INSECTS : 'WINGS AND THEIB APPENDAGES. 85 



base of the wings against the sides of the cavity in which 

 they are inserted.* 



There is a pretty little beetle (Clytus), not uncommon in 

 summer in gardens, remarkable for the brilliant gamboge- 

 yellow lines across its dark wing-cases, which makes a 

 curious squeaking sound when you take it in hand. You 

 think it is crying ; but if you carefully examine it with a 

 lens while the noise is uttered, you will perceive that the 

 cause is the grating of the thorax against the front part of 

 the two wing-cases. Several other beetles produce similar 

 sounds when alarmed, by rubbing the other end of the 

 wing-sheaths with the tip of the abdomen. Many of those 

 genera which feed on ordure and carrion do this. 



But the noisiest of all insects are those of the classes 

 Orthoptera and Homoptera, the Crickets and Grasshoppers, 

 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 

 chirping of the House-cricket (Aeheta domestica). 



We will catch one for investigation. 



Now, you see, each of the upper wings (or wing-cases) 

 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 horny ridges. 

 In front of the clear drum, and forming a curved base to 

 the triangle, a horny ridge passes across, tapering out- 

 wards, 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 then closes its wing- 



* " Introd. to Entom." ; Lett. xxiv. See, however, the quotation 

 from Bnrmeister on p. 91, infra. 



