Insect pupil mechanisms

The light-dependent pigment migration system of dragonfly ocelli was studied by optical, non-invasive techniques. The median ocellus is comprised of two lateral halves, as can be demonstrated in the intact animal since illumination of the receptors in one half of the median ocellus only induces a movement of pigment located in that half. Measurable pigment migration can occur within a few seconds, but its speed and extent depends on light intensity. Dispersal of pigment, which occurs upon light adaptation, proceeds faster than retraction, which occurs upon dark adaptation. Action spectra for pigment movement have been determined in Sympetrum and Anax. The spectrum for Sympetrum has a prominent UV peak, moderate blue sensitivity, and very low green sensitivity. A similar profile is obtained in Anax, but only after intense orange adaptation which suppresses the green sensitivity. The results conform to the known spectral sensitivities of Libellulid and Aeschnid ocellar receptors. It is concluded that the photoreceptors drive pigment movement through an unknown mechanism. The effect of the migration of pigment is the selective reduction of radiant flux on the retina from luminous sources at high elevations relative to the animal's normal flying posture.