Regeneration of axotomized tympanal nerve fibres in the adult grasshopper Chorthippus biguttulus (L.) (Orthoptera: Acrididae) correlates with regaining the localization ability

Adult males of the grasshopper Chorthippus biguttulus exhibit a stereotyped turning behaviour towards the direction of the female song. This behaviour has been used to study the regeneration of synaptic connections used for pattern recognition and sound localization. Unilaterally deafened animals are not able to localize the sound direction and turn exclusively towards the intact side, regardless of the speaker position. This behaviour does not change with postoperative time. After an axotomy of the tympanic nerve fibres the sensory axons regrow and regenerate their synaptic contacts which is deduced from the recovery of the ability to localize sound. The behavioural threshold for stimulation from the operated side is increased by approx. 5 dB SPL. The probability of correct turning towards the operated side increases with postoperative time. The ability for lateralization improves with postoperative time and may reach values of intact animals (discrimination of 1–2 dB SPL difference. Animals with two operations (axotomy of one tympanal nerve and blocking of the other ear) do not react to the female song, which suggests that recognition of the species-specific song pattern is not possible with the regenerated fibres and their synaptic connections alone. Neuroanatomical studies show that the regeneration of localization ability is correlated with an ingrowth of sensory fibres into the frontal auditory neuropil of the metathoracic ganglion.