Odour plumes and odour-mediated flight in insects

Flying insects often follow odour plumes to find resources. Some insects may employ an 'aim-and-shoot' strategy using mechanoreceptors before flight to determine wind direction. Once airborne, insects must use optomotor anemotaxis to set a course upwind. This mechanism uses a visual appraisal of how wind modifies the insect's path. A straight upwind course yields a front-to-rear image flow directly below the insect. Details of this process in male moths flying to female pheromone have emerged mainly from wind tunnel studies. Loss of the pheromone triggers 'casting', or wide lateral excursions without upwind progress, whereas contact with a plume usually induces a zigzag path upwind. The temporally regular counterturns in casting and zigzagging seem to be generated by a central programme. Brief contact with a filament of odour induces a heading towards upwind, and an optimal rate of encounter promotes a rapid, straight upwind course. Other insects, such as parasitoid wasps seeking a host and tsetse flies seeking a blood meal, seem not to have a temporally regular pattern of counter-turns and often fly straight upwind. The availability of visual cues from the odour source itself, the aerial distribution of odour set by turbulent diffusion, and light and wind levels all influence the success of these manoeuvres.