In its solitary phase the desert locust is green, avoids other locusts, and lives at low density. In the gregarious phase it is yellow or pinkish, actively seeks company, has different body proportions and a proportionally larger brain, and forms bands as a nymph and swarms as an adult.
The cue for the transition is mechanical. Repeated tactile stimulation of the outer face of the hind femur, of the kind that happens when insects are pushed together onto shrinking patches of vegetation after rain, initiates the change. Serotonin in the thoracic nervous system rises sharply and drives the early behavioural shift, with gregarious behaviour appearing in a matter of hours. Colour and shape follow over moults, and the acquired phase state is partly passed to offspring through the egg.
Monitoring agencies give a scale for the result: a swarm of one square kilometre holds on the order of tens of millions of locusts and can eat roughly the daily food of tens of thousands of people. Predicting which local outbreak will escalate to a regional plague remains difficult, since it turns on rainfall sequences that are themselves hard to forecast.