The Phase Void — Locust Swarm Transition as In Vivo Pe Boundary Crossing

Desert locusts (Schistocerca gregaria) exist in two discrete behavioral phases: solitary (Pe<1, low O/R/C) and gregarious (Pe>4, O=3/R=3/C=3). The phase transition is triggered by crowding-induced serotonin release — a neurochemical signal opaque to the individual. This is the first substrate demonstrating the Pe V* boundary crossing as a discrete biological switch rather than a continuous gradient. Solitary phase = COHERENT regime. Gregarious phase = DRIFTING regime. The serotonin trigger IS the V* crossing mechanism. Spearman anchor: population density at phase transition vs. Pe_gregarious across Schistocerca and related species.