THE PERIODICAL CICADA
muscle which arises below from a special support on the ventral wall of the second abdominal segment (Figs. 123, 124, TmMcl). It is by the contraction of these muscles that the drum membranes are set in motion.
Fig. 124. The abdomen and sound-making organs of the male periodical cicada
A, the abdomen cut open from above, exposing the air chamber (AirSc), and showing the great tympanal muscles (TmMcl) inserted on the tympana (Tm). The arrows indicate the position of the first spiracles opening into the air chamber (see fig. 123, ISp)
B, inner view of right hall of first and second abdominal segments, showing the ribbed tympanum (Tm), and the muscles that vibrate it (TmMcl)
AirSc, air chamber; DMcl, dorsal muscles; IS, IIS, IIIS, sternal plates of first three abdominal segments; ISp, first abdominal spiracle; IT, IIT, IIIT, tergal plates of first three abdominal segments; N2, tergal plate of third thoracic segment; Tm, tympanum; TmMcl, tympanal muscle; W3, base of hind wing; VMcl, ventral muscles
But a muscle pulls in only one direction; the drum muscles produce directly the inward stroke of the drumhead membranes; the return stroke results from the outward convexity and the elasticity of the heads themselves and the stiff ribs in their walls.
When a cicada starts its music, it lifts the abdomen a little, thus opening the space between its ventral drum-
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