Although a house can feel empty when you're the only one home, it isn't really. A typical human household includes roughly 100 species of insects, spiders and other arthropods, and according to a new study, there isn't much we can do about it.

Arthropods might make lousy company, but their presence shouldn't necessarily bug us. Many come inside by accident, researchers say, and very few cause any trouble. The vast majority are harmless, and some can even be helpful.

The new study is part of a multi-year, seven-continent research project focused on the tiny ecosystems within our homes. These creatures have been living inside human dwellings for at least 20,000 years, and as the study shows, they still live with us no matter how clean we keep our homes. Insects and arachnids are a normal part of virtually every human household, the researchers say.

"We are just beginning to realize — and study — how the home we create for ourselves also builds a complex, indoor habitat for bugs and other life," says lead author Misha Leong, postdoctoral researcher at the California Academy of Sciences (CAS), in a statement. "We're hoping to better understand this age-old coexistence, and how it may impact our physical and mental well-being."

In fact, the bugs we often view as intruders could benefit our indoor biomes, points out study co-author and CAS entomologist Michelle Trautwein.

"While the idea of uninvited insect roommates sounds unappealing, bugs in houses may contribute to health in a roundabout way," Trautwein says. "A growing body of evidence suggests some modern ailments are connected with our lack of exposure to wider biological diversity, particularly microorganisms — and insects may play a role in hosting and spreading that microbial diversity indoors."

The great indoors

pie chart of household arthropodsThe average diversity of arthropods collected from all room types. (Image: Matthew A. Bertone/PeerJ)

The new study is based on investigations of 50 houses in North Carolina, which yielded roughly 10,000 specimens from 554 rooms, representing nearly 600 types of arthropods in 300 taxonomic families. It's the latest update from the "Great Indoors" project, which has already outlined "the complete arthropod fauna of the indoor biome" and found that arthropod diversity is higher in more affluent homes.

While these studies use North Carolina as a sample region, the findings echo what researchers have been seeing in homes worldwide, Trautwein tells the Washington Post. "We've been sampling houses all over the world, and it's true globally," she says. "Bugs don't respect the limitations, the borders we've created. They just view our houses as extensions of their habitat."

ant Ants are one of four arthropod families found in 100 percent of homes surveyed. (Photo: Steve Jurvetson/Flickr)

The average house has about 121 arthropod "morphospecies," or species easily distinguished by appearance, the project has found. The most common are flies in the order Diptera, which accounted for 23 percent of arthropods in an average room. Next were beetles (19 percent); spiders (16 percent); ants, bees or wasps (15 percent); lice (4 percent); and "true bugs" in the order Hemiptera (4 percent).

"Arthropods were found on every level of the home and in all room types," the researchers reported in an earlier study, published in 2016. They found book lice in 49 homes, while four other arthropod families were detected in all 50: cobweb spiders, carpet beetles, gall midge flies and ants.

Snug as a bug in a rug

Anthrenus carpet beetle larva Carpeted rooms host a wider array of insects, like this carpet beetle larva. (Photo: D.K. Kucharska/Shutterstock)

According to the new study, published in the journal Scientific Reports, arthropods abound in even the cleanest homes. Tidiness does not play a significant role in a home's arthropod diversity, the study found (with the lone exception of cellar spiders, which thrive in cluttered areas of basements and crawlspaces). Neither does the presence of dogs, cats, houseplants, dust or pesticides.

There are plenty of good reasons to keep your home clean and uncluttered, but as the CAS explains in a press release, human behavior plays a "minimal role" in determining which insects and spiders share our homes.

The study did reveal some patterns, though. Arthropods seem to prefer a building's lower levels, with research showing greater diversity on ground floors and in basements, especially large rooms. A wider range was also found in carpeted rooms versus those with bare floors, and in "airier" rooms with more windows and doors. Common areas like living rooms host more biodiversity than bedrooms, bathrooms or kitchens, while basements tend to host unique communities of cave dwellers, like spiders, mites, millipedes, camel crickets and ground beetles.

'A complex ecological structure'

house centipede eating a fly A common house centipede feeds on a captured fly. (Photo: Andrew Skolnick/Shutterstock)

In every type of room, the researchers found "a complex ecological structure of predator and prey," with key roles played by resident arthropods as well as strays that wander in from outside.

Some arthropod species have evolved to live indoors, although the researchers say many of the specimens they collected were accidental intruders. Gall midges, for example, feed on outdoor plants and can't survive inside for very long.

"While we collected a remarkable diversity of these creatures, we don't want people to get the impression that all of these species are actually living in everyone's homes," said team member Matthew Bertone, an entomologist at North Carolina State University, in 2016. "Many of the arthropods we found had clearly wandered in from outdoors, been brought in on cut flowers or were otherwise accidentally introduced. Because they're not equipped to live in our homes, they usually die pretty quickly."

As for the intentional intruders, most are upstanding citizens. "The vast majority of the arthropods we found in homes were not pest species," Bertone added. "They were either peaceful cohabitants — like the cobweb spiders found in 65 percent of all rooms sampled — or accidental visitors, like midges and leafhoppers."

Good bugs, bad bugs

Parasteatoda tepidariorumAn American house spider, Parasteatoda tepidariorum, with a leafhopper. (Photo: Wikimedia Commons)

The survey did find pests, just not as many. German cockroaches were in 6 percent of houses, subterranean termites were in 28 percent, fleas were in 10 percent and bed bugs weren't found at all. About 74 percent of houses did have cockroaches, but only three had American cockroaches — a "true pest," the researchers write. The rest were smoky brown cockroaches, which have a slightly better reputation.

Not only are indoor arthropods mostly benign, but some could be beneficial. On top of Trautwein's point about their role in promoting microbial diversity — which can strengthen the human immune system — some also offer more direct perks. House spiders eat a variety of pests like flies, moths and mosquitoes, for example, and house centipedes are known to prey on crickets, earwigs, roaches and silverfish.

By investigating the diversity of this domestic wildlife, scientists hope to shed more light on the unique ecosystems inside our homes. And that's no trivial task: According to a 2015 study, the indoor biome is Earth's fastest-growing environment.

"Even though we like to think of our homes as shielded from the outdoors, wild ecological dramas may be unfolding right beside us as we go about our daily lives," Leong says. "We're learning more and more about these sometimes-invisible relationships and how the homes we choose for ourselves also foster indoor ecosystems all their own."

Editor's note: This story has been updated since it was first published in January 2016.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.