Tonic Immobility in a Cricket: Behavioral Characteristics, Neural Substrate, and Functional Significance

The field cricket (Gryllus bimaculatus), a model organism for neurobiological studies, shows tonic immobility (TI) upon restraint of its legs and maintains the restrained posture for several minutes. All bodily movements including abdominal ventilations are inhibited during TI. Catalepsy, in which the animal maintains a new posture forcibly given by an experimenter, is observed in all appendages. Due to the cataleptic nature, the cricket can assume any posture. Ablations of sense organs revealed that exteroceptors such as short trichoid sensilla on the pronotum and campaniform sensilla on the legs were necessary for the flexed-leg posture, and chordotonal organs that detect the tibial movement/position were critical for the induction of TI. Cooling of the brain led to immediate termination of TI, suggesting that immobility of the entire body is maintained by continuous activity of descending neurons originating from the brain. In the natural habitat, TI occurs during escape in which the cricket creates self-imposed restraint by crawling into a small opening made by pebbles or plants, supporting the view that TI is functional for dazzling predators that primarily use vision for prey detection.