Humans share their homes with hundreds of species of flies, spiders, beetles, lice and other arthropods, a new study reports.

Researchers visited 50 houses within 30 miles of Raleigh, N.C., and collected all of the arthropods they could find. They found 579 different morphospecies — meaning they were all physically different — from 304 different families.

The study, which appears in the journal PeerJ, found there were more than 100 species per house.

One arthropod found in all homes was the gall midge, a fly just four-one hundredths of an inch long. Although gall midges live only outdoors, they are probably blown often into homes, said Michelle Trautwein, an entomologist at the California Academy of Sciences and one of the study’s authors.

She and her colleagues also found many booklice, harmless cousins of head lice. “Basically, we are all are living with lice,” Dr. Trautwein said.