NOISES OF INSECTS. 407 



omitted. On each side of the drum-cavities, when the 

 opercula are removed, another cavity of a lunulate 

 shape, opening into the interior of the abdomen, is ob- 

 servable''. In this is the true drum, the principal or- 

 gan of sound, and its aperture is to the Cicada what 

 our larynx is to us. If these creatures are unable 

 themselves to modulate their sounds, here are parts 

 enough to do it for them : for the mirrors, the mem- 

 branes, and the central portions, with their cavities, 

 all assist in it. In the cavity last described, if you re- 

 move the lateral part of the iirst dorsal segment of the 

 abdomen, you will discover a semi-opaque and nearly 

 semicircular concavo-convex membrane with trans- 

 verse folds— this is the drum''. Each bundle of mus- 

 cles, before mentioned, is terminated by a tendinous 

 plate nearly circular, from which issue several little 

 tendons that, forming a thread, pass through an aper- 

 ture in the horny piece that supports the drum, and are 

 attached to its under or concave surface. Thus the 

 bundle of muscles being alternately and briskly relaxed 

 and contracted, will by its play draw in and let out the 

 xlrum : so that its convex surface being thus rendered 

 concave when pulled in, when let out a sound will be 

 produced by the effort to recover its convexity ; which, 

 striking upon the mirror and other membranes before 

 it escapes from under the operculum, will be modulated 

 and augmented by them ^. I should imagine that the 



" Reaum. uM siipr.f. 3. II. " Ibid./. 6. tt. f. 9. 



" Plate VIII. Fig. 19. cc. The figure given in this plate does not 

 show the drums clearly ; but the principal object of it was to exhibit the 

 bundles of muscles, which are of a different form from those in Reaumur's 

 figures. In the above figure, a. is the mirror; bb, the bunches of mus- 

 cks; cc. the drums} d, the back of the abdomen; c. the belly. 



