auditory system of non-calling grasshoppers (Melanoplinae: Podismini) and the evolutionary regression of their tympanal ears

• Published in: Journal of Comparative Physiology 2010-11, Vol.196 (11), p.807-816
• Main Authors: Lehmann, Gerlind U. C, Berger, Sandra, Strauß, Johannes, Lehmann, Arne W, Pflüger, Hans-Joachim
• Format: Article
• Language: English
• Subjects: Acoustic communication; Animal Physiology; Animals; Auditory Pathways - anatomy & histology; Auditory Pathways - physiology; Auditory system; Auditory Threshold - physiology; Biological Evolution; Biomedical and Life Sciences; Ear; Ear, Middle - anatomy & histology; Ear, Middle - innervation; Ear, Middle - physiology; Electrophysiological Phenomena - physiology; Evolution; Evolution of nervous system; Female; Flight; Grasshoppers - anatomy & histology; Grasshoppers - physiology; Hearing; Insect hearing; Life Sciences; Male; Melanoplinae; NATURAL SCIENCES; NATURVETENSKAP; Nerves; Neurosciences; Original Paper; Orthoptera; Podismini; Predation; Sound production; Vestigialisation; Wings; Zoology
• ISSN: 0340-7594; 1432-1351; 1432-1351
• DOI: 10.1007/s00359-010-0560-2

Reduction of tympanal hearing organs is repeatedly found amongst insects and is associated with weakened selection for hearing. There is also an associated wing reduction, since flight is no longer required to evade bats. Wing reduction may also affect sound production. Here, the auditory system in four silent grasshopper species belonging to the Podismini is investigated. In this group, tympanal ears occur but sound signalling does not. The tympanal organs range from fully developed to remarkably reduced tympana. To evaluate the effects of tympanal regression on neuronal organisation and auditory sensitivity, the size of wings and tympana, sensory thresholds and sensory central projections are compared. Reduced tympanal size correlates with a higher auditory threshold. The threshold curves of all four species are tuned to low frequencies with a maximal sensitivity at 3-5 kHz. Central projections of the tympanal nerve show characteristics known from fully tympanate acridid species, so neural elements for tympanal hearing have been strongly conserved across these species. The results also confirm the correlation between reduction in auditory sensitivity and wing reduction. It is concluded that the auditory sensitivity of all four species may be maintained by stabilising selective forces, such as predation.