title: Polarization sensitivity as a visual contrast enhancer in the Emperor dragonfly larva, Anax imperator.
abstract: Polarization sensitivity (PS) is a common feature of invertebrate visual systems. In insects, PS is well known for its use in several different visually guided behaviours, particularly navigation and habitat search. Adult dragonflies use the polarization of light to find water but a role for PS in aquatic dragonfly larvae, a stage that inhabits a very different photic environment to the adults, has not been investigated. The optomotor response of the larvae of the Emperor dragonfly, Anax imperator Leach 1815, was used to determine whether these larvae use PS to enhance visual contrast underwater. Two different light scattering conditions were used to surround the larval animals: a naturalistic horizontally polarized light field and a non-naturalistic weakly polarized light field. In both cases these scattering light fields obscured moving intensity stimuli that provoke an optokinetic response in the larvae. Animals were shown to track the movement of a square-wave grating more closely when it was viewed through the horizontally polarized light field, equivalent to a similar increase in tracking ability observed in response to an 8% increase in the intensity contrast of the stimuli. Our results suggest that larval PS enhances the intensity contrast of a visual scene under partially polarized lighting conditions that occur naturally in freshwater environments.
authors.affiliation: School of Biological Sciences, Bristol Life Sciences Building, Tyndall Avenue, University of Bristol, Bristol BS8 1TQ, UK camilla.r.sharkey@gmail.com.
authors.forename: Camilla R
authors.initials: CR
authors.lastname: Sharkey
authors.affiliation: School of Animal Biology and the Oceans Institute, Faculty of Science, University of Western Australia, 35 Stirling Highway, (M317), Crawley, Western Australia 6009, Australia.
authors.forename: Julian C
authors.initials: JC
authors.lastname: Partridge
authors.affiliation: School of Biological Sciences, Bristol Life Sciences Building, Tyndall Avenue, University of Bristol, Bristol BS8 1TQ, UK.
authors.forename: Nicholas W
authors.initials: NW
authors.lastname: Roberts
mesh_terms: D000818:Animals; D001522:Behavior, Animal; D007814:Larva / Q000502:physiology; D008027:Light; D009068:Movement; D009799:Ocular Physiological Phenomena; D063191:Odonata / Q000502:physiology*; D014785:Vision, Ocular; D014796:Visual Perception
keywords: Odonata; Optomotor response; Polarization vision; Rayleigh scattering; Turbidity