The locust swarm of lore is one life phase of a type of short-horned grasshopper. It's a phase that can become a plague under the right conditions. (You might recall the one mentioned in the Book of Exodus.) The swarms can cover hundreds of square miles and consist of many billions of locusts. They are highly migratory and can quickly strip whole fields of vegetation. The swarm is initiated by a population burst caused by rain followed by drought, pushing greater numbers of the insects into a smaller area. Scientists in England and Australia say these close quarters trigger a chemical response. Unfortunately, the change also creates a domino effect — swarming locusts are driven to breed and eat at an increased rate.