By Yelena Mandenberg With a staggering trillion cicadas expected to surface across the United States this spring, experts have revealed a few clear signs that this rare double-brood event that hasn’t occurred in over two centuries is about to hit. These insects typically emerge around mid-May, but in Missouri, they could make an early appearance due to warmer temperatures potentially waking them from their lengthy sleep. Historically, periodical cicadas have been known to emerge when soil temperatures 8 inches below the surface hit 18C, as per a 1968 research study. Current data from the Natio…