Mechanical response of the tympanal membranes of the tree cricket Oecanthus henryi

Abstract

Crickets have two tympanal membranes on the tibiae of each foreleg. Among several field cricket species of the genus Gryllus (Gryllinae), the posterior tympanal membrane (PTM) is significantly larger than the anterior membrane (ATM). Laser Doppler vibrometric measurements have shown that the smaller ATM does not respond as much as the PTM to sound. Hence the PTM has been suggested to be the principal tympanal acoustic input to the auditory organ. In tree crickets (Oecanthinae), the ATM is slightly larger than the PTM. Both membranes are structurally complex, presenting a series of transverse folds on their surface, which are more pronounced on the ATM than on the PTM. The mechanical response of both membranes to acoustic stimulation was investigated using microscanning laser Doppler vibrometry. Only a small portion of the membrane surface deflects in response to sound. Both membranes exhibit similar frequency responses, and move out of phase with each other, producing compressions and rarefactions of the tracheal volume backing the tympanum. Therefore, unlike field crickets, tree crickets may have four instead of two functional tympanal membranes. This is interesting in the context of the outstanding question of the role of spiracular inputs in the auditory system of tree crickets.

Active auditory mechanics in female black-horned tree crickets (Oecanthus nigricornis)

“…The proximal end of the anterior TM is predominantly flat, with 2 bumps located at the proximal end of the groove. Overall, it is similar but less pronounced than that described for O. henryi (see Mhatre et al 2009). Above this furrow and the midpoint of the anterior TM there is a small ridge.…”

“…As in other tree crickets (Mhatre et al 2009), O. nigricornis have 2 tympanic membranes on the tibia of each foreleg. The anterior TM is both longer and wider than the posterior TM (anterior TM length 770 µm ±48; width 204 µm ±25; n = 13; posterior TM length 536 µm ±34; width 77 µm ±5; n = 5) (Fig.…”

“…Animals were mounted for laser vibrometry using the modified protocol from Mhatre et al (2009;. O. nigricornis were anesthetised using CO2 and mounted to a damped metal rod using liquid latex as described previously, but instead of using a balsa block or Blu-Tack (Bostik Ltd) to immobilise the femorotibial joint, wooden posts affixed to either side of the mount served to stabilise the leg and joint using dental wax.…”

“…The signal was flattened to provide equal power output across all frequencies, amplified (US amplifier, Avisoft, Berlin, Germany) and subsequently played back through a loudspeaker (ScanSpeak, Avisoft, Berlin, Germany). The loudspeaker was positioned perpendicular to the length of the animal as illustrated in Mhatre et al (2009). A reference microphone (4138, Brüel and Kjaer, Naerum, Denmark) positioned 5 mm above the femorotibial joint was used to monitor the acoustic stimuli throughout the acquisition of anterior TM vibration velocities.…”

“…Acoustic signals are important phonotactic cues for females and comprise a long-range call with a carrier frequency between 3-4.6 kHz depending on temperature (Sismondo 1979) and a courtship song used at close range which is less well described (Bell 1980). Like other tree crickets (Mhatre et al 2009), O. nigricornis have tympanal ears located on the proximal tibiae of the forelegs and although calls (Brown et al 1996), sound production (Williams 1945;Sismondo 1979;Sismondo 1993) and many features of sexual selection (see for example Brown 1999;Brown and Kuns 2000;Bussiere 2004) in this species have been described, the capabilities and morphology of the female receiver have not yet garnered attention. This paper therefore provides an analysis of the female auditory system as well as probing the extent of active auditory mechanics in the in Oecanthinae.…”