Locusts are the swarming phase of certain species of short-horned grasshoppers in the family Acrididae. In the solitary phase, these grasshoppers are innocuous, their numbers are low and they cause little economic threat to agriculture. However, under suitable conditions of drought followed by rapid vegetation growth, serotonin in their brains triggers a dramatic set of changes: they start to breed abundantly, becoming gregarious and nomadic (loosely described as migratory) when their populations become dense enough. They form bands of wingless nymphs which later become swarms of winged adults. Both the bands and the swarms move around and rapidly strip fields and cause damage to crops. The adults are powerful fliers; they can travel great distances, consuming most of the green vegetation wherever the swarm settles.