THE SONG OF THE CICADA. 
365 
dry and parchment-like naturally, has no need of being put in 
tension. More than this, it is convex, and a tensor muscle 
would but hinder it from returning to its natural convexity ; in 
other words, would act in opposition to its vibrations. 
Membranes vibrating secondarily by influence. The mirror 
and the folded membrane. — Keaumur was aware that the 
mirrors served to reinforce the sound produced by the vibra- 
tions of the drums. Without doubt, too, they are useful in 
preventing the entrance of foreign bodies into the abdominal 
cavity, which would interfere with its power of resonance. The 
vibrations of the mirrors may be demonstrated by sprinkling fine 
dust over their surface, which will dance as soon as the song 
begins. 
The folded membrane would seem, at first sight, to be but 
ill adapted for the reinforcement of sound, as it is lax and soft 
in the fresh state. Sufficient tension, however, is brought about 
by its special tensor muscle, which is, in turn, antagonized by 
the stemo-entogastric muscle. The former acts by drawing the 
apophysis, at the point where it has its movable insertion, 
outwards and backwards. The absence, moreover, of this 
muscle in the female insect is an indirect proof of its action, 
which can be distinctly shown by dividing it with a fine pair of * 
scissors, when a diminution in the intensity of the sound will 
be perceived. As regards the sterno-entogastric muscle, its role 
is somewhat complex ; but M. Carlet concludes, after elaborate 
experiments, that in its active condition, or that of contraction, 
it approximates the thorax to the abdomen, at the same time 
puckering the folded membrane ; while in its passive state, or 
that of relaxation, it draws this membrane closer and renders 
it plane throughout its whole area, thus coming to the aid of 
the tensor muscle in facilitating its vibrations. 
The Functions of the Sonorous Cavities and the Stigmata. 
— The thoraco-abdominal cavity differs from the others in that it 
alone is brought into communication with the outside by stig- 
mata, while the other cavities communicate freely and directly 
with the exterior. The first-named cavity represents a drum 
with two membranes, of which the case is chitino-membranous, 
the function of the two skins being fulfilled by the “ timbales.” 
As for the role of the stigmata, it can be fairly likened to that 
of the opening of the Eustachian tube in the tympanic cavity of 
the organ of hearing, their office being to bring about equi- 
librium in tension between the external air and that confined 
in the cavity into which they lead. The difference in tension 
between the imprisoned air and that outside must certainly be 
at its maximum during the contractions of the motor muscles 
of the drum, because of the heat set free by their molecular 
action ; and the eyelid-like stigmata are accordingly to be seen 
