This paper re-examines the physical basis of frequency analysis and adequate stimulation of the receptors of the locust ear.
The anatomy of the locust ear was examined by dissection, serial sections and SEM. The compliance of the tympanum and Müller's organ were measured both with a force transducer and by application of known air pressure. The motion of the tympanum and of Müller's organ excited by a calibrated closed sound field was measured under stroboscopic illumination. This allowed the relative phase between the driving force and resultant motion, and the amplitude of motion, to be measured simultaneously.
The mass of the tympanum is irregularly distributed. Laterally, there is a thick region with endocuticle. Mesially, the sclerotized folded body and styliform foot are surrounded by regions of mesocuticle about 3 μm thick, but separated from the mesial rim of the membrane by an arc less than 1 μm thick and from each other by an area 1.4 μm thick which surrounds the pyriform vesicle.
The different sclerites and their surrounds have different compliances and masses. For the folded body region these are 0.38 m. N−1 and 3.9 μg, for the styliform body 0.21 m.N−1 and 2.9 μg and for the pyriform vesicle 0.22 m.N−1 and 0.32 μg. The calculated resonant frequencies of these regions are: folded body 3.5–4.1 kHz, styliform body 5.5–6.5 kHz and pyriform vesicle 16–19 kHz. Müller's organ has a mass of 10 μg, a compliance, with respect to the tympanum, of about 7.5 m.N−1 and a calculated resonant frequency of 0.58 kHz. The ‘thick membrane’ is more compliant than the ‘thin membrane’ and appears to act as a hinge between the thin membrane and the lateral edge of the ear.
When driven from one side by sound pressure, the tympanum vibrates. Resonances were seen, on the folded body at 3.25 kHz and on the styliform body at about 5.5 kHz. The styliform body region appears to be driven by coupling from the folded body resonance, but at its own resonance is relatively ineffective at driving the folded body which has become mass-limited.
When the tympanum vibrates, Müller's organ vibrates on the tympanum and shows complex rotatory motion and squeezing strains of its attachments to the tympanum. The strains differ in different parts of Müller's organ and change with frequency. Tuning curves of the strains differ from those of the tympanal vibration. The strains appear to be the basis of adequate stimulation of the receptors.
When a simple mass load is attached to the folded body its resonant frequency falls, but removal of the mass of Müller's organ does not cause an increase in the resonant frequency of the folded body, although the amplitude of its vibration increases markedly. Müller's organ thus acts as a damping or resistive load on the tympanum. The change in sharpness of tuning of the resonance of the folded body region indicates that about half the power required to sustain the vibration of the tympanum is dissipated in Müller's organ.
Müller's organ has different frequency-dependent drives acting on it, which are normal to each other. The resulting motion resembles a Lissajous' figure. Such figures can be measured along different axes to produce different tuning curves. There are three groups of receptor cells in the ‘ganglion’ region of Müller's organ oriented normal to, at 45° to and parallel to the tympanum. It is suggested that these are driven by different adequate stimulus spectra; this is borne out by independent work on the response spectra of single receptor cells.