Archived content - Cicada (5 of 46)
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A. Introduction (Continued)
2. Strange Lives (Continued)
2B. Songs with deafness?
Male cicadas are the usual "singers." The call they make attracts females of the same species. It is thought that every cicada species in the same area has a different mating call. Other males may gather when they hear another member of their own species singing, and join in an ear-splitting chorus. Males of some species sing in solitary splendour. Possibly these males disperse rather than gather when they hear another's song.
The louder the song, the greater the distance males can call to others of their species. They have evolved a complex set of sound-producing, amplifying and hearing structures.
The sound-producing and hearing structures lie at the base of the abdomen. On the sides of the back are a pair of oval, convex, ribbed membranes called "tymbals" that produce the sound. These tymbals lie under the folded wings, and in some cicadas (including Neocicada and Tibicen) they are also protected by a covering plate from the next abdominal segment. A muscle is attached to the base of the tymbal. When it retracts, it bends the convex membrane downwards, and each strong rib in turn buckles with an audible popping noise. The number of ribs that buckle with each muscle pull, and the frequency with which the muscle is pulled, generate the pitch of sound: from a low buzz (Tibicen auletes) to a high-pitched whine (T. canicularis) or a fluting trill (Okanagana rimosa). Some cicadas are able to work the pair of tymbals alternately, thus doubling the frequency. Some species make a single, continuous sound that rises in intensity, then tapers away again (Tibicen canicularis, T. chloromerus, T. lyricen) while others utter a single phrase over and over again (Tibicen auletes, T. dealbatus, T. pronotalis, T. robinsonianus). Others are able to vary the pitch of sound with a raspy, alternating pair of notes (Tibicen dorsatus, Neocicada hieroglyphica) or a pair of notes in sequence (Tibicen linnei, T. pruinosus, Magicicada septendecim, Okanagana canadensis) . Some western species of the genera Platypedia and Okanagana and the eastern Magicicada septendecula merely chirp. Only Magicicada cassinii has a truly musical performance, complete with staccato and legato phrases.
There is a narrow cavity between the base of the abdomen and the thorax on the underside more or less covered by a pair of plates ("opercula") extending backwards from the thorax. This cavity serves both to modulate the male call, and to receive sounds. Within the cavity there is the best-developed auditory apparatus of any known invertebrate. This consists of two broad, shiny, disc-shaped membranes ("mirrors") that extend almost right across the abdomen and serve as ear drums (“tympana”). Uniquely, the membrane is kept taut by a muscle. Males can "turn off" their hearing while singing merely by relaxing this muscle. As the great French entomologist Henri Fabre  found, a cannon fired in their vicinity will not interrupt their song Footnote 1.
The male abdomen is largely filled by air sacs that act as a sounding board. The smaller cavity in front of the "mirrors" also serves to modify the sound. The larger the covering opercula, the larger the resonating chamber they form on the underside. Periodical cicadas, which have rather small opercula and sing in the 70-80 decibel level, are about as loud as a vacuum cleaner, which is tolerable until they all get going together. Cicadas of the genus Tibicen (harvestflies or jarflies) have the largest opercula, and the loudest song in North America. Their songs have been measured above 106 decibels (as have hard-rock bands). In tropical Asia there are cicadas with opercula as long as the whole abdomen.
For reasons best known to themselves, cicadas of the tribe Tettigadini have abandoned the use of tymbals in favour of wing clicking, which they do by rubbing the wings against processes on the thorax. The sound is amplified by communal “singing.”
Not all loud, buzzing insects are cicadas! If the song you hear is at night and sounds like Morse code (either two short buzzes or four, endlessly repeated) then what you hear is the song of the leaf-winged katydid Pterophylla camellifolia, which is common throughout eastern North America as far north as southern Ontario. It has also been heard from several locations in Ottawa, Ontario, which is its farthest northern record.
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