Curiously enough, no special auditory organs have yet been discovered, and it has been suggested that these insects do not hear in our sense of the word, but feel rhythmical vibrations. But whether the males “sing in rivalry,” as Dr. David Sharp suggests, is another matter. The purpose of the “song” in the first place is no doubt intended as a guide to the females seeking mates. But it is quite conceivable that the call of one male may stimulate every other male in the neighbourhood. Darwin, commenting on this aspect of the music, gives a quotation from Dr. Hertman, who says: “The drums are now ... heard in all directions. This I believe to be the marital summons from the males. Standing in thick chestnut sprouts about as high as my head, where hundreds were around me, I observed the females coming around the drumming males.... This season a dwarf pear-tree in my garden produced about fifty larvæ of _Cicada pruinosa_; and I several times noticed the females alight near a male while he was uttering his clanging notes.” The structures, he remarks, from which these sounds proceed, “must be ranked amongst the most remarkable voice-organs in the animal kingdom. They are totally different from the stridulating organs that are found in many other insects.... Some difference of opinion has existed as to the manner in which the structures act, but the account given by Carlet ... will, we believe, be found to be essentially correct.” They are partly thoracic and partly abdominal. On examining a male Cicada there will be seen, on the under surface, two plates, meeting in the middle line of the body and overlapping the base of the abdomen. They can be slightly moved away from the abdomen, and thereby a wide fissure will be produced, displaying the mechanism beneath. If the whole operculum be removed, three membranes will be seen, an external, called the “timbal,” an anterior folded and soft membrane, and a posterior “mirror.” This last is a very beautiful object, tensely stretched and pellucid, yet reflecting all the hues of the rainbow. The sound is primarily produced by the vibrations of the timbal, to which a muscle is attached; the other membranes are probably also set vibrating, and the whole skeleton helps to increase or modify the sound, which is probably also influenced by the position of the operculum, which varies in different species. A further control of the tension of the air is exerted by “stigmata” or pores, and certain air-chambers connected therewith. Throughout these pages comment has been made on the apparently “fortuitous” character of complex patterns and structures. The “musical-box” of the Cicada affords yet another instance. Nevertheless there is an impressive harmony between the several parts; an interdependence which is not fortuitous. There is obviously a nexus of growth-controlling factors preserving harmony between each separate part which as yet has escaped all endeavour to discover.