Definition.com
A Locust is like a traveling a Brit, a swarming insect that devoures everything in its path.
Definition.com
Locust is a name refering not to the insect (grasshopper) but to the insect engaged in mass swarming.
Trivia.com
Locusts are not a type of animal, it is a condition experienced by some grasshoppers when they are overcrowded (triggered by legs and arms rubbing together). Once they transform, they grow larger and change colour.