A previous paper (Canadian Entomologist 40(2): 81–89, 1958) considered the general nature of an air-borne scent and suggested a mechanism by which a flying insect could be guided towards the source of an attractive odor. At first sight, it might seem that a repellent is merely an attractant with a minus sign, but this is an oversimplification, because an attractant may and often does bring the attracted insect in from a distance of a few miles, whereas the repellent has done its work if it removes the unwanted insect to a distance of no more than a few inches. The quantitative difference in the effect is so great as to imply a qualitative difference in the mechanism. On the naive view, the attractant draws the insect upwind and therefore the repellent should drive it downwind. A qualitative difference implies some third result, possibly a tendency to move the insect across the wind. This may, in fact, be the way repellents work in some circumstances, but it can scarcely be the whole story and is mentioned at this point merely to bring out the inherent difference between the attraction and repulsion of insects by odors.