The responses of insects to olfactory stimuli and to air-borne signals generally can be correctly understood only when the stimulus is presented in a natural way, and, moreover, one which is fully understood by the experimenter.

Molecular diffusion is an extremely slow process, and diffusion gradients are relatively unimportant in providing directional guidance. Aluch more important are the mass movements of the scented air, so that, for example, Drosophila melanogaster give one kind of response to an attractive scent when it is presented in a moving air current and a comp1etely different response to the same smell when the air is still.