Vibratory Sense Organs And Behaviour

The distance of communication between mates on a plant depends on the intensity of the emitted signal, sensitivity of receptors and attenuation of signals during transmission through a medium. Behavioural experiments demonstrated that stink bugs communicate at distances well above 1 m when on the same plant. Long distance efficient communication is enabled by 20 dB to 40 dB

difference between intensity of emitted signals recorded on the leaf immediately below the singing bug (3 to 31 mm/sec) (Cokl et al., 2004b) and threshold-velocity sensitivity of leg vibratory receptors in the frequency range below 200 Hz (Figure 4.2). Attenuation of plant-transmitted vibratory signals was measured in several plants and can be as low as 0.3 dB/cm, as measured for 75 Hz vibratory signals transmitted through a banana leaf (Barth, 1998). In plant rod-like structures such as stems and stalks, we can expect standing wave conditions (Michelsen et al., 1982) with the occurrence of readily repeated signal amplitude minima and maxima with increasing distance from the source. Investigations of amplitude variations with distance of artificially-induced pure-tone vibratory signals transmitted through Cyperus stem demonstrated that amplitude minima of 124 Hz signals do not fall more than 20 dB below the input value and the readily repeated amplitude maxima reach the input value at distances above 30 cm (Cokl, 1988). The amplitude of the main induced spectral component at 84 Hz varied above the input value at most measuring points exceeding values of the 124 Hz input signal.

There are no data about communication distances by broad band stridulatory signals transmitted through plants. The vibration velocity of broad band songs normal to the surface of the plants was between 0.1 and 1 mm/sec in most recordings from plant stems and leaves (Michelsen et al., 1982). The threshold sensitivity of subgenual vibratory receptors in Cydnid bugs ranged at frequencies below 3000 Hz between 0.01 and 0.001 mm/sec (Devetak et al., 1978). Attenuation of broad band stridulatory signals during transmission through plants has not been measured yet, so that the question of their value in long range communication remains open. It is possible that long distance mate location on a plant is enabled by low frequency components of the broad band signals and the stridulatory components are involved in species recognition at short distance or take part in another behavioural context.

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