As if spiders aren't terrifying enough for some people, now we have to worry about ones that can "fly."

A hotel in Chicago warned of some uninvited "guests," this time of year. Hilton Chicago Magnificent Mile Suites sent a note suggesting guests keep their windows closed, "to avoid the annual migration of high-rise flying spiders, a Chicago phenomenon."

Known as "bridge spiders" or "gray cross spiders," they don't technically take flight, but instead spin balloon-like webs that are swept up by the wind and carried great distances. The spiders can grow to the size of a half-dollar coin.