INSECTS : WINGS AND THEIR APPENDAGES. 89 



of an irregular shape, the bottom of which is divided into 

 three portions ; of these the posterior is lined obliquely 

 with a beautiful membrane, which is very tense — in some 

 species semi-opaque, and in others transparent — and re- 

 flects all the colours of the rainbow. This mirror is 

 not the real organ of sound, but is supposed to modu- 

 late it. The middle portion is occupied by a plate of 

 a horny substanoe, placed horizontally, and forming 

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

 terminates in a carina or elevated ridge, common to both 

 drums. Between the plate and the after-breast (post- 

 pectus) another membrane, folded transversely, fills an 

 oblique, oblong, pr 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 portion of the first and second segments 

 being removed from the side of the back of the abdomen 

 which answers to the drums, two bundles of muscles meet- 

 ing each other in an acute angle, attached to a place oppo- 

 site to the point of the rnucro of the first ventral segment of 

 the abdomen will appear. In Reaumur's specimens, these 

 bundles of muscles 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. Whilst 

 Reaumur was examining one of these, pulling it from its 

 place with a pin, he let it go again, and immediately, 

 though the animal had been long dead, the usual sound 

 was emitted. On each side of the drum-cavities, when 

 the opercula are removed, anothef cavity of a lunulate 

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

 servable. In this is the true drum, the principal organ of 



