The Anatomical and Mechanical Basis of Stimulation and Frequency Analysis in the Locust Ear

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 Muller's organ were measured both with a force transducer and by application of known air pressure. The motion of the tympanum and of Muller'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 fim thick, but separated from the mesial rim of the membrane by an arc less than 1 /im thick and from each other by an area 1 -4 fim 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 fig, for the styliform body 0-21 m.N" and 2-9/ig and for the pyriform vesicle 0-22 m.N 1 and 0*32 fig. 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. Muller's organ has a mass of 10 fig, a compliance, with respect to the tympanum, of about 7-5 m. N" and a calculated resonant frequency of 058 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, Muller's organ vibrates on the tympanum and shows complex rotatory motion and squeezing strains of its attachments