Specialized ommatidia of the polarization-sensitive dorsal rim area in the eye of monarch butterflies have non-functional reflecting tapeta |
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Authors: | Thomas Labhart Franziska Baumann Gary D Bernard |
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Institution: | (1) Institute of Zoology, University of Zurich, Winterthurerstrasse 190, CH 8057 Zurich, Switzerland;(2) Department of Electrical Engineering, University of Washington, Seattle, WA 98195-2500, USA |
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Abstract: | Many insects exploit sky light polarization for navigation or cruising-course control. The detection of polarized sky light
is mediated by the ommatidia of a small specialized part of the compound eye: the dorsal rim area (DRA). We describe the morphology
and fine structure of the DRA in monarch butterflies (Danaus plexippus). The DRA consists of approximately 100 ommatidia forming a narrow ribbon along the dorsal eye margin. Each ommatidium contains
two types of photoreceptor with mutually orthogonal microvilli orientations occurring in a 2:6 ratio. Within each rhabdomere,
the microvilli are well aligned. Rhabdom structure and orientation remain constant at all retinal levels, but the rhabdom
profiles, as seen in tangential sections through the DRA, change their orientations in a fan-like fashion from the frontal
to the caudal end of the DRA. Whereas these properties (two microvillar orientations per rhabdom, microvillar alignment along
rhabdomeres, ommatidial fan array) are typical for insect DRAs in general, we also report and discuss here a novel feature.
The ommatidia of monarch butterflies are equipped with reflecting tapeta, which are directly connected to the proximal ends
of the rhabdoms. Although tapeta are also present in the DRA, they are separated from the rhabdoms by a space of approximately
55 μm effectively inactivating them. This reduces self-screening effects, keeping polarization sensitivity of all photoreceptors
of the DRA ommatidia both high and approximately equal. |
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