Organization of the circadian system in insects.

The circadian systems of different insect groups are summarized and compared. Emphasis is placed on the anatomical identification and characterization of circadian pacemakers, as well as on their entrainment, coupling, and output pathways. Cockroaches, crickets, beetles, and flies possess bilaterally organized pacemakers in the optic lobes that appear to be located in the accessory medulla, a small neuropil between the medulla and the lobula. Neurons that are immunoreactive for the peptide pigment-dispersing hormone (PDH) arborize in the accessory medulla and appear to be important components of the optic lobe pacemakers. The neuronal architecture of the accessory medulla with associated PDH-immunoreactive neurons is best characterized in cockroaches, while the molecular machinery of rhythm generation is best understood in fruit flies. One essential component of the circadian clock is the period protein (PER), which colocalizes with PDH in about half of the fruit fly's presumptive pacemaker neurons. PER is also found in the presumptive pacemaker neurons of beetles and moths, but appears to have different functions in these insects. In moths, the pacemakers are situated in the central brain and are closely associated with neuroendocrine functions. In the other insects, neurons associated with neuroendocrine functions also appear to be closely coupled to the optic lobe pacemakers. Some crickets and flies seem to possess central brain pacemakers in addition to their optic lobe pacemakers. With respect to neuronal organization, the circadian systems of insects show striking similarities to the vertebrate circadian system.