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How many types of bugs live in your house right this minute? Science weighs in.

Ground beetles are happier outside, but they'll move in if you let them. (Matt Bertone)
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If you’re reading this at home, you are currently surrounded by arthropods — invertebrates with exoskeletons and jointed legs. But just how many spiders, crustaceans and other creepy crawlies live in the average American home? Until now, that question has been difficult to answer.

A new study published in the scientific journal PeerJ seeks to change that. Unsatisfied with mere speculation about the types of arthropods that coexist with humans, entomologist Matt Bertone and his colleagues decided to find out for themselves. So they headed to 50 homes within 30 miles of Raleigh, N.C.

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This was no ordinary census. Armed with knee pads, headlamps, tweezers and vials of alcohol, groups of entomologists scoured the superficial surfaces of each home, putting at least one of each different type of bug they could find in a vial to analyze later. They came away with over 10,000 arthropods of all sizes.

The first part of the research took place in private homes, but the analysis that followed put researchers and bugs alike under scrutiny. Bertone and a colleague delved into the work of identifying each specimen in a lab behind a glass pane at the North Carolina Museum of Natural Sciences’s nature research center. They hooked their microscope up to a monitor so visitors could see each specimen as it was identified — and ooh and ahh over creatures like spiders, lice and flies.

The team didn’t count each individual arthropod found in a home; a potentially impossible task. Rather, they focused on types of bugs. The results indicate a surprisingly diverse ecosystem within each house. The final count revealed no fewer than 579 arthropod morphospecies, or species that can be distinguished by their structures alone. The average human household lives alongside around 100 distinct morphospecies.

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Don’t freak over those numbers, said Bertone. “Most of these bugs are only a few millimeters long,” he said. Indeed, the most common arthropods found were tiny. Over 98 percent of homes contained book lice, 96 percent housed dark-winged fungus gnats, and every home contained cobweb spiders, carpet beetles, gall midge flies and ants. Of these, only a few of the spiders ever grow more than an inch long.

“They’re just milling around at the edges of room, eating little bits of hair and dead insects. This isn’t something that should change people’s behavior," he said. Rather than reaching for the bug spray, said Bertone, people should be excited that they live alongside so many other species — more than he imagined were possible inside these relatively inhospitable bug habitats.

Pests (or insects known to cause some kind of injury, stress, or property damage) were relatively uncommon in the homes studied. Though a majority of the homes (78 percent) did contain large cockroaches, the variety that’s considered a true pest was only found in three homes overall. No bed bugs were found during the study, either.

If you don’t fancy the idea of living alongside up to 750 different types of arthropods, consider downsizing: Though a few large houses contained relatively small bug populations, the vast majority of houses with fewer than 400 morphospecies were below 2,000 square feet in size. And don’t bother changing rooms to escape: Every type of room surveyed contained bugs, and only five rooms (four bathrooms and a bedroom) of the 554 studied contained no critters at all.

Bertone admitted that a superficial look at the arthropod populations of homes in one region only shows the tip of the bug iceberg. But, he said, a sense of what’s normal gives researchers a future basis with which to compare arthropod populations among different regions. Now that the cast of characters within homes has been identified, scientists can try to figure out why they are able to live in homes to begin with. How do arthropods adapt to human environments? Do they depend on specific microorganisms to help them survive? How do homes themselves filter or trap bugs?

There’s an upside for home dwellers, too. Bertone hopes that with a better sense of the arthropod population in the average American home, people can spot outliers — like rare flesh flies or strange beetles — more easily. “If you know a little bit about what’s in the home, it can give you clues,” he said. Despite the fact that our homes are apparently anything but bug-free, said Bertone, shining the spotlight on the arthropods that cohabitate with us might be able to help humans relax about the bugs they share space with.

Erin Blakemore (@heroinebook) is a freelance journalist from Boulder, Colo. She is the author of "The Heroine’s Bookshelf" (Harper). 

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