Fig. 5. Changes in the monoamines dopamine, serotonin and octopamine during phase change. Dopamine (A) increased three- to fivefold throughout the central nervous system following a 24 hperiod of isolation of gregarious locusts, but declined to near gregarious values after 1 stadium of isolation. Locusts isolated for longer periods had amounts just above those of gregarious locusts. There was an increase in brain dopamine levels 4–24 h following crowding, but a decline in the optic lobes and thoracic ganglia. Mean amounts of optic lobe serotonin increased eightfold on initial isolation, accompanied by more modest changes in the brain and no change in the thorax. Amounts declined to near gregarious levels throughout the central nervous system on further isolation. 4 h of crowding produced a ninefold increase in thoracic serotonin, followed after 24 h by smaller (fourfold) increases in the brain and optic lobes. There were no significant changes in octopamine (C) during the entire isolation process, but crowding third generation solitarious locusts for 24 h caused a 13-fold increase in the optic lobes and a sevenfold increase in the thoracic ganglia. The amounts of octopamine in the optic lobes remained high after 1 stadium of crowding. Values (mean ± S.E.M.) are the amounts in the three sampled regions of the central nervous system, standardized to give the mean long-term gregarious value as 1. Stages as in Fig. 1.