The first study to evaluate the biodiversity of arthropods in U.S. homes finds that humans share their houses with any of more than 500 different kinds of arthropods - at least on a short-term basis. Arthropods are invertebrate animals with exoskeletons, segmented bodies and jointed limbs, such as insects, spiders, mites and centipedes. "This was exploratory work to help us get an understanding of which arthropods are found in our homes," says Matt Bertone, an entomologist at North Carolina State University and lead author of a paper describing the work. "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 work was done by researchers at NC State, the California Academy of Sciences (CAS) and the North Carolina Museum of Natural Sciences.

Under an initiative called the "Arthropods of Our Homes," the researchers visited 50 free-standing houses within 30 miles of Raleigh, North Carolina, between May and October of 2012. Going room by room, the research team collected all of the arthropods it could find, both living and dead.

Across all 50 homes, the researchers identified no fewer than 579 different morphospecies of arthropod from 304 different families. Individual homes had, on average, about 100 morphospecies (between 32 and 211) and between 24 and 128 distinct families. The most commonly collected groups of arthropods in the homes were flies, spiders, beetles, ants and book lice. The term morphospecies is used to characterize animal types that are readily separable by morphological differences that are obvious to individuals without extensive taxonomic training.

"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."

For example, researchers found gall midges (Cecidomyiidae) in all 50 homes. But these millimeter-long flies feed on outdoor plants and can't survive indoors.

"The vast majority of the arthropods we found in homes were not pest species," Bertone says. "They were either peaceful cohabitants - like the cobweb spiders (Theridiidae) found in 65 percent of all rooms sampled - or accidental visitors, like midges and leafhoppers (Cicadellidae)."