The circadian clock system of hemimetabolous insects

Publisher Summary The circadian system of hemimetabolous insects, such as crickets and cockroaches, has been extensively studied neuroethologically. It is a unique model for the study of the circadian timekeeping mechanism. The circadian clock is located one in each optic lobe, regulates a variety of physiological functions, such as locomotor activity, sensitivity of the visual system, and neuronal activity, and synchronizes to a light dark cycle perceived in most cases by the compound eye. The exact location of the clock cells is unknown. The oscillatory mechanism of the clock is still largely unknown, but experiments with a cytosolic translation inhibitor, cycloheximide, have revealed that at least protein synthesis is a required process for the clock functioning in crickets. The bilaterally redundant clocks retain their synchrony and provide a stable temporal structure to the animal's behavior by interacting with one another through neural pathway. In the interaction, the so-called medulla bilateral neurons, serotonergic neurons, and pigment dispersing hormone-immunoreactive neurons are suggested to be involved. The hemimetabolous clock system may contribute to the understanding of the circadian organization that maintains the well-ordered temporal structure in physiology and behavior in multioscillator systems. This system helps to understand how the clock regulates the overt rhythm at the neuronal level, but further progress depends on the identification of the clock cells in the brain.