A diffuser for distributing insects such as mosquitoes from an aircraft, comprises a distribution tube connected to an insect source at a first end and open to the outside at a second end in a direction away from that of travel of the aircraft, for distribution of the insects; and a profile surrounding the second end, the profile being shaped to define a steadily changing airspeed. The pipe may be aligned with airflow at the point of exit of the pipe and protection may be provided around or just upstream of the exit to provide a point of minimal air velocity at the exit, the exit being the point of release of the insects.