Nature
A single touch on its leg can turn a lone desert locust into a swarming monster
Desert locusts exist in two dramatically different forms - shy solitary insects and gregarious swarming ones - controlled by the same genome. Touching, seeing, or smelling other locusts triggers a surge of serotonin in their nervous system within hours, flipping them from avoidance to attraction and rapid activity. This single neurochemical switch is what turns scattered insects into a coordinated plague of billions.