56 
THE PERIODICAL CICADA. 
note of the species, which once heard is never likely to be forgotten, 
or, if heard again, mistaken for that of some other insect. The true 
sound organs are entirely exposed in the seventeen-year Cicada except 
for the covering afforded by the closed wings of the resting insect. 
In other Cicadas these drums are usually protected by overlapping 
valves or expansion of the body wall. 
The sounding drum, or "timbal," as Reaumur termed it, of the peri- 
odical Cicada is a tense, dry, crisp membrane numerously ribbed or 
plated with the convex surface turned outward. The ribs are chitinons 
thickenings or folds in the surface of the parchment-like drum, and 
strengthen the drum while perhaps rendering it at the same time more 
elastic. The sound is produced by the rapid vibration, or undulation, 
caused by the spring- 
ing or snapping in and 
out of these corrugated 
drums. Two powerful 
muscles of very pecu- 
liar structure situated 
within the base of the 
abdomen set these 
drums in motion, pro- 
ducing the rattling so- 
called song of the Ci- 
cada, very much, as 
has been suggested, as 
sound is produced by 
pressing up and down 
the bottom of a tin pan 
which is somewhat 
bulged. 
Beneath each "tim- 
bal" in the base of the 
abdomen of the insect 
is a large sound or air 
chamber, and a third occurs in the thorax joining the first two. These 
are closed by the body walls and membranes, and the two abdominal 
ones beneath by the very peculiar "mirrors," or " spectacles" — the 
tense, mica-like membranes situated at the base of the abdomen and 
protected and covered by the semicircular rigid disks projecting from 
the thorax. These transparent membranes are often mistaken for the 
true sound organs, but they are rather sounding-boards, or drums, to 
increase and transmit the sound vibrations induced by the play of the 
timbals. That they are not essential to the production of sound can be 
shown by slitting them or removing them altogether without there being 
any cessation of the note. Much more important modifiers of sound are 
the semicircular disks projecting from the thorax over the " mirrors," 
Fia. 27. — The musical apparatus of the periodical Cicada: a, view 
from beneath, showing the plates (light colored) covering the 
sounding disks; b, dorsal view, the timbals showing as light- 
colored areas; c, section at base of abdomen, showing attach- 
ment of large muscles to timbals; d, timbal greatly enlarged, in 
normal position ; e, same drawn forcibly in by the action of one 
of the muscles, as in singing (original). 
