ABSTRACT
This chapter discusses the role of extraretinal photoreceptors among the invertebrates only in insects since they have received more intensive study than other invertebrate groups. The daily light-dark cycles associated with the earth's rotation and the annual changes in the length of the photoperiod are used by many organisms to time important physiological and behavioral events. Most of the research into the role of extraretinal photoreceptors (ERRs) in insects as well as vertebrates concerns entrainment of the biological clock and photoperiodic photoreception. In most insects the compound eyes are apparently not involved in the perception of entraining light cycles. The compound eyes have no role in the entrainment of the silkmoth flight activity rhythms. Extraretinal photoreceptors mediating a host of different kinds of light-dependent responses have been demonstrated in both vertebrates and invertebrates. It is likely that many extraretinal receptors evolved before the evolution of the lateral eyes of vertebrates or the compound eyes or ocelli of insects.
