Are the Hindwing Chordotonal Organs Elements of the Locust Flight Pattern Generator?

Abstract

ABSTRACT Anatomical and electrophysiological techniques were used to examine the structure, central nervous connections and discharge patterns of afferents arising from the hindwing chordotonal organs in the locust, Locusta migratoria. The hindwing chordotonal organ afferents were found to be sensitive to low-frequency sounds (approx. 3 kHz) and to vibrations of the supporting surface. During flight in tethered animals these afferents were strongly activated. This flight-related activity was weakly modulated and the maximum occurred between depressor bursts. Low-frequency sound pulses were used to activate selectively the hindwing chordotonal organ afferents during rhythmic flight activity in deefferented preparations (all motor nerves cut). Phasic stimulation of the chordotonal organs, time-locked to every cycle, had no effect on the frequency of the centrally generated rhythm as recorded from proximal nerve stumps. Staining of single afferents from the hindwing chordotonal organ showed that they bifurcate in the metathoracic ganglion, with one branch terminating in the ring tract and the other branch projecting to the mesothoracic ganglion via the ipsilateral connective. None of the terminal processes of chordotonal afferents was observed to be located in dorsal neuropile regions containing the processes of flight neurones. Consistent with this finding, the chordotonal afferents were not observed to make synaptic connections to flight interneurones or motoneurones. The main conclusion of this investigation is that the hindwing chordotonal organs do not contribute to the patterning of motor activity for flight. The hindwing chordotonal organs probably function as detectors of vibration and perhaps low-frequency sound.