In insects, for instance in moths and butterflies, and no doubt in most if not all others, the sense of smell is astonishingly keen, and serves as the great guide and attraction in courtship and the appeasement of mate-hunger. A single female emperor moth was placed in a box covered with fine net in a room with an open window in a country house. In three hours a dozen males of this species had entered the room, but no other moths. In twenty-four hours there were over a hundred, all fluttering around the net-covered box in which was the female. In this and other similar experiments it was found that the odour of the female moth, though imperceptible to man, clung to the box after she was removed, and that, for some days following, the empty box was nearly as powerful an attraction to the males as when it contained the female. The antennæ which carry the olfactory sense-organs are far larger in the males than in the females, as is also the case in many other lower animals where smell is a guide to mating. A single female of the vapourer moth, which is common in the London squares and parks, has been found to attract when placed in a box in an open window in Gower Street a number of males from the neighbouring plantations; and such is the penetrating and powerful character of these odorous substances produced by female moths that in one species, in which the female is wingless and lives under water, the odour escapes through the water and attracts the males in quantities to its surface. The females then arise from the depths, and, like mermaids or the witch of the Rhine, draw the infatuated males beneath the water to love and death. In several butterflies it has been shown that the males produce sweet perfumes on the surface of the wings, which can be detected as such by man, and act as stimulants to the mate-hunger of the female butterflies, which follow the scented male in numbers. The sense of smell is thus seen to be a much more powerful guide in insects than might be supposed, and it is of equally great importance to them in other enterprises and activities of life besides those of courtship. It has also a leading importance in all the lower and lowermost animals, and is the ultimate guide (for smell and taste are not separable in such simple forms) of the motile spermatic filament in its journey to the egg cell.