INSECTS 



of sound produced. In the periodical cicada, the drum- 

 heads 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 bv 

 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 oval 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 desig- 

 nated 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 drum- 

 heads 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 auto- 

 matic movement. An inspection of the interior of the 

 body of a dead specimen, however, shows that con- 

 nected with the inner face of each drumhead is a thick 



I 208 1 



