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."