Daddy long-legs—spider-like arachnids technically known as “harvestmen”—are already plenty unnerving, with their too-long-to-be-real limbs, jerky movements, and habit of clustering into a horrible, hairy mass. But researchers have recently unveiled a variety of harvestman totally new to science, and it cranks the creepy up to 11.The creature, discovered deep in a cave in Argentina, looks little like the familiar, gangly pals that sway across summer sidewalks. This arachnid sits squarely where the heebies meet the jeebies: ghostly white, with slender, grasping arms festooned with rows of spikes. Its exoskeleton is doughy; its eyes reduced to tiny, dark freckles. If the daddy long-legs squatting in your garage is Sméagol, then this Argentinian cave beast is Gollum. Like Tolkien’s famous recluse, the harvestman hails from a high-altitude grotto, but in the Andes, not the Misty Mountains.Cave biologist Marcela Peralta collected several of the arachnids while exploring Doña Otilia Cave between 2006 and 2012. The cave, which sits in a mountainous, volcanic region of Argentina’s Mendoza Province, is a lava tube, a tunnel left behind when the outer layers of a subterranean vein of lava harden and cool, like the skin that forms on a bowl of soup. Today, Doña Otilia is a half-mile long, pitch-black hallway that runs parallel to the dusty cinders above. Doña Otilia cave, about 350 meters from the entrancePhoto: Luis E. AcostaPeralta sent the weird critters to Luis Acosta, a zoologist and harvestman expert at the National University of Cordoba.“At first sight I thought I had juvenile specimens in my hands,” Acosta told Gizmodo by email. “They are normally useless for descriptive work, so I asked Marcela to ‘try to collect an adult male.’”But when Acosta looked closer he was shocked to discover the specimens, despite looking developmentally immature, were adults, both male and female.It was clear to Acosta that the harvestmen were troglobites—animals so specialized for life in caves that they can no longer survive on the surface. Their permanent pallor, near lack of eyes, and stretchy, grippy, feely parts are pretty standard for eons of evolution in the blackness. Other species of cave harvestmen were known, but these new ones were objectively weird as hell.There are thousands of harvestmen species, and many look nothing like the spindly daddy long-legs common in the Northern Hemisphere. Some have huge lobster claws or grabby bits covered in thorns. Others—especially in South America—have stubby legs and hulked-out, armored bodies. Acosta thought the cave harvestmen might belong to this latter group, but further investigation showed that no, they were even stranger.Few differences existed between the males and females, and features of the legs and genitals didn’t line up with any known family of harvestman. The new species—which Acosta named Otilioleptes marcelae, describing it in a study published last month in PLOS ONE—also appears to be in an entire family of its own.The fact that Doña Otilia harbors harvestmen at all is a little incredible. Above ground, the volcanic terrain is so parched that it’s almost “inconceivable” that a harvestman might found on the surface, said Acosta, noting their need for moist conditions.The cave harvestman may be a relict from an ancient time when the local climate was wetter, surviving for ages in its subterranean shelter. There, it waited in perpetual silence, feeding and breeding, making babies with ever-smaller eyes and ever-longer claws. In time, Otilioleptes forgot the warmth of the Sun and the kiss of a breeze.Arid above-ground landscape in the Payunia volcanic belt, near the site of the cavePhoto: Luis E. AcostaAcosta thinks the arachnid’s separation from its overworld brethren may have happened between 11 million and 16 million years ago but noted that examining the harvestman’s genetics would help narrow down the timing.Caitlin Baker, an evolutionary biologist at Harvard University not involved with the discovery, agreed that DNA analysis would answer a lot of questions. The new species is so cave-adapted, she said, that it doesn’t have a lot of the anatomy you’d normally use for classification. Such research “could shed light on how this lineage of harvestmen got into this lava tube system in the first place,” Baker told Gizmodo in an email.There’s plenty to learn about about Otilioleptes, from what it eats to its most basic of habits. But Doña Otilia is an extremely fragile habitat. Its pale residents are gravely vulnerable to even minor intrusions, and unregulated recreational caving activities are common in the area, Acosta said.“An increased traffic of visitors inside the cave would [result] in a rapid deterioration of the delicate microclimatic conditions,” warned Acosta, emphasizing that the cave needs prompt protection.Without a way to temper impacts from surface dwellers at Doña Otilia and elsewhere, a whole parade of other fascinating, gloomy beings could be lost to the dark before they’re ever discovered by us.