INSECTS
of sound produced. In the periodical cicada, the drumheads are exposed and are easily seen when the wings are lifted; in our other common cicadas each drum is concealed by a flap of the body wall.
The sound made by an ordinary drum is produced by the vibration of the drumhead that is struck by the player, but the tone and volume of the sound are given by the air space within the drum and by the sympathetic vibration of the opposite head. The air within the drum, then, must be in communication with the air outside the drum, else it would impede the vibration of the drumheads.
All these conditions imposed upon a drum are met by the cicada. The abdomen of the insect, as we have seen, is largely occupied by a great air chamber (Fig. 123), and the air within the chamber communicates with the outside air through the spiracles of the first abdominal segment (ISp). In addition to the two drumheads whose activity produces the sound, there are two other thin, taut membranous areas set into oral frames in the lower side walls of the front part of the abdomen (not seen in the figures). These ventral drumheads have such smooth and glistening surfaces that they are often designated the "mirrors." The wall of the air sac is applied closely to their inner surfaces, but both membranes are so thin that it is possible to see through them right into the hollow of the cicada's body. The ventral drumheads are not exposed externally, however, for they are covered by two large, flat lobes projecting back beneath them from the under part of the thorax.
The cicada does not beat its drums or play upon them with any external part of its body. When a male is "singing," the exposed drumheads are seen to be in very rapid vibration, as if endowed with the power of automatic movement. An inspection of the interior of the body of a dead specimen, however, shows that connected with the inner face of each drumhead is a thick
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