A narrow ear canal reduces sound velocity to create additional acoustic inputs in a microscale insect ear

Abstract

Located in the forelegs, katydid ears are unique among arthropods in having outer, middle, and inner components, analogous to the mammalian ear. Unlike mammals, sound is received externally via two tympanic membranes in each ear and internally via a narrow ear canal (EC) derived from the respiratory tracheal system. Inside the EC, sound travels slower than in free air, causing temporal and pressure differences between external and internal inputs. The delay was suspected to arise as a consequence of the narrowing EC geometry. If true, a reduction in sound velocity should persist independently of the gas composition in the EC (e.g., air, CO2). Integrating laser Doppler vibrometry, microcomputed tomography, and numerical analysis on precise three-dimensional geometries of each experimental animal EC, we demonstrate that the narrowing radius of the EC is the main factor reducing sound velocity. Both experimental and numerical data also show that sound velocity is reduced further when excess CO2 fills the EC. Likewise, the EC bifurcates at the tympanal level (one branch for each tympanic membrane), creating two additional narrow internal sound paths and imposing different sound velocities for each tympanic membrane. Therefore, external and internal inputs total to four sound paths for each ear (only one for the human ear). Research paths and implication of findings in avian directional hearing are discussed.

Ear pinnae in a neotropical katydid (Orthoptera: Tettigoniidae) function as ultrasound guides for bat detection

“…Moreover, completely occluding the spiracle would prevent natural venting of metabolic gases (i.e. CO 2 ) and inhibit equilibration of tracheal gases with atmospheric pressure 12, 45 . In C. gorgonensis , the spiracle area is large, naturally open on average three times larger than the total area of the tympanal slit (1 mm 2 : 0.3 mm 2 ) which is inversely related to the scale of pseudophyllines.…”

“…As previously shown for C . gorgonensis 46 , and in other species 18, 41 , the spiracular port and the EC with its exponential horn geometry act as a bandpass filter limited in providing pressure gains to high ultrasonic frequencies (>50 kHz) 12, 13 and designed to favour the specific carrier frequency. The level of sound entering the spiracle depends on how the incident soundwave accesses the spiracular opening 49 .…”

“…The level of sound entering the spiracle depends on how the incident soundwave accesses the spiracular opening 49 . The effect of the EC geometry in reducing sound velocity is as much as ∼20% or ca 60 µs, and coupled with the contralateral EC, collectively a total of eight inputs are possible with each causing a vibration with variable delays in four internal and four external tympanal surfaces to the same stimulus 12 . Though the reduction in velocity contributes exceptional binaural directional cues, the external port provides real-time sensitivity to exploit unamplified, fading ultrasound.…”

“…gorgonensis 46 , and in other species 20,44 , the spiracular port and the EC with its exponential horn geometry act as a bandpass filter limited in providing pressure gains to high ultrasonic frequencies (> 50 kHz, for C. gorgonensis ) 14,15 and designed to enhance detection of the specific carrier frequency. The effect of the EC geometry in reducing sound velocity is as much as ~20% or ca 60 µs, and coupled with the contralateral EC, collectively a total of eight inputs are possible with each causing a vibration with variable delays in four internal and four external tympanal surfaces to the same stimulus 14 . Though the reduction in velocity contributes exceptional binaural directional cues, the external port provides real-time sensitivity to exploit fading bat ultrasounds.…”

“…Similar to the mammalian ear, the tettigoniid ear has an air-filled ear canal (EC), also called acoustic trachea, delivering the sound to the tympana. Further, in the tettigoniid ear the EC amplifies the sound signals 11–18 and reduces propagation velocity 11, 12, 18 due to its narrowing, exponential horn design. Sound enters the EC through a specialised opening in the prothorax known as the acoustic spiracle 19 .…”