Sound Register and Biotope

Seemingly the environment in which every species normally lives has an influence on the type of sound emission. For example, forest species generally call in a frequency range totally different from that of grassland species. In fact the relative size of the various cicadas is the determining factor. Forest species are mostly large or larger than average. By contrast, species of very open areas are almost always small or smaller than average.

figure 25.8 Purana tigrina (Walker). Acoustic ethological card based on the transition call/courtship signal: (a) Temporal transcription. Oscillogram reflecting the end of one calling appeal sequence (CALL) at once followed by the courtship sound, which the male stops as soon as it is coupling the female (c); (b) Average spectrum; (d) Spectrogram corresponding to the previous oscillogram. The harmonic III and IV turn out largely disrupted throughout the courtship.

figure 25.8 Purana tigrina (Walker). Acoustic ethological card based on the transition call/courtship signal: (a) Temporal transcription. Oscillogram reflecting the end of one calling appeal sequence (CALL) at once followed by the courtship sound, which the male stops as soon as it is coupling the female (c); (b) Average spectrum; (d) Spectrogram corresponding to the previous oscillogram. The harmonic III and IV turn out largely disrupted throughout the courtship.

For the most part, forest cicadas have a relatively low frequency range with a fundamental of 1500 to 3000 Hz, strengthened by harmonics only very rarely exceeding 12,000 Hz. In grassland species, on the contrary, the fundamental is high pitched, often exceeding 4000 Hz, and the harmonics may exceed 20,000 Hz. However, especially in this latter group, there are exceptions, notably Spoerryana llevelyni Blrd, Paectira feminavirens Blrd (Boulard, 1977), Purana khuanae Blrd (Boulard, 2002a) and Pomponia fuscacuminis Blrd (Boulard, 2004b).

Other aspects of sound quality are related to solitariness and gregariousness.

Distinctive Signals of Sibling Species

It may seem surprising that the tymbal, constructed according to the same plan in all cicadas, can allow for such a range of specific sounds. This organ has proved, in fact, to be so sensitive that either the least morphological variation or the slightest change in the rhythms activating the two tymbals, either synchronous or alternately (Pringle, 1957; Fonseca and Bennet-Clark, 1998) is enough to alter the sounds produced. So much so that it has been possible, thanks to acoustic analysis, to discriminate between twin species, that is to say, species morphologically identical or very close but of elusive, unconfirmed, or even unsuspected specific identity. In this respect, the acoustic recognition of various N. American Magicicada species was pioneered by Alexander and Moore (1962).

Amongst the most sensational applications of the comparative analysis of sound production and the creation of Acoustic Identity cards, here are some examples, either little known or as yet unpublished:

• Sound detection of Lyristes gemellus Boulard, occurring sympatrically, syntopically and on the same food plant with its sibling species L. plebejus (Scopoli) in Asia Minor (Boulard, 1988) (Figure 25.10 and Figure 25.11 DVD). A third species, Lyristes isodol Blrd, its call very accelerated as compared with the former two species, was discovered

figure 25.9 Cicada orni Linne. Pluriwindowed Acoustic Etiological Card (AEC) showing various acoustic signals according to its etho-physiological conditions: (a) Visualisation of 25 sec of a nuptial call recorded in nature and ecological adventures of which were carefully noted. This temporal signal, transcribed in real time, includes: Five ranges of the normal calling (an), which is a serial access of speech in a continuation of "closer and regular sound modules", as already shown in the previous card, but here amplified in the window. (b) Two ranges translating more or less violent protests (p1 and p2) caused by an ant having nibbled the wing, the rostrum, then touched one of the legs of the cicada; A demonstration of anxiety (i) due to the fast passage of a bird in the field of vision of the insect and being translated by one brief slowing down tymbalic. A reaction of opposition (p3), protest activated by the pressing approach of a second male. This signal of opposition, constituted by numerous and deep tymbals bangings moved closer and crumpled up as indicated in the window (c), had the effect to stop homosexual attempt. (c) Irregular aspect of modules of the cymbalisation, portion taken in (a) and 10 times amplified; (d) Image of a temporal signal of 4.30 sec showing a calling signal abruptly interrupted by sudden danger and swiftly followed by a hasty departure, accompanied with a brief "signal of flight" f. (e) Concomitant recordings, for 4 sec, of two calling males at short distance from each other; the first, having been caught, emits muddled "signals of distress" (small irregular central range) d, whereas the second continues to call regularly (wide range composed of normally structured signals) N. This last document gives clear evidence of a direct observation verified many times: the signals of distress of the captured male do not affect the behaviour of the free male. In cicadas the distress call has no ecological value of alarm. (From Boulard and Mondon, 1995.)

in Turkey sympatrically with L. plebejus and L. gemellus, but not syntopically and on a different food plant (Boulard, 1988).

• The replacement of Cicada orni L. by C. mordoganensis Blrd (Boulard and Lodos, 1987) in the islands of Samos and Ikaria in the East Mediterranean, confirmed by acoustic analysis (Simoes et al., 2000).

• Tosena albata, occurring sympatrically, syntopically and on the same food plant with its sibling species T. melanoptera of which it had been considered a variant for more than a century, has recently been described as a valid species (Boulard, 2004d, 2005) (Figure 25.12 and Figure 25.13 DVD).

• The discovery of Cicadetta cerdaniensis, replacing its sibling species Cicadetta brevipennis (Scopoli) in the mountainous South of France (Puissant and Boulard, 2000). This species has recently been reported also from other parts of Europe (Gogala and Trilar, 2004; Trilar and Holzinger, 2004).

• The detection in Slovenia of three Cicadetta species, C. montana (Scopoli), C. cerdaniensis (Puissant and Boulard, 2000) and C. brevipennis (Fieber). C. brevipennis is more widely distributed in France where it had been confused with C. montana. In Macedonia, there exists another species of this species complex Cicadetta macedonica Schedl discovered by its unique signal pattern (Gogala and Trilar, 2004).

• The acoustic distinction in a rain forest of the north of the Thailand between Pomponia dolosa Blrd and P gemella Blrd that are almost identical in general and genital morphology, but occur allopatrically (Boulard, 2001c, Figure 25.14 and Figure 25.15 DVD).

• Alternate presence of two sibling species in Ranong Province in the south of Thailand; two large Pomponia have been found at different times in the same forest. As like as identical twins (the same size and appearance, the same emerald green eyes), they are impossible to differentiate through examination of museum specimens. It is only in their natural environment that they can be separated on the basis of their very different calls. The call of the first species, Megapomponia pendelburyi (Blrd), prevailing in January and February sounds like a trumpet signal (Figure 25.16 DVD); the second, Mp. clamorigravis Blrd, sounds more like a tuba and is mainly found in November and December (Figure 25.17 DVD).

• Maybe even more spectacular, museum material labelled "Pomponia fusca" has been shown to contain three other species, morphologically almost identical but differentiated by their calls, namely, P. fuscoides Blrd P. dolosa Blrd, and P. gemella Blrd (Boulard, 2004b).

By contrast, entities considered as different species have been returned to subspecific rank. This is notably the case for two French allopatric Tibicina today named as T. corsica corsica (Rambur) and T. corsica fairmairei (Blrd) (Boulard, 2000a). Another example is supplied by Salvazana imperialis Distant from Indo-China, which was put in synonymy with S. mirabilis Distant (Chou Yao et al., 1997) although morphologically it is very different by virtue of the contrasting colours of their wings; acoustics confirmed that it was indeed a case of two morphs of the same species (Boulard, 2002).

Functions of Acoustic Signals Calling Signal and Acoustic Specificity

Cicadas' secondary sexual dimorphism is centred on the tymbal complex, as indicated above. Thus, males exclusively produce sound and calling as an invitation from males to females. These, when receptive, are attracted by the males' invitation and react phonotactically, i.e. they move towards a calling male (Boulard, 1965; Fonseca, 1993). In fact, in the cicadan environment as in other Cicadomorpha, acoustic signals are above all, if not the only means of intraspecific sexual recognition and constitute a premating isolating mechanism (Claridge, 1990).

In cicadas, sight does not play an important role in specificity of contact and courtship. Males are unable to distinguish by sight between species or sex. They cannot even distinguish a dead from a living cicada near them. For example, in an assemblage of Dundubia, a male of D. feae which is calling will attract a female of its own species, not one of D. nagarasingna that lives on the same food plant. But, if a male of D. feae by chance approaches a female of D. nagarasingna, he will attempt to mate. Attempts at interspecific mating, usually brief, have been observed and filmed in nature (Boulard, 1973a). In gregarious species it is quite common to see males approach each other and attempt to mate (Figure 25.50 DVD). This will cause the courted male to protest vehemently (Boulard, 1995, and here Figure 25.9a).

Courtship

The sexual encounters of a species are dominated by the distinctiveness of their signals. In strictly gregarious cicadas, the calling signal alone is sufficient, the individuals already being close and easily visible to one another. In solitary species, however, the female that has been attracted does not necessarily reach the calling male straight away and may settle some distance from it. On responding to the female, the male changes his call to a courtship signal which represents an urgent invitation. This courtship signal, sometimes referred to as "the whispered call", generally leads to the meeting and physical contact of male and female. This is clearly shown by Cicadatra atra (Olivier), Pagiphora yanni Blrd, Angamiana floridula Distant (see Boulard, 1992, 1993a, 2001a) and also by Meimuna durga Distant (unpublished) (Figure 25.17 DVD and Figure 25.18). Calling signals have been shown also to induce substrate transmitted vibratory signals

figure 25.18 Acoustic behaviour and function of an heliophilous species, Meimuna durga (Distant). A = photographic ID of a calling male; B = calling signal; C = courtship signal; D = male and female coupling. Thailand.

(Stolting et al., 2002). It is therefore possible that vibratory signals also play a role in short-distance localising and recognition of the partner.

As a general rule, courtship signals stop once contact has been made. I know of only one case where the male continued his serenade (in this case even while in the act of mating) and that was Pomponia dolosa, a medium-sized crepuscular, almost nocturnal species given the name of "Handbell cicada" because it sounds very much like a vigorously shaken bell (Figure 25.19 DVD).

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