Neuroanatomy of the complex tibial organ in the splay-footed cricket Comicus calcaris Irish 1986 (Orthoptera: Ensifera: Schizodactylidae).

The subgenual chordotonal organ complex in insects is modified in ensiferan taxa like Gryllidae and Tettigoniidae into hearing organs with specific sets of auditory receptors. Here, this sensory organ complex is documented in the nonhearing splay-footed cricket Comicus calcaris. The tibial chordotonal organ consists of three parts: the subgenual organ, the intermediate organ, and the crista acustica homolog. The latter is an array of linearly organized neurons homologous to auditory receptors in the tibial hearing organs of Tettigoniidae. The tibial organ is structurally similar in all three leg pairs, with similar neuron numbers in the fore- and midleg, but lower numbers in the hindleg. The foreleg crista acustica homolog consists of 34±4 neurons, the highest number in an atympanate Ensiferan. Additionally, an accessory chordotonal organ with 15±5 neurons innervated by nerve 5B1 is present in the foreleg. The central projection of the tibial organreveals ipsilateral sensory terminals in the primary sensory neuropil, the medial ventral association center with terminations close to the midline. As determined from extracellular recordings, the entire tibial organ is vibrosensitive. The organization of the tibial organ is compared to other ensiferan auditory and nonauditory tibial organs. Spatial orientation of neurons in the crista acustica homolog is not reminiscent of auditory structures, and the neuroanatomy is discussed with respect to stridulation behavior and the evolutionary origin of hearing in Ensifera.