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. ... 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.
locust swarm serotonin depression