A quantitative analysis of rhythmic leg movements during three different behaviors in the cockroach, Periplaneta americana

Abstract

We report a quantitative comparison of three motor rhythms involving the legs of the cockroach Periplaneta americana. Each rhythm is associated with one of three behaviors—walking, grooming, or righting. Previous studies have defined a group of central oscillatory interneurons which appear to drive rhythmic leg movement in this species. It has been assumed that these cells drive walking behavior. Either walking or grooming is evoked by topical irritation of a cercus or lateral margin of the abdomen. Six legs are used for walking, but only one metathoracic leg is used for grooming. By experimentally limiting visual or antennal inputs, or by decapitation, the incidence of an insect's grooming behavior in response to chemical irritation is increased. The movements of single metathoracic legs are quantitatively very similar in walking and grooming. Only very slight differences are seen in the position of leg segments and in the total excursion of the leg joints in the two behaviors. Electrical activity in antagonistic leg muscles is very similar in walking, grooming and righting behaviors. Muscle burst durations, interburst intervals, and phasing between antagonists vary slightly among the behaviors. Each of these parameters is sensitive to sensory feedback from the moving leg itself. Experimental immobilization of the coxal-femoral joints during grooming alters some of the same parameters by which the three behaviors differ. When an animal grooms with one metathoracic leg and walks simultaneously with the contralateral leg, the relative timing of movements between contralateral legs is the same as when both legs are used together in walking. The close similarity among rhythmic leg movements and motor outputs among these three behaviors requires a reassessment of earlier work on the central oscillatory system for walking in this species. It now seems equally possible that the oscillator previously studied is, in fact, that for walking or for grooming. Alternatively, it is possible that the same central oscillator might control walking, grooming, and righting, or any two of these.