This search is conducted by the sense of smell. This fact, familiar enough to-day to the entomologist and the student of Evolution, was unknown to the earlier naturalists. Neither Darwin nor Wallace suspected it. It would have been wonderful if they had, for there is nothing in the general appearance of these insects which suggests an organ of smell, nor is there anything in the structure of the nervous system which would indicate this subtle sense. During recent years, however, the number of workers engaged on the investigation of the senses of animals has increased immensely, and great strides have been made in perfecting instruments of research. To the efforts of these workers we owe the discovery of the seat of the scent-detecting organs and the source of the scent. The former are furnished by the antennæ, which lodge also the senses of taste and touch. Among the Lepidoptera these constitute important secondary sexual characters, the antennæ, among the Moths at any rate, presenting striking differences in male and female. The scent-producing organs are very elusive structures, and so far have been definitely traced, among Butterflies, only in the males, where they are formed by certain peculiarly modified scales known as “androconia.” They may be either irregularly scattered over the wing, or may form complex structures. Sometimes they are arranged in the form of brightly-coloured, bristle-like tufts on the hind-wings, sometimes in a fringe along the edge of the hind-wing. In some of the Moths they are arranged to form a thick, glistening white felt, which fills a folded-over portion of the edge of the hind-wing, and in many cases “the perfume can be retained,” Weismann remarks, “and then, by a sudden turning out of the wing-fold, be allowed to stream forth.” In the Ghost-moth (_Hepialus humuli_), the hind-legs of the male have become pressed into service and have become transformed into scent-bottles, since they are swollen and filled with glands for the manufacture of odorous matter. The naturalist Fritz Müller discovered the fact that some of the Butterflies which haunted his Brazilian garden exhaled a flower-like fragrance. Anyone can test this curious trait for himself who will take the trouble to brush his finger over the wing of a newly-caught male Garden-White Butterfly (_Pieris napi_). The white powder which will adhere to the finger will be found to be made up of the wing-scales, which will exhale a delicate perfume of lemon or balsam! Among the Moths the strong odour of musk is exhaled by the Convolvulus Hawk-moth (_Sphinx convolvuli_).