Aside from pets, family members, or roommates, many of us often go weeks without seeing another living thing in our homes. But appearances can be deceiving. We are, in fact, surrounded by arthropods—insects, spiders, centipedes, and other animals with hard external skeletons and jointed legs. They are the most successful animals on the planet, and the walls that shield our homes to the elements are no barriers to them.

In the first systematic census of its kind, a team of entomologists combed through 50 American houses for every arthropod they could find, and discovered a startling amount of diversity. Each home had between 32 and 211 species, belonging to between 24 and 128 families. Most are not pests. Many were found everywhere, and yet are so obscure that only keen naturalists know about them. These bugs are our closest creaturely neighbors, and we barely register their existence.

“I hope this doesn't put fear in people's minds that they’re being overrun or that they live in unclean homes,” says Matthew Bertone from North Carolina State University, who led the study. “People have been living with these animals for centuries. This is just something that is.”

To conduct his census, Bertone teamed up with Rob Dunn and Michelle Trautwein (who had recently sampled the mites that live on our faces). Together, the team swept through 50 houses in Raleigh, North Carolina. Room to room, floor to ceiling, under shelves and behind knick-knacks, they grabbed every arthropod they could find using nothing more advanced than headlamps, forceps, nets, and aspirators for sucking up fast-moving ones. “And kneepads,” says Bertone. “Those were the best pieces of research equipment we invested in.”

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.

“It’s the way of the future,” says Emily Hartop from the Natural History Museum of Los Angeles, who recently identified 30 new species of fly in L.A.’s backyards. “So many people already live in urban environments and that proportion is increasing all the time. We need to know what's surrounding us. We’re so used to learning about the cockroaches in our bathroom or the ants in our sugar, but in terms of the benign or beneficial insects in the home, we know next to nothing.”

McIntyre points out that the team only looked at houses in Raleigh, a temperate-zone city surrounded by forest. They might find fewer species in sparser environment, or more species in the tropics, or more pests in a dense city. And what about climate, house design and age, open windows, or the number of green spaces around? “We need more cross-city comparisons,” she says.