Fly spray is a chemical insecticide that comes in an aerosol can that is sprayed into the air to kill flies. Fly sprays will kill various flying insects such as house flies and wasps. Fly sprays often contain the powerful insect toxin dichlorvos which is often targeted by environmental groups as a carcinogenic compound. Fly sprays kill insects by interrupting the nerve signals from the motor neurons in the insect. This in turn causes them to go into a permanent contraction which makes flying and respiration impossible, and the insects then die of asphyxiation.