INSECTS: WINGS AND THEIR APPENDAGES 109 



horny substance, placed horizontally, and forming the 

 bottom of the cavity. On its inner side this plate termi- 

 nates in a carina or elevated ridge, common to both drums. 

 Between the plate and the after-breast (postpectus) another 

 membrane, folded transversely, fills the oblique, oblong, 

 or semilunar cavity. In some species I have seen this 

 membrane in tension; probably the insect can stretch or 

 relax it at its pleasure. But even all this apparatus is 

 insufficient to produce the sound of these animals; one still 

 more important and curious yet remains to be described. 

 This organ can only be discovered by dissection. A por- 

 tion of the first and second segments being removed from 

 that side of the back of the abdomen which answers to the 

 drums, two bundles of muscles meeting each other in an 

 acute angle, attached to a place opposite to the point of 

 the mucro of the first ventral segment of the abdomen, will 

 appear. In Eeaumur's specimens, these bundles of mus- 

 cles seem to have been cylindrical; but in one I dissected 

 (Cicada Capensis) they were tubiform, the end to which 

 the true drum is attached being dilated. These bundles 

 consist of a prodigious number of muscular fibres applied 

 to each other, but easily separable. While Eeaumur was 

 examining one of these, pulling it from its place with a 

 pin, he let it go again, and immediately, though the ani- 

 mal had been long dead, the usual sound was emitted. 

 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 observable. In this is the 

 true drum, the principal organ of sound, and its aperture 

 is to the Cicada what our larynx is to us. If these creat- 

 ures are unable themselves to modulate their sounds, here 

 are parts enough to do it for them; for the mirrors, the 



