EXTRARETINAL PHOTORECEFTION IN INSECTS

Abstract

Abstract— Extraretinal photoreceptors are widespread among insects and function in the photoperiodic control of development and in the entrainment of circadian rhythms. The effects of light on the daily and seasonal regulation of brain neuroendocrine activity are mediated solely through extraretinal photoreceptors. In primitive insects, the eyes participate in the entrainment of nonendocrine circadian rhythms such as the locomotor rhythm. In more advanced forms, however, extraretinal pathways appear to be the only pathway for the entrainment of all rhythms thus far examined. But even in this latter case, the eyes sometimes effect a masking of the expression of the overt rhythm. An exact localization of the extraretinal receptors has not been accomplished, but in all studies to date they appear to be associated with the cerebral lobe region of the brain. Action spectra for photoperiodic responses have been determined for a number of insects. In general the responses are maximally sensitive in the blue with a marked decline in the red although exceptions do exist. Complete action spectra for circadian responses have been determined only for two insects. In both cases a plateau of sensitivity extends through the blue with a steep drop at longer wavelengths. From the action spectra data, the extraretinal receptors appear to have a threshold sensitivity less than 3 times 10 ‐2 J/m 2 . The pigment nature of the receptor is unknown although it appears not to be a carotenoid derivative.