Think of it as good news in bad packaging: The swarms of mayflies that are coating Midwestern towns this summer are a sign that the region’s waterways — most notably the Mississippi River — are healthier than they’ve been in decades. The flies don’t bite or sting; they just mate and die, all in the course of one day, and they do so in such large volumes that they have to be removed with power washers, shovels, and snowplows. Still, their presence is welcome: “They are an indication of the general health of the river,” said John Lindell, district manager for the U.S. Fish and Wildlife Service in Marquette, Iowa. As efforts to clean Midwestern waterways of pollution from sewage and farm runoff have paid off, mayfly populations have boomed. This year’s swarms are bigger than they’ve been in a half-century, with some even visible on weather-radar screens.