Anatomical Wonder: “Daddy Longlegs”

Depending on the type of person you are, you may be either terrified or enamored by those spindly creatures commonly known as “daddy longlegs.” You may have heard that, by weight, daddy longlegs (otherwise known as harvestmen) are “the most venomous spider in the world”—phrase full of farce, as daddy longlegs are a) not spiders and b) absent of any glands or fangs that would secrete venom.

Daddy longlegs belong to a family of arachnids of around 6,500 species more closely related to scorpions than spiders. They have eight legs, but lack silk glands, which may explain why they do not spend time in webs unless of course they are in the unfortunate circumstance of being eaten. Instead, they amble about on the forest floor, in between rocks and logs, feeding on small insects, mites, spiders, vegetable matter—whatever’s available.

Naturalist Theodore Savoy wrote in 1938 that “the study of harvestmen is the study of legs.” Indeed, if there was an Anatomy Hall of Fame, or a collection of anatomical wonders of the world, daddy longlegs’ legs would surely form part.

Photo credit: Tarsomeres on the leg of a daddy longlegs. Source: NPR

The most immediate and obvious feature? Their size. A harvestman’s body usually ranges between three and seven millimeters—the size of a single grain of rice. Their legs, however, can span up to 5.9 inches, or 28 times the size of their bodies. For perspective, imagine a human being traipsing about on 80-foot limbs!

Far from being clumsy stilts, the long legs offer the harvestman multiple ways of locomotion. Scientists have identified four "gaits" common to the arachnids: they can walk, run, bob and zig-zag, and stot like a gazelle (they can lift all four legs simultaneously and spring into the air.)

Harvestmen walk on six legs. If you watch closely as they move about, they wave the remaining two in the air like wands, probing their surroundings for potential threats and food. The second pair of legs comes racked with spines, useful for stabbing a flatworm or for jousting with a compatriot. Conveniently, in a survival tactic also seen in reptiles, daddy longlegs can lose this limb without much detriment to the overall organism.

Harvestmen’s legs do what, in other creatures, would be the work of multiple organs. Certain species’ legs are lined with receptors that sense changes in heat, pressure, different chemicals—a bit like “having tongues, noses, and fingertips all over your knuckles,” says Prashant Sharma, a harvestman biologist at University of Wisconsin Madison.

The daddy longlegs’ legs beg a paradox: how can an organism with such an outsized anatomical oddity be so nimble and quick?

The answer lies in small joints known as tarsomeres. "Tarsomeres are present in other arthropods," continues Sharma, “but only harvestmen use them in such a broad range of behaviors— sensing, climbing, fighting, courtship."

Think of the space on your finger between knuckles. Now imagine that piece of bone being composed of small “articulating” segments. In theory, such an adaptation would enable you to wrap your fingers around a tree branch (or a pencil) two to three times.

Thanks to tarsomeres, harvestmen’s legs essentially become prehensile, “like a curling marsupial tail,” says Vanessa Gonzalez, a computational genomics scientist at the Smithsonian Museum of Natural History. Gonzalez teamed up with Sharma and other researchers to sequence the genome of daddy longlegs. The research helps scientists a) explain why and how daddy longlegs’ legs came to be and b) understand the “basis of extreme adaptations.”

So, there you have it: daddy longlegs, anatomical wonder of the world! Oh, and did we mention they can also grow immaculate beards? We’ll let the video explain…

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