Abstract
We investigated the neuroanatomy and physiology of the complex tibial
organ of an atympanate ensiferan, the Gryllacridid Ametrus tibialis.
This represents the first analysis of internal mechanoceptors in
Gryllacridids. The complex tibial organ is tripartite consisting of a
subgenual organ, intermediate organ and a homologue organ to the crista
acustica of tympanate ensiferan taxa of Tettigoniidae, Haglidae, and
Anostostomatidae. The crista homologue contains 23 8 2 receptor neurons
in the foreleg. It is associated with the leg trachea and found
serially in all three thoracic leg pairs. Central projections of the
sensory nerve of the complex tibial organ bifurcate in two lobes in the
prothoracic ganglion, which do not reach the midline. The axonal
endings project into the mVAC, the main vibratory-auditory neuropile of
Ensifera. Recordings of the tibial nerve show that the tibial organ is
sensitive to vibrational stimuli with a minimum threshold of 0.02 to
0.05 ms(-2) at 200-500 Hz, but rather insensitive to airborne sound.
The main function of the tibial organ is therefore vibration sensing,
although the specific function of the crista homologue remains unclear.
The presence of the crista acustica homologue is interpreted in
phylogenetic context. Because ensiferan phylogeny is unresolved, two
alternative scenarios can be deduced: (a) the crista homologue is a
precursor structure which was co-opted as an auditory system and
represent a morphologically highly specialized structure before
acquisition of its new function; (b) a previously functional tibial ear
is evolutionary reduced but the neuronal structures are maintained.
Based on comparison of neuroanatomical details, the crista acustica
homologue of A. tibialis could present the neuronal complement of an
ear evolutionary precursor structure, which was successively made
sensitive to airborne sound by elaboration of cuticular tympana,
auditory spiracle and trachea for sound propagation. Copyright (C) 2008
S. Karger AG, Basel.
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