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The behavior of the desert locust, Schistocerca gregaria (Forskal) (Orthoptera: Acrididae), is adjusted rapidly to population density and is a phase characteristic. We used discriminant analysis to quantify the extent of phase transition from the solitary to the gregarious phase and accurately classify the phase status on the basis of two decisive behavioral parameters: nymphal activity and social interaction. Fecal extracts, examined by olfactometry, attract solitarious nymphs but do not contribute to behavioral phase transition. Neither do visual stimuli alone. Short-range olfaction of airborne volatiles slightly affects behavioral phase transition. Antennectomy abolishes density response. Cuticular lipid extract, containing presumptive contact pheromones, does not attract nymphs, but does significantly affect behavioral phase transition.