• Clock models;
  • insect photoperiodism;
  • photoreceptors;
  • spectral sensitivity

This review examines the spectral sensitivities of photoperiodic responses in insects and mites in relation to circadian-based models for the photoperiodic clock. It concludes that there are probably a number of different photoreceptors at both the organ and molecular levels. These latter probably fall into two classes: (i) a blue-light sensitive photoreceptor and (ii) a range of opsins (i.e. opsin proteins conjugated with a vitamin A based pigment) absorbing light at a range of wavelengths. In flesh flies (Sarcophaga spp. and possibly other higher Diptera), which are considered to exemplify the ‘external coincidence’ model, entrainment of the photoperiodic oscillator probably involves a blue-light photoreceptor of Drosophila-type CRYPTOCHROME (CRY1) absorbing maximally at approximately 470 nm, whereas opsins absorbing at longer wavelengths may be involved in the photo-inductive process (diapause/nondiapause regulation) that occurs when dawn light coincides with the photo-inducible phase. In the parasitic wasp Nasonia vitripennis, on the other hand, a species that lacks CRY1 but expresses the nonphotosensitive ‘mammalian-type’ CRY2, and is considered to exemplify ‘internal coincidence’, entrainment of the dawn and dusk oscillators may involve opsin-based photoreceptors absorbing light at longer wavelengths as far as the red end of the spectrum. In the Lepidoptera, which express both CRY1 and CRY2, properties of both external and internal coincidence may be evident. The presence or absence of cry1 in the genome may thus emerge as a key to the photoperiodic mechanism on its light input pathway.