Although a house can feel empty when you're the only one home, it isn't really. According to a new study, a typical U.S. household includes not just humans, dogs and cats, but also roughly 100 species of insects, spiders and other arthropods.

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

Described as the first comprehensive survey of its kind, the study scoured 50 houses in North Carolina to reveal "the complete arthropod fauna of the indoor biome" for that sample region. Researchers collected some 10,000 specimens in 554 rooms, representing at least 579 types of arthropods from 304 taxonomic families.

The average house had about 121 arthropod "morphospecies," or species easily distinguished by appearance alone. The most common were flies from 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" from the order Hemiptera (4 percent).

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

"This was exploratory work to help us get an understanding of which arthropods are found in our homes," explains lead author Matt Bertone, an entomologist at North Carolina State University, in a press release about the research. "Nobody had done an exhaustive inventory like this one, and we found that our homes host far more biodiversity than most people would expect."

The study, published Jan. 19 in the journal PeerJ, is based on searches conducted over six months at 50 free-standing homes near Raleigh, North Carolina. The houses were randomly selected, ranging from seven to 94 years old and 840 to 4,833 square feet. The researchers checked the "visible surfaces" in all interior rooms, including behind and underneath furniture but not inside drawers or cabinets.

"Arthropods were found on every level of the home and in all room types," they write. Of the 554 rooms surveyed, only five (four bathrooms and one bedroom) didn't contain any arthropods. Book lice were found in 49 homes, while four other arthropod families were found in all 50: cobweb spiders (Theridiidae), carpet beetles (Dermestidae), gall midge flies (Cecidomyiidae) and ants (Formicidae).

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

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," Bertone says. "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 adds. "They were either peaceful cohabitants — like the cobweb spiders found in 65 percent of all rooms sampled — or accidental visitors, like midges and leafhoppers."

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. House spiders eat a variety of pests like flies, moths and mosquitoes, and house centipedes are known to prey on crickets, earwigs, roaches and silverfish. By taking a closer look at the diversity of this domestic wildlife, scientists hope to shed more light on the ecosystems inside our homes. And that's no small task: According to a 2015 study, the indoor biome is Earth's fastest-growing environment.

"Now that we have a better idea of which species are most common in homes, we can focus on studying them," says study co-author Michelle Trautwein, an entomologist and fly expert at the California Academy of Sciences. "Do they provide important services that we don't know about in the ecosystems of our homes? Do any host microbial organisms that affect our health, for good or bad? And we can also begin to explore their traits to see if they share evolutionary characteristics that have made them better suited to live with humans."