Cicadas are the worldâ€™s longest-living insects. Of the 3,000 species of cicadas globally, only 7 share synchronised life cycles that allow them to emerge from underground once in 12-17 years. They can survive by feeding on the tree roots during their life underground. A group of cicadas known as Brood X are expected to emerge across the mid-Atlantic for the first time since 2004. It is among the largest cicada broods with a 17-year life cycle.