Laboratory experiments are described that demonstrate that the antennal aristae of muscoid Diptera are sensitive to slight fluctuations in pressure, acting as external baroreceptors. Further experiments show that the increase in activity exhibited by flies at low pressure is of a kinetic nature, lacking any directional element, while the reaction of flies to manually-produced pressure waves that vibrate the aristae is tactic in a baronegative sense. It is suggested that the erratic prethunderstorm flight of muscoid Diptera results largely from such a baronegative response to localized pressure changes. This suggestion is based on laboratory observations of the reactions of flies under simulated storm pressure patterns.