Changes from solitarious to gregarious behaviour occur rapidly and are mediated by serotonin. (A) Behaviour of a solitarious locust (top) and a gregarious locust (bottom) in the test arena is shown. A group of gregarious locusts is presented behind a clear, perforated wall to the right. Shown are representative tracks over ten minutes. After an initial jump, the solitarious locust moves slowly away from the group of locusts and spends a large proportion of the time motionless. The gregarious locust covers much more ground, spends a significant period close to the locust group and is rarely still. (B) Long-term solitarious and long-term gregarious locusts differ markedly in the amounts of key neurotransmitters and neuromodulators found in the CNS. Data from solitarious locusts (blue) are expressed as multiples of the amount found in gregarious (red) CNS (error bars are standard errors of the mean). (C) Time course of changes in serotonin in different brain regions during the entire phase change process, from the initial separation of long-term gregarious locusts through up to three generations of isolation, followed by increasing durations of crowding of long-term solitarious locusts; optic lobes (blue), central brain (red), thoracic ganglia (green). Data for (B) and (C) are from . (D) Serotonin is (i) necessary and (ii) sufficient to induce behavioural gregarization. Histograms showing proportions of locusts displaying fully solitarious (Pgreg = 0 to 0.2) through to fully gregarious (Pgreg = 0.8 to 1) behaviour as measured in the arena (A). In (i), locusts were injected with the serotonin synthesis inhibitor α-methyl tryptophan (AMTP) or a saline control and then subjected to gregarizing stimuli for two hours; AMTP-treated locusts remained solitarious, unlike the controls. In (ii), serotonin or saline was topically applied to the thoracic ganglia for two hours in the complete absence of gregarizing stimuli; serotonin promoted gregarious behaviour. (E) Diagrammatic summary of the behavioural gregarization pathway and the role of serotonin as revealed by pharmacological manipulations such as those shown in (D).