Hexacentrus unicolor

The song of Hexacentrus unicolor consists of short chirps (approximately 50—100ms long) presented in a rhythm of about 2Hz (1.6-3.3 Hz). One chirp consists of a crescendoing group of pulses, which follow one another with a frequency of 320-415 Hz. The variation is principally due to different ambient temperatures (22.5-28.5°C). The frequency spectrum shows two narrow maxima at 11 and 33 kHz and two submaxima at 22 and 44 kHz.

Data known from studies of other bushcrickets (e.g. Busnel, Busnel & Dumortier, 1956) suggest that during one opening and closing sequence one chirp, comprising a group of pulses, is produced. However, recordings of the wing movements show that this is not the case in this species: with each cycle of movement only one pulse is created. Accordingly the frequency of the wing movements is very high (320-415 Hz, see above). The course of the stridulatory movement is shown in Fig. 2. Fig. 2A shows a long recording of the song, Fig. 2B gives a more detailed account of the generation of a chirp. Some time before the generation of sound, the elytra are moved with very high frequency; only after a short period of 'introductory swinging' does the crescendoing tone start. The pulses always emerge during the second half of the closing movement of the wings (Fig. 2C). At the end of a pulse group the wings remain in an opened position for some time. During the successive closure of the wings an isolated sound pulse is created.

Throughout the song the animal is positioned in such a manner that its slightly opened elytra render the alae visible (see fig. 6 of Heller, 1983). The amplitude of the movement is very small and only a vibration is perceptible with the naked eye. An identical wing vibration can be recorded before the song. At this stage the movement is soundless, with almost the same rhythm, frequency and amplitude. The only difference from singing - except for the missing sound production - is that in most cases the elytra spring back without delay to the neutral position at the end of a chirp.

The frequency of the wing beat during flight is, as expected, far lower: 22-23 Hz (also measured opto-electronically on two females at 26 °C). [1]

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