Abdominal grooming in the cockroach: Development of an adult behavior

Chemical or tactile irritants delivered to the dorsomedial region of the abdomen of the cockroach Periplaneta americana. evoke a complex grooming response. In larvae the response consitsts of extension of all six legs and arching of the abdomen which is rubbed against any solid surface presented. In adults, leg extension and arching are accompanied by side to side wagging of the abdomen and rapid scissoring of the wings. In some decapitated late instar larvae, abdominal wagging behavior can be evoked by chemical or tactile stimulation of the dorso-medial region of the abdomen. The form of the wag is somewhat different in these animals than in the adults, and it occurs at a lower frequency. Neuronal circuitry underlying the rhythmic wagging response of the adult may therefore be laid down or made functional in the mid- to late-larval stages. The wagging behavior is not normally expressed until the final ecdysis. A population of sensory setae is found on the abdomen at all stages of development, and some of these are presumed to trigger the grooming behavior when stimulated. The morphology and distribution of some of the adult receptors differs from some of the larval receptors. All larvae, including those late-instar individuals which show an adult-like response (wagging) have identical receptors on the abdomen in terms of morphology, distribution, and general responsiveness. Thus, no obvious changes in abdominal sensory receptors are correlated with the emergence of the wagging behavior. Anatomical and/or functional changes within the central nervous system are most likely responsible for the emergence of adult wagging behavior.