It's probably safe to say that not everyone would be thrilled to come across a massive spider lurking in the darkness of an old mineshaft. But for a collaborative team of North and South American scientists, it was a fantastic discovery: not only was the spider they found unusually large, but it was also a species none of them had ever seen before.

The Sierra Cacachilas wandering spider. Image: San Diego Natural History Museum

After years of research and peer review, the new arachnid received its official name just last month: Califorctenus cacachilensis, the Sierra Cacachilas wandering spider. It's named for the mountain range where it was discovered in Baja California Sur, Mexico, a part of the world where wandering spiders are quite rare, and not that well understood.

Wandering spiders are fast-moving ground predators that chase after prey rather than trapping it in webbing. Perhaps the most famous of them all is the Brazilian wandering spider, which is notorious for its aggression and dangerous venom. This new spider doesn't seem to be quite as formidable, but it is impressive in size, with an inch-long body and legs stretching four inches across.

"In all my experience over the years collecting spiders on the peninsula, I had never seen a spider this large," said Maria Luisa Jiménez of the Centro de Investigaciones Biológicas del Noroeste in Mexico, who led the study of the new species.

Image: San Diego Natural History Museum
Image: San Diego Natural History Museum

The very earliest discovery of the enormous arachnid actually happened back in 2013 – and the spiders weren't even there for it! A research expedition came across an unusual-looking shed exoskeleton in the cracks of a rock overhang. Recognising that this shed belonged to a wandering spider, and knowing that these spiders tend to be nocturnal, the team returned after dark to find the first living representative.

Once the researchers knew what to look for, they were able to find more members of the new species, and to collect live specimens to take back to the lab for study. Most of the spiders were discovered hanging out in the darkness of mineshafts, a fitting substitute for the caves they're probably more accustomed to.

The scientists suspected the spiders were new to science, but confirming that hunch took quite a lot of work. Identifying a new species starts with comparing it to all other related species to make sure.

"Spiders have been named since the 18th century, so you really need to look for all the literature since then," Daniele Polotow of the Universidade Estadual de Campinas in Brazil told me in an email. "The internet is a big ally nowadays, facilitating access to large libraries around the world."

The spider's egg mass. Image: San Diego Natural History Museum

"After you are sure no one before you has named this species," she added, "you can start thinking about how it relates to all other species from the same group." To do this, the researchers compiled a long list of the arachnids' features and generated a "tree of life" to tease out the evolutionary relationships between the various species.

The most famous relative of the Sierra Cacachilas species might be a mean customer, but it seems the new arachnid is not so bad. In fact, one of the expedition scientists, Jim Berrian of the San Diego Natural History Museum, reported via a blog post that he was bitten by one of the spiders without suffering any nasty side effects.

Still, being careful around possibly dangerous study subjects can be tricky. "While dealing with spiders, there is no one-size-fits-all approach," Polotow explained. "I try to be careful and use basic equipment, like long forceps or tweezers, and place [the spiders] in a secure vial."

So far, no one has studied the venom of the new species, so we can't yet be sure of its effects. "But it's always good to keep in mind that most spiders are not aggressive or dangerous [to people]," Polotow added. "There are more than 45,000 spider species in the world and only a few can be a threat to humans."

In most cases, the relationship is quite the opposite: humans are a danger to wildlife. In the case of the Sierra Cacachilas wandering spider, more study will be needed to determine its conservation status, but Michael Wall of the San Diego Natural History Museum notes that at least some of these spiders have been found in the Sierra La Laguna Biosphere Reserve. Wall organised the expedition that originally tracked down the new species. 

"We can't really say much about [this spider's] relative rarity," Wall said, "but we at least know that part of its range includes a protected area."

* In an omission of similarly large proportions, an earlier version of this article neglected to mention the large spider's size. Those elusive dimensions have now been added.


Top header image: San Diego Natural History Museum