Surprisingly, the team had no shortage of volunteers. Although some people seemed to want the researchers to act as exterminators, most were genuinely curious. But even the most arthropod-positive participants were shocked at how many specimens the team found—over 10,000 across the 50 homes. Every room had something. Afterwards, the team would line up their haul and walk the home-owners through the specimens. Bertone recalls, “A lot of people said, ‘Wow that's a lot, I need to do something!’ We said, ‘No, this is typical, we're just searching a lot harder than you would.’”

These tenants are inconspicuous because they’re very good at hiding in pieces of furniture, and tend to be very small. Common residents like book lice, springtails, and carpet beetle larvae are just a millimeter long, if that. “You don't have a giant scarab beetle living under your TV,” says Bertone, reassuringly.

Most previous studies of house-bound arthropods have focused on pests, like cockroaches, fleas, and bed bugs. But these were in the minority; the team found them in just 6, 10, and 0 percent of the houses, respectively. “The negative collective reputation of insects in our homes is undeserved,” says Dieter Hochuli from the University of Sydney. Indeed, most home arthropods were benign visitors and strays that had wandered in from the surrounding environment. “Arthropods are incredibly adaptable, so the habitats we create for ourselves quickly become habitats for them,” Hochuli adds.

The most common arthropod groups, found in all or almost all the homes, included usual suspects, like cobweb spiders, ants, and carpet beetles. There were also more obscure groups like the book lice (wingless, harmless, fungus-eating relatives of parasitic lice), gall midges (creators of tumor-like swellings in plants), and the dark-winged fungus gnats (er, dark-winged gnats that eat fungus). The gall midges turned up in every house, but they're not even mentioned among the 2,000 species listed in a recent handbook of urban arthropods.

By contrast, some of the home arthropods were incredibly rare. “I encountered organisms I’ve never seen before as an entomologist collecting for 15 years in North Carolina,” says Bertone. His team found a telephone-pole beetle, the sole survivor of a long-extinct family, which can reproduce as larvae. They saw a larval-beaded lacewing, a predator that typically lives among termites, paralyzing them with toxic farts. And in five homes, Bertone found spitting spiders, which immobilize their prey by spraying them with venomous silk. “There could be a really cool spider just under your feet!” he says, perhaps less reassuringly.

“This study examined a fauna that was quite literally all around us but deemed of little interest,” says Nancy McIntyre from Texas Tech University. “It's a miniature version of urban ecology as a whole.” She means that ecologists have long ignored cities, due to some imagined gulf between man-made, ‘unnatural’ environments and wild, ‘natural’ ones. But we’re a part of nature too, and it's increasingly clear that our urban ecosystems are worth studying.