Harvestmen boast limbs that can taste, smell, breathe, seduce, and even coil themselves around twigs two or three times over.
By Katherine J. Wu – staff writer at The Atlantic, where she covers science.
“Let’s start with what harvestmen are not. They are not men, nor do they harvest; they probably get their name from the time of year they’re commonly spotted, in late summer or early fall. Though eight-legged, they are not spiders, but they are part of the broader arachnid group, which also includes ticks and mites. And despite some truly slanderous rumors, they are not venomous and pose absolutely no threat to us who walk upright.
What harvestmen are, though, are extraordinary assemblages of legs—stupendously long, spindly legs that can outstrip their bodies by a factor of 28, a bit like a human tromping around on 80-foot limbs. Some 6,500 species of harvestmen exist, including several commonly referred to as daddy longlegs (again, not spiders), as well as species whose legs are actually quite short; they have swept the planet, inspiring myths and legends about the luck that their astounding appendages bring. Their legs perform the work of several organs at once, serving as weapons, tools of seduction, and sensors that ferry crucial intel to their central, rice-size bodies. That these legs exist at all is a testament to the extremes to which evolution will go to ensure a species’ success. “The study of harvestmen is the study of legs,” the naturalist Theodore Savory once wrote. And the feats their limbs manage are anything but pedestrian.
Humans boast insufferably of their bipedalism, but it’s harvestmen that truly know how to walk. When birds and lizards give chase, harvestmen will fire up their getaway sticks to trot and scamper, scuttling up walls and wedging themselves into cracks. They might zig and zag to disorient their assailants, or even pause to bob frenetically in place—a tactic that’s thought to make them harder to spot and snag.
When harvestmen forage for their own food, their legs come in handy as well. Their eyesight is usually poor—they manage the difference between dark and bright, and can discern hulking objects, but not much else—so they use their limbs to gain a sensory foothold as well. Of their eight legs, the front four are the most sensitive. Their tips are densely freckled with receptors that, in a variety of species, can detect heat, water, pressure, and a panoply of chemicals, a bit like having tongues, noses, and fingertips “all over your knuckles,” Prashant Sharma, a harvestman biologist at the University of Wisconsin at Madison, told me. Many harvestmen will scuttle about on six legs, while keeping their “legs two”—the longest pair— constantly aloft, “waving around and touching everything,” helping them locate meals, Rodrigo Willemart, who studies harvestman sensory ecology at the University of São Paulo, in Brazil, told me. The other six legs function primarily to motor the arachnid around, though the fourth pair can also sport seriously stabby spines, used by some harvestmen to pinch predatory flatworms in two or to joust for access to mates. Some species’ hindmost legs can grow so long that competing suitors will line up to compare them. “Whichever male has the longest leg wins, and it’s the one that is going to mate,” Guilherme Gainett, a developmental biologist at the University of Wisconsin at Madison, told me.
Supporting such lengthy limbs requires anatomical innovation: Some harvestmen’s bodies are so far away from their extremities that the limbs’ lower segments, or tibias, must outfit themselves with air holes to keep themselves oxygenated. But for creatures with such cartoonish proportions, these arachnids are surprisingly stable and adroit. The limbs of long-legged species are perpetually bent, keeping them in a crab-walk that slings their bodies low to the ground. “Imagine if your center of gravity was below your knees,” Mercedes Burns, a harvestman biologist at the University of Maryland at Baltimore County, told me. “It’s extremely tough for them to get flipped over.” Many species’ feet are also composed of dozens of joints—the equivalent of a human finger with about 100 knuckles—that allow them to curl the ends of their limbs around branches two or three times over, like a spider monkey’s prehensile tail. “They climb on trees, stems, twigs, just coiling around,” Willemart said.
This wraparound trick also comes in handy during sexual foreplay. Males of some species will lounge with their legs splayed out, waiting for a female to bang into their limbs. Upon contact, the male will grip his mate with armlike appendages called pedipalps, and snake his legs around hers to “hold them in place,” Kasey Fowler-Finn, a biologist studying harvestmen mating behaviors at St. Louis University, told me. It’s a game of footsie gone awry, a tactic to ensure that the lovers can mate face-to-face, since their genitals are stationed just below where their throats would be. Grabbing onto her legs might help the male “figure out which end is the right one,” Burns told me.
Males will sometimes maintain their hold on the female for hours, long after the deed is done, perhaps to keep her from “bolting off to mate with another male,” Burns said. “It’s basically impossible to separate them; you can’t tell whose leg is whose.” Fowler-Finn is also studying a bizarre behavior in which some postcoital males from one species will nibble on their mate’s second leg and … jiggle it. “I don’t know why it happens,” she told me. “One potential reason is he’s demonstrating how high-quality of a mate he is—the harder he shakes, the better.”
All this leggy labor makes it of tantamount importance for harvestmen to keep their glorious gams clean. Like fastidious felines, they’ll groom themselves by running their legs back and forth through their mouths, whirling and twirling them as if “eating ears of corn,” Ignacio Escalante, a harvestman biologist at the University of Wisconsin at Milwaukee, told me. “It’s super cute.”
But Escalante’s primary interest is in harvestmen that lose their legs—sometimes very much on purpose. Super-long limbs, while dead useful, can also be a liability. So when their limbs are threatened, many harvestmen will simply slough them off and scram. “You can hear a little click,” Fowler-Finn said.
This self-sacrificial act, called autotomy, is actually pretty common—reptiles jettison their tails, and honeybees their stingers, for instance. Escalante describes the harvestman version like a shoulder violently dislocating—“you have a kind of contraction, and it just breaks.” A hard little plate then slides into place to stanch the wound. Many predators are left agog at the now-loosed leg, which can twitch of its own accord for several minutes after it’s severed. Researchers who gather harvestmen in the field have learned to collect them carefully. “You’re always worried you’ll cause them to drop a leg,” Fowler-Finn said. Females seem to use this detachability to their benefit, biting or tugging on the limbs of irksome males until they simply fall away. It’s a good way to end an unwanted tryst, she told me: “Just pop a limb off during sex.”
The whole ordeal sounds traumatic, but Escalante is convinced that most harvestmen amputees actually fare pretty well, at least if they don’t lose more than a couple of legs. (The limbs can’t grow back.) The arachnids will shift their stance and can still jog around at roughly similar speeds. Having an unbalanced body can actually encourage a sort of disorganized canter that confuses predators, possibly making the wobbly creatures even tougher to snatch.
Even around the junctures where their legs meet the body—spots Escalante fondly refers to as arachnid “armpits”—harvestmen have very particular adaptations, including little pores that exude rank chemicals meant to drive enemies away. “They smell super strong,” Escalante told me. “Sometimes when you grab them they will release a little drop of milky, yellow-y solution … It can have a sweet, gasoline smell.” Certain harvestmen will even tuck their legs inward to anoint themselves in the solution, raking their limbs across the body to ensure maximum stank.
Harvestmen first appeared at least 400 million years ago, and have since splintered into very diverse forms. Scientists have plenty of open questions about even their well-documented acts: It’s still unclear, for instance, why some harvestmen will cluster in thick, woolly knots, often on the sides of buildings, where their tightly packed legs will dangle in remarkable impressions of beards. Some researchers think it’s a defensive tactic, a strength-in-numbers strategy that concentrates their potent antipredator smell. Others say these aggregations have the makings of an orgy, or maybe a humidifying cuddle puddle that keeps the harvestmen’s skinny limbs from drying out when the weather turns cool. “It’s one of the cons of having really long legs: You have a surface-area-to-volume problem,” Burns told me. It’s part of the reason the arachnids’ limbs feel so “stiff and crispy” when you pick them up, like paint-coated hairs, Fowler-Finn said.
Some answers might be hidden in the harvestmen genome. Gainett and Sharma, of the University of Wisconsin at Madison, recently teamed up with the genomics expert Vanessa González, of the Smithsonian National Museum of Natural History, to cobble together the first-ever draft of the genome of Phalangium opilio, the world’s most widespread harvestman species—a breakthrough that will “help us understand the basis of extreme adaptations,” said Fowler-Finn, who wasn’t involved in the work. The trio and their colleagues tracked down some of the genes that dictate how the legs take on such distinctly different fates, and others that help harvestmen feet become flexible and grippy. Harvestmen, they confirmed, pattern their bodies a lot like insects do. Certain genes get flipped on in horizontal stripes that encircle the animals’ torsos, telling each leg to acquire its final identity. Only a couple of genes stop the front-most legs from becoming armlike pedipalps, they found. The researchers were even able to prove this by shutting some of the leg genes down, shortening the forelimbs of some laboratory specimens into downright stubs. “We made daddy shortlegs,” González told me. “That was the most amazing outcome.”
Sharma’s team also thinks that the harvestman genome is one of our best bets at understanding how arachnid ancestors—the ones who begat spiders, ticks, mites, harvestmen, and more—might have structured their DNA. “What we find in the harvestmen reflects the early arachnid,” Gainett told me. “It’s what we need, if we want to understand the basic body plan” of this group. As strange as they might seem to us, harvestmen and their legs are a modern variation on a very old motif.”
Katherine J. Wu is a staff writer at The Atlantic, where she covers science.