THE SONG OF THE CICADA. 
363 
surface of the “ tetrahedron,” while the stigmata of the second 
pair (s", fig. 3) are sometimes obliterated, and consequently less 
easily seen than the others. 
The stigmata of the thorax differ from those of the abdomen 
not only by their position, but also by their shape and struc- 
ture ; the former being well developed, furnished with a horny 
rim (joeritrema *), edged with hairs, and with a movable 
valve which opens and shuts like an eyelid, while the latter are 
mere punctures, devoid of the above accessories. The stig- 
mata in connection with the sonorous cavities, to wit, those of 
the metathorax and the first of the abdominal pairs, claim spe- 
cial attention. The first named has a chitinous peritrema , and 
two eyelid-like valves, furnished with lashes, of which the lower 
alone is movable. In G. plebeia it debouches directly into the 
metathoracic cavity, but in C. hcemcttodes communicates with 
a large air-vessel, which ramifies over the upper surface of the 
muscles of the drum. 
The first stigma of the abdomen (s', fig. 3), is situated in the 
internal surface of the “ tetrahedron,” a little above the mirror, 
and consists of a little round orifice, debouching into a tracheal 
trunk, which eventually breaks up into numerous branches. 
The Mechanism of Stridulation. — In order rightly to under- 
stand its functions, it will be best to study separately the role 
of each of the factors of which it is made up. 
It is a mistake to suppose that the framework of the vocal 
organ acts merely as a support and a guard for the vibrating 
-elements. On the contrary, it is in itself an admirably arranged 
organ of vibration ; it can, in fact, be said that the whole body 
of the Cicada vibrates when it is in song. For the ter gum of 
the prothorax may be seen to tremble during the chant, and if 
the vibrations of other parts of the body are less obvious, they 
can at any rate be rendered apparent by putting them to the 
proof of touch. As regards the role of the membranes and their 
muscles, a distinction must be made between those which vibrate 
directly , and those which do so secondarily by influence. The 
former, or the drums, are put in motion by special muscles, 
while the latter — the mirrors and folded membrane, are excited 
by the vibrations of the drums. With regard to the drums — if 
these be removed while the insect is singing, the sound will 
immediately cease ; but if they be left intact, while the others 
are removed, the song, though weakened, will continue to be 
heard. The drum is therefore the sound-producing organ, the 
vibrating body, which, throwing the air into waves, acts upon 
the neighbouring membrane. Beaumur discovered this fact by 
pulling the muscle of the drum in a dried specimen of the 
* From the Greek 7repi, a around,” and rpr^a, u a hole.” 
B B 2 
