Copiphora rhinoceros

behaviour · bio.acousti.ca

Calling-song Analysis The calling song is a rapid succession of short buzzes, each about 0.5 to 1.0 s in duration, given at a highly variable rate of 40 to 50/rain. At 5 cm distance, dorsal aspect, the sound pressure level of 76-3, measured with the meter on linear response, just exceeded 100 dB (re 20 laPa). Measurement of 76-2 at the same distance and aspect gave a maximum attained level of 92.6 dB during each of three buzzes. On an oscilloscope the buzz can be resolved to a series of phonatomes. ('Phonatome' is defined as all of the sound produced during one cycle of wing movement (Morris & Walker 1976); it coincides with the term 'syllable' (Broughton 1963).) Each phonatome is presumed here (Fig. 1) to consist of a pulse train of incompletely dis- crete, lower-amplitude, rapid-decay pulses (see Morris & Pipher 1972 for terminology). This minor train is about 14 ms in duration and is followed by a single prolonged (major) pulse of much higher amplitude, lasting about twice as long. Specimen 76--3 exhibited a phonatome rate of 14/s at 24 C. The phonatome rate is low enough at temperatures near 20 C to impart a quavering quality to the song. C. rhinoceros" frequency spectrum (Plate I, Fig. 2, 76-2) has a maximally intense energy peak at 8.7 kHz with pronounced second and third harmonics (17.4 and 26.1 kHz). A spectro- gram of 76-3 (Fig. 3A), which is also the cumula- tive product of many phonatomes, has the same features. When the two phonatome components are analysed separately, one obtains distinctive spectrograms (Fig. 3B, C, D). The spectrogram of the minor pulse train (Fig. 3B) contains sound energy across a wide band (14 to 23 kHz); the 8.7 kHz fundamental is absent. The minor pulse train has a much lower amplitude than the major pulse and the gain was greatly increased in obtaining the spectrogram of Fig. 3B; hence, although the frequencies of the minor trains are present in the spectrogram of Fig. 3A, they are at such a low intensity relative to the major peaks as almost to escape notice. Fig. 3C is a spectrogram from the middle portion of a single major pulse. The 8.7 kHz fundamental is present and likewise its two harmonics; these peak more narrowly than in the cumulative spectrograms of Figs. 2 and 3A. The second and third overtones are manifest in the departure of the waveform from a simple sinusoidal shape, i.e. in the 'shoulders' of a high resolution trace taken from the middle region of a major pulse (Fig. 3E). The same complex waveform can be seen in the trace of Fig. 3C. Toward the final third of the major pulse, this waveform becomes increasingly simple, the shoulders less in evidence (see the trace of Fig. 3D). Coincidentally there is a dropping away of the harmonics. The second harmonic, for example, which is almost half as intense as the fundamental in mid-pulse (Fig. 3C), is reduced to less than one-fifth the intensity of the fundamental by pulse end (Fig. 3D). From the sinusoidal waveform of its major pulse, C. rhinoceros appears to possess a high Q, narrowly tuned, resonant system (Sales & Pye 1974). The existence of a strong second harmonic in such a system is interpreted by Sales & Pye as follows: the fundamental frequency corresponds to the tooth-scraper contact rate; the resonator or 'filter' (the modified lattice of veins and tegminal cells) is tuned to twice this contact rate; two resonator oscillations are interposed between each successive tooth-scraper interaction during the generation of a pulse. By an extension of the above explanation, the 26-kHz third harmonic of C. rhinoceros may be supposed to result from an interposition of three waves between each tooth-scraper engage- ment. In other words, the teeth are contacted at a rate of 8700/s during the major pulse and the resonator is tuned to three times this contact rate, i.e. about 26 kHz. Other copiphorines also exhibit prolonged 'grylloid' pulses, e.g.N, melanorhinus (Walker 1975) and Neoconocephalus ensiger (Harris) (Gwynne 1977). Calling Display The calling display of male C. rhinoceros incorporates both tremulation and stridulation. Every few minutes (sometimes more frequently) the caller changes from one modality to the other; thus a time interval containing repeated tremulation bouts, interspersed with silence, is alternated with an interval of successive buzzes. Stridulation and tremulation do not occur simultaneously. Several lone C. rhinoceros males were seen signalling in this manner, each on a plant that was apparently Philodendron; they were perched on the leaf underside near the prominent mid-rib, about 0.5 m above the ground. (Tremulation is also executed from the upper surfaces of leaves.) The tremulations are stereotyped. Each con- sists of three oscillations that are just possible to discriminate with the human eye. The plane of elevation about which the body oscillates appears to be farther removed from the substrate than that which obtains during the non-tremulating stance. That is, the insect seems first to lift its body away from the plant (the distance between the abdomen and the substrate increases by about 0.5 cm and the abdomen seems to be slightly more raised than the anterior end of the body) and then, continuing smoothly, to execute the oscillations about this more elevated stance. Such behaviour increases the moment of the forces imparted to the substrate. C. rhinoceros produces tremulations in bouts of two to five. Three tremulations per bout is common; four tremulations per bout occur but substantially less often; two and five tremula- tions per bout are rare. Often the first tremula- tion in a bout is less pronounced. Each tremula- tion is presumed to produce in the plant a pulse of substrate-borne vibration, i.e. the behaviour generates a transverse wave in the plant tissue. A tremulation bout thus produces a pulse train of substrate vibrations. One male, his display measured extensively in the field, produced an average of 3.1 tremula- tions per bout (33 bouts). He had an average bout duration of 2.4 s (7 bouts). From these figures one can calculate a rate of 1.3 tremula- tions/s. The recurrence of tremulation bouts is variable and can be much affected by acoustic interaction with nearby conspecific singers. A male, relatively undisturbed in this sense, was observed to produce tremulation bouts every 15 to 20 s. One male tremulated repeatedly in the labora- tory within the tiny cylindrical analysis cage. That this behaviour can take place in the absence of any evoking conspecifics attests further to a rhetorical calling function under endogenous control.

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