger.
The arachnids also secrete a potent — and sometimes noxious — substance that’s thought to ward off hungry hunters and might be more powerful when produced by many harvestmen.
Kasey Fowler-Finn, an arachnid expert at Saint Louis University, recognizes the smell, but isn’t particularly perturbed by it. “It’s like sweet but bitter chocolate, maybe a bit buttery,” she said. “I tried to train my dog on it.”
If the harvestmen are forced to scatter, there might be strength in numbers, if only by the sheer discombobulating power of a thousand legs dashing off in different directions. “They do this body-bobbing behavior that tends to freak people out,” Dr. Fowler-Finn said. “It’s hypothesized to be a distraction mechanism.”
Tight-knit jumbles could also come in handy when it comes time for certain leggy lotharios to mate.
“Really, this is all conjecture,” Dr. Fowler-Finn said. “We don’t have a good answer for it.”
The congregations also come in many different forms. They can range in number from dozens to hundreds of creepy crawlers. Some might bring together multiple Opiliones species. Others will repeatedly dissolve and reform over time.
And when researchers come across the communes, they can benefit, too.
“I’ve collected them by literally scooping up hundreds of individuals,” Dr. Fowler-Finn said. “And they just drip from your hand.”
The aggregations might look dense, but really, it’s all legs — writhing appendages that are dry and brittle and can be shed by some species at will.
Dr. Burns loves her arachnids. But in all honesty, she said, “It feels pretty gross.”