The free walking behaviour of the tettigonid Neoconocephalus robustus provides a unique quantitative description of the interactions that can occur between a rear and middle leg on the same side of the body in an insect. The rear legs step at approximately half the frequency of the anterior legs, and right and left sides of the body can function almost independently. Relative co-ordination produces a slow drift in the timing of the rear and middle leg protractions so that all the possible values of inter-leg phase may be observed.
A statistical examination of this behaviour shows that the duration of the middle leg step cycle is increased by the presence of a rear leg protraction, only when it occurs in the last half of the retraction stroke of the middle leg. If it occurs in the earlier part of the cycle there is no corresponding contraction of the middle leg step period. This and several other features of the walk exclude a ‘magnet effect’ or an ‘excitatory’ interaction between homologous points in the leg cycle and provide a verification of the ascending ‘inhibitory’ interaction proposed by Wilson (1966).
Co-ordinated walking in this insect appears to be a discrete process in which the performance of a protraction stroke activates a co-ordinating influence and, in conjunction with a frequency hierarchy, maintains the required time relationship between the legs.