Locusts vs. Grasshoppers

There are many stories in the news lately about the deadly locust invasions, especially in Africa. Locusts belong to the same order as grasshoppers, katydids, and crickets - the Orthoptera (derived from the Greek words orthos meaning straight or rigid and ptera meaning wing).
Grasshoppers congregate in huge swarms that can do severe damage to crops. These swarming grasshoppers are called locusts. There are more than 20,000 species of grasshoppers known to science, but only about a dozen of these transform into locusts and produce damaging swarms.
Locusts and grasshoppers are the same in appearance, but locusts can exist in two different behavioral states (solitary and gregarious), and most grasshoppers do not. When the population density is low, locusts behave as individuals, much like grasshoppers. However, when locust population density is high, individuals undergo physiological and behavioral changes, known as phase polyphenism, and they form gregariously behaving swarms of adults.
In addition to changes in behavior, phase change may be accompanied by changes in body shape and color, and in fertility, physiology, and survival. These changes are so dramatic in some species that the swarming and non-swarming forms were once considered to be different species.  The scale of population increase and migrations also distinguish those species known as locusts from grasshoppers.

Locusts are large herbivorous insects that can be serious pests of agriculture due to their ability to form dense and highly mobile swarms. They are species of short-horned grasshoppers that periodically form large populations in dense migrating groups, where individuals differ in several characteristics from those living separately. A locust has longer and stronger wings and a smaller body than a grasshopper.