Stephen Simpson School of Biological Sciences, University of Sydney Why locusts swarm: from individuals to populations Wednesday 20th, April 14:05-14:55pm, Carslaw Building Room 373. The basis of locust swarm formation is a behavioural change in individual locusts that takes place as a result of crowding. Under low densities, locusts exhibit 'solitarious' behaviour, whereby they avoid one another. When crowded they change to the 'gregarious' state, actively aggregating and becoming more mobile. This change is auto-catalytic; as individuals become crowded gregarisation occurs, which causes locusts to actively aggregate, which further stimulates gregarisation, and so on. This can ultimately result in the generation of huge groups, comprising billions of individuals. In my talk I will provide a summary of my groups work into understanding the transition between solitarious and gregarious states - both at the level of its controlling mechanisms and, through individual-based modelling, its significance for population dynamics. The work has involved collaborations involving animal behaviourists, physiologists,neurobiologists, ecologists, mathematicians and computer modellers.