SUMMARY
Mechanoreceptors involved in the escape jumping evoked by hindwing stimulation have been investigated in the field cricket Gryllus bimaculatus. By partial ablation of the hindwing, we found that a mechanosensory system relevant to the escape behaviour was localized on specific veins of the hindwing tip. Scanning electron microscopy revealed three types of mechanoreceptive sensillae on the corresponding region. Based on their morphology, type I and type III sensillae were judged to be trichoid and chaetic sensillae, respectively. Type II sensillae were newly found in this study, having a twisted shaft with a socket-like structure at its base. They existed almost exclusively on the tip and middle regions of the hindwing. The conduction velocity of type II units was significantly smaller than that of type I units. One cycle of sinusoidal deflection of a single type II sensilla at frequencies in the range of 10-120 Hz caused the sensory unit to discharge a single or a few spikes that were not directly correlated with any specific direction of hair movement nor specific deflection angle. The response probability decreased with the stimulus frequency to be less than 0.1 at 0.2 Hz. The results suggest that type II sensillae would serve as contact mechanoreceptors with a low-cut filter property to obtain general information on the presence of stimuli on the hindwing tip rather than specific information on their precise positioning or movement.
