INSECTS : WINGS AND THEIE APPENDAGES. 97 



those genera wliicli feed on ordure and carrion do 

 this. 



But the noisiest of all insects are those of the classes 

 Orthoptera and Homoptera, the Crickets and Grasshop- 

 pers, and the Treehoppers. And these shall bring us 

 back to our microscope, to which we shall return with 

 the more zest, after this little interval of repose for our 

 strained eyes. 



Listen ! we hear coming up the kitchen-stairs, the 

 stridulous chii-ping of the House-cricket {AeJieta do- 

 mestioa). 



" The cricket chirrups on the hearth." 



The cook shall catch us one for investigation. " Please 

 sir, here's the crickets : here's half a dozen on 'em. I 

 don't like 'em, I don't ; nasty noisy varmint !" Thank 

 you, cook ; we'll try and turn them to some useful pur- 

 pose to-day, at least. 



Now, you see, each of the upper wings or wing-eases 

 has a clear space near the centre, of a triangular form, 

 crossed by one or two slender nervures. This space 

 has received the name of the tympanum or drum. It 

 is bounded externally by a broad dark nervure, which 

 with a low power we see is scored with three or four 

 longitudinal furrows, of course separated by as many 

 homy ridges. In front of the clear drum, and form- 

 ing a curved base to the triangle, there passes across 

 a homy ridge, tapering outwards, which is roughened 

 throughout its length by close-set teeth exactly like a 

 file. When the insect chooses to be musical it partially 

 opens and closes its wing-sheaths, causing the two files 

 to rub across each other ; and this gives rise to the 

 peculiar ringing vibration the intensity of which is 

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