The research group chose a representative selection of homes, big and small, from 7 to almost 100 years old. They limited themselves to visible surfaces under and behind furniture, baseboards, ceilings, and shelves. I asked researcher Matt Bertone what it was like to knock on a someone’s door, introduce yourself, and then spend hours crawling around their floor with a headlamp and tweezers. He said it wasn’t awkward until the end (well, more awkward than what you’d expect if a complete stranger looked under your couch and bed).
Before leaving, the collection team reviewed what they found with the homeowners. It was a bit of a shock for some. “I think they thought we would only find a few things, and we would come back with hundreds of specimens,” said Bertone. His team collected and identified more than 10,000 specimens during the course of the study. On average, more than 93 different species lived in each home. And that is an extremely conservative estimate.
All the houses examined contained flies, beetles, spiders, and ants. Almost all of the houses (98 percent) contained booklice, harmless tiny insects you’ve probably never heard of. Booklice eat mold and mildew, commonly found in starchy paste of books and wallpaper.
Don’t start pricing flamethrowers on Amazon just yet. “It’s very benign,” says Michelle Trautwein, co-author on the study. “We all have these quiet roommates that really don’t have a lot of negative impact on our lives.” Most of the arthropods found were described by the researchers as “air plankton”; they were accidental entrants into the home that got trapped and died there.
For example, the group found gall midges (Cecidomyiidae) in every home, even though their entire life cycle takes place outdoors. There is nothing for them to eat in a human home. Gall flies are also representative of many arthropods found in one other way: They are best seen with a microscope. A big gall fly is about 2 mm long.
“So much of what we find in houses is incredibly tiny, and even if you’re looking for it, you wouldn’t be able to find it,” said Trautwein. Someone who isn’t a specialist might never see them unless they know what to look for. “I’d be crawling around and I’d see fecal specs and I go, ‘Okay, there’s a spider under there,’” says Bertone. “People don’t know what those are. They see those little spots, but they have no idea that it’s spider poop.”
Now that you are suspicious about every speck you might see in your house, here’s what the researchers didn’t find: not a single brown recluse spider or bed bug. They didn’t find very many fleas. Most of the arthropods they found were harmless, minute, accidental visitors lured in from the outdoors to a quiet death from starvation, dehydration, or cobweb spiders.
The number of bugs in our houses makes more sense when you consider that human-built spaces encompass an area as large as some natural biomes globally. But we know far more about the animals outside our houses than in them. “The finding that ‘non-pest’ species made up the majority, … and the sheer number and prevalence of arthropod groups found highlights [our] current lack of knowledge,” says University of Liverpool biologist Crystal Frost.
Many of the insects identified in the study weren’t able to be identified to species—and some of them will probably turn out to be new species. Our homes aren’t the Galapagos Islands, but they are unique environments that organisms may specialize in. We know that some fungi are found only in bathrooms and washing machines, and the same is quite likely of a few insects and spiders from this study. Carpet beetles, clothes moths, and cobweb spiders have limited abilities to disperse on their own, but we move them from place to place with our belongings.
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