Cicadas are large insects that periodically emerge in massive numbers, known primarily for the intense, sustained sound they produce during the warmer months. These insects do not generate noise by rubbing body parts together, a method called stridulation seen in crickets or grasshoppers. Instead, the cicada possesses one of the most complex and efficient sound-generating systems in the insect world, allowing a relatively small creature to create a sound that can reach over 100 decibels at close range. The mechanism relies on a pair of specialized membranes and an internal body structure that works together to produce and amplify the loud, buzzing chorus that defines their seasonal appearance.
The Tymbals: Anatomy of Sound Production
The process of sound creation begins with a pair of specialized organs called tymbals, which are located on the sides of the male cicada’s abdomen. Each tymbal is a corrugated, dome-like membrane that is part of the exoskeleton, featuring a series of thin, thickened ribs. These structures are complex acoustic generators connected to some of the most powerful and fastest muscles found in the insect kingdom.
The cicada generates sound by rapidly contracting the strong tymbal muscles attached to the base of the membrane. This muscle contraction pulls on the tymbal, causing the stiff ribs to buckle inward sequentially, which generates a distinct, sharp click or pulse of sound. As the muscle relaxes, the elastic tension of the tymbal’s structure causes the membrane to snap back to its original, outward-facing position, producing a second click.
This inward buckling and outward snapping happens exceptionally fast, sometimes repeating hundreds of times per second. The rapid, repeated succession of individual clicks is what the human ear perceives not as a series of separate pulses but as the continuous, high-pitched buzz, whine, or rhythmic call characteristic of the species. For some species, this process excites a damped oscillation in a resonant cavity multiple times per single muscle contraction.
Amplification: The Cicada’s Resonance Chamber
The majority of the cicada’s abdomen is essentially a large, hollow cavity, dominated by a massive air sac derived from the tracheal system. This empty space acts as a resonance chamber, similar to the hollow body of a guitar, where the sound waves generated by the tymbals are intensified.
This abdominal cavity operates as a Helmholtz resonator, a system where air vibrates in a cavity open to the outside via a small opening. In the cicada, the air sac serves as the cavity, and a pair of acoustically transparent membranes on the underside of the abdomen, called tympana, act as the acoustic windows or “necks” of the resonator. The natural frequency of this abdominal resonator is tuned to match the frequency of the sound pulse created by the tymbals, resulting in sympathetic resonance that significantly boosts the volume.
To further modulate the sound, the cicada can adjust the tension of its abdominal cavity by changing its posture. The sound radiated from the tympana can be up to 20 times louder than the sound produced by the tymbal alone, allowing the cicada to be heard from long distances. Small, stiff flaps known as opercula cover the tympana and can be adjusted by the insect to control the sound’s radiation and direction.
The Language of Cicadas: Interpreting Their Calls
The loud, sustained noise produced by the cicada serves a communicative purpose. The most familiar sound is the congregational or calling song, which is a loud, species-specific pattern used to attract females and establish chorusing centers. Multiple males often synchronize their calls to form a massive chorus, making the sound easier for females to locate.
Once a female is nearby, the male switches to a quieter, shorter courtship song designed for close-range communication. Females, who lack the large tymbals, often respond to the male’s advances using a simple, species-specific sound, such as a wing-flick or click. This acoustic exchange confirms species identity and willingness to proceed with reproduction.
Cicadas also employ a distress call or squawk, which is a harsh, sudden buzzing noise typically emitted when the insect is captured or disturbed by a predator. This uncoordinated burst of noise may serve to startle a predator, allowing the cicada a moment to escape.