It is one of nature's most radical transformations - the moment a crowd of harmless desert locusts begins to swarm into a devastating plague.
Now scientists from the UK and Australia say they have discovered the trigger - the brain chemical serotonin.
The molecule is best known in humans as a target of anti-depressant drugs.
The discovery could lead to new control strategies for the pests, which plague 20% of the world's land, they write in Science journal.
Desert locusts are known to swarm by the billions, inflicting severe hardship on farmers in parts of Africa, China and other areas.
But the insects actually spend much of their life in a harmless, "solitary" phase.
"To find that serotonin is what causes a normally shy, antisocial insect to gang up in huge groups is amazing"When food runs short, they slowly become clustered together and enter their "gregarious" phase, culminating in an aggressive swarm.
Prior to swarming, the locusts undergo a series of dramatic physical changes - their body colour darkens and their muscles grow stronger.
To find out the chemical signal that triggers this metamorphosis, scientists from Oxford University, Cambridge University and Sydney University began monitoring locusts in the laboratory.
They triggered the gregarious behaviour by tickling the beasts' hind legs, to simulate the jostling they experience in a crowd.
They found that locusts behaving the most gregariously (in swarm-mode) had approximately three times more serotonin in their systems than their calm, solitary comrades.
"The question of how locusts transform their behaviour in this way has puzzled scientists for almost 90 years," said co-author Dr Michael Anstey, from Oxford University.
"We knew the [physical] stimuli that cause locusts' amazing Jekyll and Hyde-style transformation.
"But nobody had been able to identify the changes in the nervous system that turn antisocial locusts into monstrous swarms.
"Now we finally have the evidence to provide an answer."
The team say their finding opens up a new idea for a locust control strategy - a chemical that inhibits serotonin and thus converts swarming locusts back to their solitary phase.
In humans, by contrast, keeping serotonin levels high is the aim of many anti-depressant drugs.
"Serotonin profoundly influences how we humans behave and interact," said co-author Dr Swidbert Ott, from Cambridge University.
"So to find that the same chemical is what causes a normally shy, antisocial insect to gang up in huge groups is amazing."
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