214 A NATURALIST IN HIMALAYA 



escape through the artificial hole. Yet the sound con- 

 tinues. The air pressure within the sac is therefore 

 not the essential cause. 



The only organs now left for investigation are the 

 muscle and the drum. 



It is simple to experiment on these. I remove both 

 drums and the insect remains permanently silent. I 

 divide both muscles ; the result is the same. The 

 cicada cannot utter another note. I take the divided 

 muscle in a forceps and gently pull it and vibrate 

 it. Even these clumsy efforts may generate a faint 

 note. The conclusion is clear. The muscles throw 

 the drums into vibration, and the vibrating drums 

 originate the note. 



The matter must be considered a little further. A 

 simple membrane thrown into vibration by a muscle 

 could not produce this powerful sound unless there 

 was something particular about its structure. Let us 

 consider it in a little detail. The drum is a somewhat 

 pear-shaped membrane about five-sixteenths of an inch 

 in its lono-est and three-sixteenths of an inch in its 

 shortest diameters. It has a pearly white appearance 

 and is crossed by a series of brown parallel bands. 

 Its mechanism is, I think, more easily understood if 

 the drum is considered as consisting of two parts. 

 First, there is the part of the drum which bears the 

 ridges. It comprises the anterior half of the organ. 

 The membrane is here soft and delicate. It is crossed 

 transversely by five chitinous bars. The first of these 

 bars is undeveloped and imperfect ; the second is more 

 complete ; the remaining three are firm and strong 

 and each is thickened in the centre so as to form a 

 5tout oval knob. Such is the ridged area of the drum, 



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