BUTTERFLIES, MOTHS AND CICADAS 215 



The second part is the area which bears the tooth. 

 It comprises the posterior half of the organ. It is 

 composed of a hard elongated plate prolonged above 

 into a distinct tooth that fits into a recess immediately 

 beneath the knob on the last bar. The diagram 

 (Fig. 8) ought to make clear the relationship of these 

 two essential parts, the thickened ridges and the 

 tooth. 



Now for the attachment of the muscle. When this 

 is traced to its termination it is seen to end in a smooth 



Area 

 bearing 

 ndges 



Area 



bearing 



toofh 



Bar with 

 Central Knob 



Plate with 

 centra' tooth 



FIG. 8. Drum of Cicada. 



plate. A flat tendon then connects this plate to the 

 posterior margin of that portion of the drum which 

 bears the pointed tooth (Fig. 9). The muscle there- 

 fore acts upon the drum and at each contraction pulls 

 upon the tooth. A rapid succession of muscular con- 

 tractions thus throws the tooth into a quick vibration. 

 At each movement the tooth strikes against the 

 thickened bar and a single sound is thus produced. 

 Backwards and forwards moves the tooth. The bar 

 is thrown into a quick vibration and this originates the 

 noise. 



There are left the other parts, the abdominal cavity, 

 the operculum, the shield, 



