Low pass filters and differential tympanal tuning in a paleotropical bushcricket with an unusually low frequency call

Abstract

SUMMARYLow-frequency sounds are advantageous for long-range acoustic signal transmission, but for small animals they constitute a challenge for signal detection and localization. The efficient detection of sound in insects is enhanced by mechanical resonance either in the tracheal or tympanal system before subsequent neuronal amplification. Making small structures resonant at low sound frequencies poses challenges for insects and has not been adequately studied. Similarly, detecting the direction of longwavelength sound using interaural signal amplitude and/or phase differences is difficult for small animals. Pseudophylline bushcrickets predominantly call at high, often ultrasonic frequencies, but a few paleotropical species use lower frequencies. We investigated the mechanical frequency tuning of the tympana of one such species, Onomarchus uninotatus, a large bushcricket that produces a narrow bandwidth call at an unusually low carrier frequency of 3.2kHz. Onomarchus uninotatus, like most bushcrickets, has two large tympanal membranes on each fore-tibia. We found that both these membranes vibrate like hinged flaps anchored at the dorsal wall and do not show higher modes of vibration in the frequency range investigated (1.5-20kHz). The anterior tympanal membrane acts as a low-pass filter, attenuating sounds at frequencies above 3.5kHz, in contrast to the highpass filter characteristic of other bushcricket tympana. Responses to higher frequencies are partitioned to the posterior tympanal membrane, which shows maximal sensitivity at several broad frequency ranges, peaking at 3.1, 7.4 and 14.4kHz. This partitioning between the two tympanal membranes constitutes an unusual feature of peripheral auditory processing in insects. The complex tracheal shape of O. uninotatus also deviates from the known tube or horn shapes associated with simple band-pass or high-pass amplification of tracheal input to the tympana. Interestingly, while the anterior tympanal membrane shows directional sensitivity at conspecific call frequencies, the posterior tympanal membrane is not directional at conspecific frequencies and instead shows directionality at higher frequencies.

Selective forces on origin, adaptation and reduction of tympanal ears in insects

“…But even for male calls there may be opposing pressures for providing good cues for directionality (typically higher frequencies) and for being optimally transmitted through the biotope (typically lower frequencies; Schul and Patterson 2003). Recently, a very unusual case has been described for a pseudophylline bushcricket (Rajamaran et al 2013): one of the two tympana (all bushcrickets have two tympana in each foreleg) is tuned to the lowfrequency call of the male, the other tympanum is tuned to higher frequencies, which might be contained in sounds of predators. The same is true for the directional response of the two tympana.…”

Auditory mechanics in a bush-cricket: direct evidence of dual sound inputs in the pressure difference receiver

“…However, for the subfamily Pseudophyllinae (a large group with some 1000 species described) the acoustic spiracle is reduced (a character used as diagnostic for this subfamily) and the bulla is replaced by a small chamber [18][19][20]. In other species, the AT forms a large U-shaped bend at the bulla site [6]. Although poorly understood, in Pseudophyllinae, the AT is unlikely to be the main acoustic input, and some authors suggest that the tympanal slits might play an important role as waveguides [18,20].…”

What determines the number of auditory sensilla in the tympanal hearing organs of Tettigoniidae? Perspectives from comparative neuroanatomy and evolutionary forces

“…and the species with the highest known sensillum number (Ancylecha fenestrata) has a cover at the anterior tympanum. Specific evolu-Biomechanical analysis in Onomarchus uninotatus (Pseudophyllinae) showed fascinating adaptations for the two tympanal membranes with differential tympanal tuning (acoustic partitioning) of the anterior tympanum as a low-pass filter and the posterior tympanum as a high-pass filter (Rajaraman et al 2013). The structure and mechanics of the CA and associated elements would be interesting for their organization in this case.…”