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The curious case of daddy longlegs clusters

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I’ve always been curious where insects go when temperatures dip and food gets hard to find. I know some flee to more accommodating climes. Some find a spot to lay eggs and then expire. Others dig in and wait it out—underground, inside bark layers, wedged into rock crevices. And they often do so alone.

However, a group of students at Bigfork High School, under the leadership of geoscience teacher Hans Bodenhamer, discovered firsthand that there are also insects that hunker down together. While monitoring a cave in Glacier National Park in fall 2009, the class came across a cluster of overwintering insects called harvestmen, better known as daddy longlegs.

In fact, the cluster was a 2-foot-by-7-foot mass of thousands of the leggy critters, hanging from the ceiling by their “pedipalps,” or feeding appendages, their legs dangling like a thick mat of tangled hair.

Montana Headwall. Outdoor adventure under the Big Sky.
  • Luis Fernández García

As Bodenhamer will tell you, it’s best not to disturb a harvestmen cluster. If it is disturbed, the creatures will flow from the walls or ceiling in a “living, pulsating waterfall,” he says. “It’s kind of creepy, but very cool and beautiful at the same time.”

I had never thought of these gentle backyard bugs as beautiful, but perceptions often change as you learn more about an animal.

Harvestmen belong to the order Opiliones and are related to mites, and not, as is commonly thought, spiders. They have lentil-sized, tannish, oval bodies and eight long, thin, fragile-looking legs. Although they are octopods, like spiders, their bodies are unsegmented, meaning the head and torso is one piece. Spiders have two parts. Spiders are exclusively carnivorous, but harvestmen feed on plants and fungi, in addition to other insects and dead organisms. Perhaps this is how they got the “harvest” appellation.

Unlike spiders, harvestmen don’t have silk glands, so they can’t produce webs, and I’ve always heard they do not bite. My daughter begs to differ, and of course any animal that has a mouth can bite. What is true is that harvestmen don’t produce venom and are not poisonous.

Montana Headwall. Outdoor adventure under the Big Sky.
  • Hans Bodenhamer
  • Students found this 2-foot-by-7-foot cluster of overwintering harvestmen inside a Glacier National Park cave. The photo below shows how the creatures hang from the ceiling by their “pedipalps,” or feeding appendages, and let their legs dangle like a thick mat of tangled hair.

Over the past couple of seasons, Bodenhamer and his students have seen harvestmen clusters in about a dozen caves in northwest Montana, many outside of Glacier National Park. They’ve found caves with multiple clusters, but never a cluster as large as the one they found in fall 2009. What’s unclear, even after years of study, is why the harvestmen gather in the first place.

Back in the early 1980s, a study led by Robert Holmberg of Athabasca University in Alberta, Canada, suggested a number of possibilities. It might be random congregation in a suitably protected microclimate within the cave. It might have something to do with warmth, or possibly water conservation and humidity control. It might be part of the mating game. Most likely, researchers think, it has to do with safety in numbers and a concentration of defense pheromones.

But definitive answers remain a mystery; the Canadian researchers just aren’t sure what’s going on. Bodenhamer and his students came to a similarly open-ended conclusion. When they returned to Glacier National Park in February 2010, the enormous cluster had disappeared, leaving no remains behind.

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