Sound Production the Tymbal and Its Sound Anatomy of the Tymbal

With respect to behaviour it is of particular significance that the fully developed tymbal system is only found in males. Detailed descriptions are given elsewhere (notably Carlet, 1877a, 1877b; Vogel, 1923; Pringle, 1954, 1957; Young, 1972; Michel, 1975; Bennet-Clark, 1999b). It consists of two symmetrical membranes, the drums or cymbals of Casserius (1600) and Reaumur (1740), usually known as tymbals. They are found dorso-laterally on the first abdominal segment (Figure 25.1). In some species, females have small rudimentary tymbals, which are, however, totally sclerotised and ineffective. This is the case notably in the large Tosena albata Distant. On the other hand, there are also species in which males lack a tymbal. Apart from the archaic Tettigarctidae, this is so in some rare species in a few genera of Tibicinidae, but otherwise phylogenetically only distantly related, mainly from equatorial Africa and North America (Boulard, 1976a, 1986, 1990a, 1993b, 2002b).

From outside, the tymbal appears as a slightly convex plate of cuticle, strengthened with sclerotised ribs and disks framed in a roundish rim of cuticule. In some species the tymbals are (partly) hidden under the protruding tergite of the second abdominal segment. Each tymbal is connected, eccentrically, to a very powerful muscle by a short triangular tendon. The tymbals buckle inwards by muscular contractions and then pop back with the slackening of the muscles.

These actions, in very rapid repetition, produce a train of pulses, often very loud, as the ribs are alternately or simultaneously compressed and relaxed in a rhythm varying from species to species (Pringle, 1954; Boulard, 1990a; Bennet-Clark, 1999b). The sound produced is amplified by what is truly an abdominal resonant chamber formed by a huge bag in the tracheal respiratory system, occupying the largest part of the male abdomen and pushing back the digestive and procreative viscera towards the end of the body. This resonant chamber lightens the body of the males. So, when these are seen against the light, their abdomen, astonishingly, seems to be empty (see Pomponia cyanea Fraser, in Boulard, op. cit.).

A somewhat similar organ is found in males of several groups of moths, notably Arctiidae and Nodolidae (Fullard, 1977, 1992; Conner, 1999; Skals and Surlykke, 1999).

Such studies need also to take account of tymbal-like structures found widely in other related auchenorhynchous families (e.g. Ossiannilsson, 1949; Striibing and Schwartz-Mittelstaedt, 1986), and also of the very strange "acymbalic (or atymbalic) cicadas".

The Sound of the Tymbal, Fundamental Acoustic Parameters

The comparative study of cicada acoustic signals is based on three essential and measurable variables, which define a sound:

• Duration of call units

What The Purpose The Cicada

figure 25.1 Sound and auditory acoustic system of a male cicada: (a) Base of abdomen seen from below, very stretched, with the left opercula turned upwards "to open" the ventral acoustic chambers and to show the components of the acoustic system. (b) Acoustic system (isolated abdominal segments I and II) seen for 3/4 from behind and left. ca = left auditive capsule; cp = tymbal cover; cy = tymbal; eg = entogaster (internal armature); md = right tymbal muscle; mjd = yellow membrane (distended, the organ in function); mjp = yellow membrane (wrinkled, at rest); mt = tympanum; op = operculum (latero-ventral cuticular disc); you = tendon connecting the tymbal muscle with the internal face of the tymbal (seen here as through a transparent medium); UI, UII = the first two abdominal segments (Acousticalia) carrying the acoustic organs; vi = thin memebrane within the right auditive capsule. (From Boulard, M. and Mondon, B., "Vies et Memoires de Cigales" Editions de L'^quinoxe, Barbentane, 1995. With permission.)

figure 25.1 Sound and auditory acoustic system of a male cicada: (a) Base of abdomen seen from below, very stretched, with the left opercula turned upwards "to open" the ventral acoustic chambers and to show the components of the acoustic system. (b) Acoustic system (isolated abdominal segments I and II) seen for 3/4 from behind and left. ca = left auditive capsule; cp = tymbal cover; cy = tymbal; eg = entogaster (internal armature); md = right tymbal muscle; mjd = yellow membrane (distended, the organ in function); mjp = yellow membrane (wrinkled, at rest); mt = tympanum; op = operculum (latero-ventral cuticular disc); you = tendon connecting the tymbal muscle with the internal face of the tymbal (seen here as through a transparent medium); UI, UII = the first two abdominal segments (Acousticalia) carrying the acoustic organs; vi = thin memebrane within the right auditive capsule. (From Boulard, M. and Mondon, B., "Vies et Memoires de Cigales" Editions de L'^quinoxe, Barbentane, 1995. With permission.)

In addition to these three variables is the timbre, which often allows the human ear to distinguish between two sounds having the same basic or fundamental frequency and amplitude. The timbre depends on the existence and number of harmonics, the respective frequencies of which are multiples of the basic or fundamental sound frequency.

The acoustic signals of animals and especially of insects differ from pure sounds and casual noises by their structure and the regular repetition or rhythm of emission. Insect sounds are consisted of basic units or echemes that are repeated. The echemes vary in duration and speed of attack and decay, and may be repeated, unitary or grouped in simple or complex motives or phrases. Thus there is an almost infinite range of possibilities leading through simply temporal characteristics to the establishment of a very high degree of specificity. Of particular importance, in fact determining the sound produced, are not only the form, number and spatial distribution of the constituents of the emitting device (specific dimensions, morphology and topography of the tymbals), but also its dynamics, the time of the year, the hour and the impact of the environment, in particular temperature (See Sanborn, Chapter 7).

Visualisation of Acoustic Signals: CIA and CEA

Thanks to the development of microcomputers and of software for the study of audio data, it has now become rather simple to transform digitised sounds into images and thus to allow them to be analysed in a frequency, time or amplitude domain and archived. In the first applications of this new technology, cicada specialists have been able to establish CIA and CEA which can be transformed into sonotypes and etho-sonotypes. Programs allowing the multiwindow stacking of sonograms have revealed unique graphic representations of the sounds of the species under study. Other authors use different but similar presentation of sound characteristics.

For the sake of authenticity, recordings should be made wherever possible in nature as calls in captivity are often atypical. However, recording of the smallest species is rarely possible in the field.

Once computerised, multiwindow sonograms containing several different, but complementary graphs are constructed. It is customary to include:

(a) The temporal oscillogram reproducing an ethological unity, generally the phrase or a regularly repeated sequence of a signal, or a relatively substantial portion of one, expressed in time.

(b) The curve of the average spectrum (analysis of relative intensity over frequency) or the curve of relative energies in time called "amplitude envelope".

(c) A portion of the temporal oscillogram (a) stretched in arbitrarily chosen space-time revealing the spatiotemporal structure of the emission (Figure 25.2).

(d) The spectrogram showing the temporal variation of emitted frequencies and intensities. Sometimes, it can be constructive to add a second sector, an additional card depicting the periodicity, often very narrow in cicadas, the pulses and their phonatomic composition providing the morphology and the average number of sound units transcribed in a reference span of time. This second section will contain usually also:

(e) A visualisation of the displayed signal over a time period of 1/10th of a second (sometimes more, sometimes less, according to species), a matrix for the following analysis.

(f) The structure of sound units shown over a reference span of time of about 12 msec, as defined by the computer tool.

Anatomy of the Auditory System

Cicadas of both sexes can perceive sounds thanks to some rather remarkable "ears" which they also have on their abdomen. These are two small capsules, called the auditory capsules, hemispherical or in the form of a "skullcap", implanted symmetrically on the sides of the second abdominal segment. Each auditory capsule encloses sound-sensitive organs, scolopidia, which receive and integrate the

3 impulses

2 phrases -

3 impulses

2 phrases -

Sound Emission

Sequence of a sound emission, first phrases composing the temporal signal (1 12)

figure 25.2 Schematic graphic transcription of the beginning of a sound emission (a) and structural analysis after more and more stretching in time scale and spaces (b), (c), until it is possible to visualise individual elementary oscillations, original groupings of which constitute impulses (phonatomes). (From Boulard, M. and Mondon, B., "Vies et Memoires de Cigales" Editions de L'equinoxe, Barbentane, 1995. With permission.)

Sequence of a sound emission, first phrases composing the temporal signal (1 12)

figure 25.2 Schematic graphic transcription of the beginning of a sound emission (a) and structural analysis after more and more stretching in time scale and spaces (b), (c), until it is possible to visualise individual elementary oscillations, original groupings of which constitute impulses (phonatomes). (From Boulard, M. and Mondon, B., "Vies et Memoires de Cigales" Editions de L'equinoxe, Barbentane, 1995. With permission.)

vibrations perceived by huge, rather thin, tympanic membranes (Figure 25.3) tightly embedded in the first intersegmental base of the abdomen. Often iridescent and shining, the tympana can also be completely transparent or more or less opaque according to species. Reaumur (1740) and Vogel (1923), examining the apparatus of the big European cicada, Lyristes plebejus, called them mirrors. The scolopidia with their long axons pass on to the axial nervous system the information about sound vibrations that have activated the tympana. Rather curiously, the tympana are always larger in males than in females.

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  • ben kerr
    What muscles would be activated for the production of different sounds:?
    7 years ago

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