It's only been two years since we last heard that a large number of periodic cicadas would be emerging from the ground, congregating on tree branches and singing their distinctive mating calls. This year, there's news that more will be coming out again. But every time the story is in the news, it's reported that these insects stay underground for 13 or 17 years. So why do we hear about periodic cicadas so often? Duncan McFadyen asks N.C. State University entomologist Clyde Sorenson.