School of Life Sciences, Joseph Banks Laboratories, University of Lincoln, Lincoln LN6 7TS, United Kingdom.
School of Life Sciences, Joseph Banks Laboratories, University of Lincoln, Lincoln LN6 7TS, United Kingdom;
Proc Natl Acad Sci U S A. 2021 Mar 9;118(10). doi: 10.1073/pnas.2017281118.
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, [Formula: see text]). 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 [Formula: see text] 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.
位于前腿的蝈蝈耳朵在具有外、中、内耳成分方面在节肢动物中是独一无二的，类似于哺乳动物的耳朵。与哺乳动物不同，声音通过每只耳朵中的两个鼓膜从外部接收，通过源自呼吸气管系统的狭窄耳道（EC）从内部接收。在 EC 内部，声音的传播速度比在自由空气中慢，导致外部和内部输入之间存在时间和压力差异。据推测，延迟是由于 EC 的几何形状变窄而产生的。如果是这样，那么声音速度的降低应该独立于 EC 中的气体组成（例如空气、[化学式：见文本]）而持续存在。通过对每个实验动物 EC 的精确三维几何形状进行激光多普勒测振法、微计算机断层扫描和数值分析的整合，我们证明了 EC 的缩小半径是降低声音速度的主要因素。实验和数值数据还表明，当 EC 中充满多余的[化学式：见文本]时，声音速度会进一步降低。同样，EC 在鼓膜水平分叉（每个鼓膜一个分支），为每个鼓膜创建两个额外的狭窄内部声音路径，并为每个鼓膜施加不同的声音速度。因此，每个耳朵有四个外部和内部输入的声音路径（人类耳朵只有一个）。讨论了在鸟类定向听力方面的研究路径和发现的意义。
