LETTERS ON ENTOMOLOGY. 119 
seems to open into the interior of the abdomen : 
next to this, on the inner side, is another large 
cavity of an irregular shape, the bottom of which 
is divided into three portions ; of these the pos- 
terior is lined obliquely with a beautiful mem- 
brane, which is very tense — in some species semi- 
opaque, and in others transparent — reflecting all 
the colours of the rainbow. This mirror is not 
the real organ of sound, but is supposed to mo- 
dulate it. The middle portion is occupied by a 
plate of a horny substance 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 (postpecti/s) an- 
other membrane, folded transversely, fills an 
oblique, oblong, or semi-lunar cavity. It is some- 
times seen in tension, and probably the insect 
can stretch or relax it at pleasure ; but the organ 
which produces the sound remains yet to be 
described, and can only be discovered by dis- 
section. A portion of the first and second seg- 
ments 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 the 
point of the mucro of the first ventral segment 
