Passage IV

NATURAL SCIENCE: This passage is adapted from the article "How Cicadas Make Their Noise" by Henry C. Bennet-Clark (©1998 by Science American, Inc.).

The early dusk of late spring can be quite a noisy time. Every year some of the several thousand species of cicada emerge from underground, and the males begin to sing their raucous, almost deafening, song. Among these virtuosos of the insect realm, the males of one species of Australian cicada are distinguished for having the loudest insect call measured so far.

Cicadas are plant-sucking bugs, related most closely to aphids and leafhoppers. They are large—and noisy—for insects that feast on sap: the smallest are 10 millimeters (just under half an inch) long, the largest 100 millimeters (four inches). Female cicadas lay their eggs in the stems of plants or in trees. Later, the newly hatched young drop to the ground and burrow in search of plant roots to tap. They remain underground throughout their larval life, which can last many years; the 13-year and 17-year life circles of the American periodic cicadas are among the longest. When the larvae emerge, they molt into winged adults and then live for only a few weeks. During this time, the males sing to attract mates.

Why the insects' calling song so loud is not entirely understood. One possibility is that the sound may saturate the ears of predators, make it hard to locate cicada precisely. Or it may be that larger cicadas have a larger home range and need to call louder to attract a mate. We do know that the females are not hard to hearing: they hear just fine with thresholds of 30 to 40 decibels. And it is quite likely that female cicadas can discriminate between males on the basis of song quantity and quality, just as female crickets do.

The key to understanding how the males make their powerful noise lies in isolating the different aspects of the sound-production mechanism. The primary equipment is a pair of domed tymbals, or drum-like structure, that sit on either side of the abdomen. Each of these elastic, resonant organs has a row of four convex ribs that runs longitudinally up and down its surface. These ribs are flexibly connected to one another as well as to a large oval plate at the rear of the tymbal.

Attached to each oval tymbal plate, in turn, is a large, fast muscle. The contraction of these muscles distorts the domed tymbals, creating a pulse of sound.

Because they contract so rapidly, the muscles produce enough energy in the first three milliseconds of each contraction to cause two or three of the curved ribs on each tymbal to buckle in sequence. This motion causes two or three stepwise inward movements of the oval plate. The stored elastic energy released during these stepwise movements products a brief, loud click as the individual ribs buckle. This series of clicks merges to form a train of vibrations at the cicada's song frequency.

A click from the rib of a tymbal produces high sound pressures within the cicada's abdomen. In most species of cicada, this region is filled largely by an air sac. The abdomen contains a pair of thin eardrums that extend the full width of the ventral surface and serve as acoustic windows, connecting the air sac to the outside world.

The high-pressure sound pluses produced by the tymbals and ribs create a sympathetic resonance in the abdominal air sac. Because the eardrums are larger than the tymbals ,they serve as an effective means of radiating the sound beyond the body—causing it to be about 20 times louder than if the sound were to emanate simply from the tiny tymbals. Thus, the insect manages to broadcast over a far greater range.

Although sound is radiated via the eardrums, the sensory part of the male ear is in a separate capsule, connected to the eardrum by way of a small canal. This separation may protect the males, preventing them from going deaf.

The abdomen and eardrums do more, though, than simply throw the sound into the world. They serve to maintain the quality of the all-important mating song. By extending the abdomen and by opening the opercula that cover the eardrums, the cicada can tune the abdominal resonator to the frequency of the sound pulses produced by the tymbals. The abdominal structures can therefore increase the purity or the loudness of the song. They act as what is called in musical terminology a resonant acoustic load to the tymbals—that is, they correct and balance the sound system.

A similar mechanism seems to occur in many other species of cicadas, and related to sound-production systems have been described in other loud insects as well. Yet to the best of our knowledge, the males of this Australian cicada species remain the noisiest insects around.