Hemimetabolous insects, such as crickets and cockroaches, provide a unique model system for neuroethological study of the circadian timekeeping mechanism. The circadian pacemaker is located in the optic lobe and synchronizes to a light dark cycle in most cases through the compound eyes. The location of the circadian pacemaker cells is still under investigation, but some candidates have been postulated on the basis of molecular probes such as antibodies to the Drosophila PERIOD protein and the crustacean pigment dispersing hormone (PDH). To retain their synchrony and provide a stable temporal structure to the animal`s behavior, the bilateral pacemakers interact with one another through a neural pathway. A set of neurons, the so-called medulla bilateral neurons, covey the information about light/dark and the clock through which the pacemaker entrains its contralateral partner. The system opens up opportunities to clarify how the pacemaker regulates the overt rhythm at the neuronal level.