216 A NATURALIST IN HIMALAYA 



The function of the abdominal sac is to control the 

 volume of the note. The cavity is in direct communi- 

 cation with the inner surface of each drum. When the 

 tension of the enclosed air increases then an additional 

 pressure is thrown upon the membrane ; the bars are 

 more tightly stretched, and therefore, as in the case of 

 a violin string, they give rise to a more powerful note. 



The function of the opercula is obscure. They lie 

 so close upon the abdomen that they force the sound 



■{Oorsal surface of i^.^aDaominal 

 I segment. 



Drurn. 



Chitlnous rod of drum. 



Tooth of drum. 



Tendon connectfn^ muscle fo tooth 



Plate at end of muscle. 



Muscle 



Ventral surface of i^^'abdomfnaJ 

 segment 



Fig. 9. — Diagram to show essential parts of 

 musical organ of Cicada. 



to issue through a narrow slit, and this probably 

 increases the intensity of the note. It may also be 

 possible that the opercula serve to direct the sound 

 in a special direction suitable to the purpose of the 

 insect. Nevertheless, these suggestions do not seem 

 to me sufficient to explain the function of such large 

 and conspicuous organs as the opercula. 



The shields are merely what their name implies : 

 they simply protect the fragile drums. 



vSuch is the musical organ of the cicada, a perfect 



