This Spider's "Necklace" Was Actually Predatory Parasites Feeding

EARTH, Laniakea Supercluster—Imagine discovering that your necklace was feeding on you.

That's sort of what happened when researchers at São Paulo's Butantan Institute took a closer look at a preserved juvenile spider and realized the not-so-elegant ring of pale beads around its waist wasn't jewelry. It was a ring of parasitic mite larvae, each one about half a millimeter wide, latched onto the spider's most vulnerable spot—a soft joint called the pedicel—and feeding on its lymph, the arachnid equivalent of blood.

Using microscopy and scanning techniques, the team confirmed the mites were an entirely new species, now known as Araneothrombium brasiliensis.

"With more than 3,000 species of spiders alone, Brazil has immense potential for discovering new parasitic mites," said Ricardo Bassini-Silva, the institute's mite specialist in the accompanying press release announcing the findings.

It's only the second spider-parasitic mite ever documented in Brazil and the first of its mite family recorded anywhere in the country. The genus was previously identified only in Costa Rica in 2017, in a study published in the International Journal of Acarology.

What makes the find stranger is the mites' double life—a real-life dark underworld worthy of a Pixar animation mashup of A Bug's Life with the Scream series. As larvae, they're parasites. However, once they mature, they detach from their host, disappear into the soil, and become free-living predators, hunting small insects and even other mites.

"For this group of mites, it isn't uncommon to know many parasitic species only through their larvae," Bassini-Silva said, "since in adulthood they become free-living predators, which makes them very difficult to find."

The parasitized spiders were originally collected near caves and grottos in Pinheiral, a municipality in the state of Rio de Janeiro, before being left in jars for years, unnoticed. So somewhere in the soils of rural Brazil, the freewheeling versions of the mite species are presumably still out there, hunting other tiny creatures in the dark.

A nature horror film in action.