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But the most peculiar characteristic of this family, however, consists in the structure of the mechanism by which the males make the trilling sound for which they have been so long famous. In the male of the Seventeen-year Cicada the musical instrument consists of two stretched membranes, one on each side of the body, which are plainly to be seen immediately behind the wings. These membranes are gathered into numerous fine plaits, and are played upon by muscles or cords fastened to their under surfaces. When these muscles contract and relax, which they do with great rapidity, the drum-heads, which the membranes resemble, are alternately tightened and loosened, the effect of this alternate tension and relaxation being the production of a rattling sound very much like that caused by a succession of quick pressures upon a slightly complex and elastic piece of tin-plate. Certain cavities within the body of the insect, which may be seen on raising two large valves beneath the abdomen, and which are separated from each other by thin transparent partitions of the brilliancy of mica or highly polished glass, tend to increase the intensity of the sound.