Functional relevance of acoustic tracheal design on directional hearing in crickets

Abstract

Internally coupled ears (ICEs) allow small animals, relatively to the wavelengths of sounds to be localized, to reliably determine the direction of a sound source. ICEs are found in a variety of taxa, but crickets have evolved the most complex arrangement of coupled ears; an acoustic tracheal system comprised of a large cross body trachea that connects two entry points for sound in the thorax with the leg trachea of both ears. The key structure that allows for the tuned directionality of the ear is a tracheal inflation (acoustic vesicle) in the midline of the cross body trachea holding a thin membrane (septum). Crickets are known to display a wide variety of acoustic tracheal morphologies, most importantly with respect to the presence of a single or double acoustic vesicle. However, the functional relevance of this variation is still not known. In this study, we investigated the peripheral directionality of three co-occurring, closely related cricket species of the subfamily Gryllinae. No support could be found for the hypothesis that a double vesicle should be regarded as an evolutionary innovation to increase (1) interaural directional cues, (2) the selectivity of the directional filter, or (3) provide a better match between the directional and sensitivity tuning. Nonetheless, by manipulating the double acoustic vesicle in the rainforest cricket P. podagrosus, selectively eliminating the sound transmitting pathways, we revealed that these pathways contribute almost equally to the total amount of IIDs, emphasizing their functional relevance in the system.