Actually, Luke, the cicada is almost always called a "17-year locust" rather than simply "locust." I've heard of katydids being called "locust," but really the plain old garden variety grasshopper is the true locust. When they hop, they're grasshoppers; when they swarm, they're locusts.
The plagues of "locusts" that devastated the American Midwest in the 1930s were just that: swarming grasshoppers. They ate my grandpa's barn doors, shovel handles, and fenceposts. They were grasshoppers.