evolution of directional hearing in crickets

Sound localization in small insects can be a challenging task due to imposed physical constrains in deriving sufficiently large intensity differences between both ears (IIDs). In crickets, sound source localization is based on a complicated biophysical mechanism using a modified tracheael system connecting the tympana of both ears with an external sound entrance. These acoustic trachea form the basis of a pressure difference receiver; it responds to the interaction of three different sound components at the site of the tympanum, with the result of a directionality of the ear despite an unfavorable relationship between the wavelength of sound and body size. In a previous survey on different cricket species predominantly of the tropical rainforest, we found large morphological differences of the acoustic tracheal apparatus subserving directionality. What are the factors, the biophysical and environmental constraints accounting for these differences, and how did the pressure difference receiver in crickets evolve at all? What have been the evolutionary precursors of the system, and what are the modifications which provide highest directionality of the system? The current proposal aims to investigate its evolution using anatomical, neurophysiological, biophysical and modelling approaches. We will compare anatomical variation in the acoustic tracheal system of primary non-hearing species with those using elaborate acoustic signalling, and species with secondary loss of sound communication.

Compared to all other hearing animals, insects are the smallest ones, both in absolute terms and in relation to the wavelength of most biologically relevant sounds. The ears of insects can be located at almost any possible body part, such as wings, legs, mouthparts, thorax or abdomen. The distances between both ears are generally so small that cues for directional hearing such as interaural time and intensity differences (IITs and IIDs) are also incredibly small, so that the small body size should be a strong constraint for directional hearing. Yet, when tested in behavioral essays for the precision of sound source localization, some species demonstrate hyperacuity in directional hearing and can track a sound source deviating from the midline by only 12 degrees. They can do so by using internally coupled ears, where sound can act on both sides of a tympanic membrane. In our project we used comparative anatomical, physiological and biophysical methods to study the evolution of structures responsible for directional hearing in crickets. We describe their varying anatomy and mode of operation for, exhibiting probably one of the most sophisticated of all internally coupled ears in the animal kingdom. We found large morphological differences of the acoustic tracheal apparatus subserving directionality. In the project we investigated the biophysical and environmental constraints accounting for these differences and asked how the pressure difference receiver in crickets evolved. We compared anatomical variation in the acoustic tracheal system of primary non-hearing species with those using elaborate acoustic signalling, and species with secondary loss of sound communication, to identify potential evolutionary precursors of the system and those modifications which provide highest directionality. By careful surgical modification of the tracheal system we could also quantify the impact of one part of the pressure difference receiver for the directionality, and how this changes the directional performance of female crickets, both in the laboratory and in the field.