Locusts and grasshoppers may look the same and physically - they pretty much are (besides for the fact grasshoppers hop and locusts can fly).The major difference is how they BEHAVE.
Locusts can exist in two completely different behavioural forms- as loners or in a group (solitary and gregarious) but grasshoppers are only solitary.
When the population density of a group of locusts is low and food is abundant, locusts behave as individuals,and you may not even be able to tell the difference netween a locust and a grasshopper. But, when the locust population density is high they form into groups and initiate swarming behavior. The change from one behavioural state to another is known as a phase change.
But- Tha's not the only thing that changes in a locust. In addition to behaviour, phase change can also alter body shape, color, and fertility!
Scientists have tested this out in a lab. When tapping the insect gently over and over again, a few hundred times (mimicking a high density population) the scientists were able to make a 'calm and solitary locust' phase change into a 'swarming gregarious locust.'