INSECTS: WINGS AND THEIR APPENDAGES 105 



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-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, 

 there passes across a horny ridge, tapering outward, which 

 is roughened throughout its length by close- set teeth ex- 

 actly 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 

 heightened by the tense "drum" acting as a sounding 

 board. 



So at least some say; but M. Groureau, who has pub- 

 lished some elaborate observations on the chirping of in- 

 sects, l asserts that the sound is chiefly owing to the action 

 of the "file" (which he calls the "bow") on the longitudi- 

 nally-ridged nervure, which he calls the " treble- string. " 



We see in this individual, that is so obliging as to 

 produce what cook calls its "nasty noise" before us, that 

 he elevates the wing-sheaths so as to form an acute angle 

 with the body, and then rubs them together with a very 

 brisk horizontal motion; but which of the nervures it is 

 that actually produces the sounds it would require a very 

 careful and elaborate series of experimental researches to 

 determine. It has been asserted that the legs play a part 

 1 "Ann. Soc. Ent. de France." 



