Hind femora influence directionality of the locust ear

Summary

Whereas usually sensory effects on behaviour were investigated, here the reverse, the influence of hind leg position on the acoustic input to the locust ear has been measured with a probe microphone. The large hind femur has considerable mobility in all 3 planes of rotation; it covers the ear as it is lifted (Fig. 1) and closes or opens it when straddled or tilted. In the lower frequency range of locust hearing no effects are found (Fig. 8; 7 kHz). In the high frequency range (20 kHz) femora cause sound pressure changes of a magnitude equal to the known body-induced directionality effects: With increasing straddling (σ) and lifting (λ), amplifying effects are found which develop nearly parallel for ipsi- and contralateral sound direction (Fig. 5). Opposing effects are found for caudal and frontal sound direction. The more the femora are straddled and lifted the stronger caudal sound is amplified (up to + 6 dB) and frontal sound attenuated (up to −6 dB; Fig. 5); this shifts the directionality characteristic of the ear towards the caudal hemisphere (Fig. 6).

Since the two frequency ranges are both stimulated by species-specific sound spectra, since they are both independently represented by different receptors and since they are influenced in different ways by the femora, the animal could perform the ipsi-/contralateral decision with the help of low frequencies while the frontal/caudal decision could be performed simultaneously with the additional use of high frequencies — thereby solving the old front/back ambiguity of directional hearing in the azimuth plane. In the range of leg postures observed in walking or climbing animals (Fig. 3), ear directionality is nearly unchanged (Fig. 4).