“Start with your windowsills,” advises Rob Dunn. “Light fittings are often a graveyard, too.”

As households across the United States decorate their homes with plastic spiders for Halloween, Dr. Dunn, an applied ecologist at North Carolina State University, is encouraging people to search out the real thing — and then to photograph whatever they find, rather than squash it.

His new project, Never Home Alone, aims to gather at least 10,000 observations of arthropods — insects and their kin — from around the world. Anyone can participate, using the online nature-identification platform iNaturalist; the only condition is that the bugs must be observed indoors.

That is where humans, too, are mostly to be found. “We spend more than ninety percent of our lives inside,” Dr. Dunn said recently, citing a 2001 study funded by the Environmental Protection Agency.

In the world’s densest cities, the indoor biome is bigger than the outdoor space, at least in terms of floor area. (Indoor Manhattan, Dr. Dunn calculates, now exceeds outdoor Manhattan by a factor of three to one.) Yet scientists know almost nothing about the spiders and flies and booklice that inhabit this space alongside us.