Tonotopic organisation of auditory receptors in tettigoniidae (Orthoptera: Ensifera)

Summary

• 1. Removing the anterior tympanic membrane of the tettigoniidCaedicia simplex did not significantly alter either the tuning or the intensity-response characteristics of individual auditory fibers recorded in the leg nerve (Figs. 2 and 3). However, this operation did make the auditory sensilla in the crista acustica accessible for physiological recording.

• 2. The physiological responses of individual auditory receptors in the crista acustica have been recorded and the frequency-threshold characteristics of identified sensilla have been measured for sound frequencies between 1 kHz and 40 kHz.

• 3. In response to an acoustic stimulus a slow hyperpolarising potential (1–2 mV), lasting the duration of the tone and superimposed hyperpolarising spike potentials (2–4 mV) were recorded in the attachment cell of the sensillum (Fig. 4A). In two of the 28 sensilla from which recordings were obtained, depolarising spike potentials were recorded superimposed on the hyperpolarising potential (Fig. 4B). On rare occasions, depolarising spike potentials were recorded in the absence of any slow potential. Lucifer Yellow was successfully injected into the attachment cell of a sensillum only when the slow potential was recorded (Fig. 6).

• 4. Each sensillum has a typical V shaped tuning curve with the most sensitive sensilla tuned to sound frequencies between 7 kHz and 18 kHz (Figs. 7 and 8). The sensilla of the crista acustica are tonotopically organised with the more proximal receptors tuned to the lower sound frequencies (Figs. 6–8). The frequencies of optimal sensitivity of adjacent sensilla are separated by an interval of approximately 1 kHz (Fig. 8).