We demonstrate that the probability of the crayfish, P. clarkii, to tail flip in response to a touch on the dorsal tail fan is dependent on both the size and the behavioral state of the animal. Alterations in the animal’s internal physical state, such as when the animal autotomizes its chelipeds, will cause larger-sized animals to tail flip; if they were not autotomized, then no tail flip response would occur. Altering the external environment by removal of water causes small crayfish, which normally habituate slowly, to rapidly habituate. Observation of large adult crayfish in a species, O. australis packardi, one that evolved to live in total cave darkness, revealed that they are more likely to tail flip than are the sighted, adult P. clarkii. Results indicate that the behavioral state of the crayfish can result in rapid and long-term alterations in the tail flip response and in habituation rates to repetitive stimuli. This ability to show plasticity in gain setting may be regulated by neuromodulators and can occur in large adults of the sighted crayfish. Differences between the two species indicate that size may not be the sole contributing factor to account for tail flip behaviors.