The aim of neuroethological approach is to explain animal behaviour in terms of underlying neural circuitry. Arthropod animals are suitable for this field of study, since they have a relatively simple nervous system and a smaller number of central neurones, that are frequently identifiable, than vertebrates. Small crayfish show an avoidance dart behaviour in response to gentle touching of the tailfan. The movement of the uropods in this response is controlled by local circuits within the terminal abdominal ganglion. Three distinct groups of interneurones, intersegmental ascending, spiking local and nonspiking local interneurones, receive cholinergic excitatory inputs directly from sensory afferents. The ascending interneurones make direct excitatory connections with nonspiking interneurones. By contrast, spiking local interneurones make direct inhibitory connections with nonspiking interneurones. A small amount of depolarizing current injected into nonspiking interneurones causes a sustained and smooth membrane hyperpolarization of the motor neurones. Using electrophysiological, pharmacological and immunocytochemical approaches, GABAergic and glutamategic inhibitory pathways are revealed to be involved in this circuit. Premotor nonspiking interneurones are divided into two groups, PL and AL interneurones. PL interneurones form neural pathways that close the uropod while AL interneurones form neural pathways that open it. Inhibitory outputs from PL interneurones are mediated by GABA while those of AL interneurones are mediated by glutamate. Thus, PL and AL interneurones form parallel and opposing pathways with different inhibitory transmitters. Functional differences between these two inhibitory pathways are revealed by the analysis of transverse lateral inhibition of ascending interneurones. Many ascending interneurones extend dendrites only within the hemiganglion contralateral to their somata. They receive transverse inhibitory inputs via two distinct bilateral local interneurones. Nonspiking interneurone, LDS is GABAergic and mediates slow IPSPs in the ascending interneurones, while spiking local interneurones of a medial group are glutamatergic and elicit fast IPSPs in different ascending interneurones. Bath application of picrotoxin blocks fast IPSPs but has no effect on slow IPSPs. The reversal potential of the glutamate-mediated hyperpolarization of interneurones is much lower than that of the GABA-mediated one. Slow inhibitory effects of GABA are mediated by a potassium current, while fast inhibition of glutamate is mediated by a chloride current.