Optic lobe projections of marginal ommatidia in Calliphora erythrocephala specialized for detecting polarized light

The structure of ommatidia at the dorsal eye margin of the fly, Calliphora erythrocephala is specialized for the detection of the e-vector of polarized light. Marginal zone ommatidia are distinguished by R7/R8 receptor cells with large-diameter, short, untwisted rhabdomeres and long axons to the medulla. The arrangement of the R7 microvillar directions along the marginal zone is fan-shaped. Ommatidia lining the dorsal and frontal edge of the eye lack primary screening pigments and have foreshortened crystalline cones. The marginal ommatidia from each eye view a strip that is 5 °-20 ° contralateral to the fly's longitudinal axis and that coincides with the outer boundaries of the binocular overlap. Cobalt injection into the retina demonstrates that photoreceptor axons arising from marginal ommatidia define a special area of marginal neuropil in the second visual neuropil, the medulla. Small-field neurons arising from the marginal medulla area define, in turn, a special area of marginal neuropil in the two deepest visual neuropils, the lobula and the lobula plate. From these arise local assemblies of columnar neurons that relay the marginal zones of one optic lobe to equivalent areas of the opposite lobe and to midbrain regions from which arise descending neurons destined for the the thoracic ganglia. Optically, the marginal zone of the retina represents the lateral edge of a larger area of ommatidia involved in dorsofrontal binocular overlap. This binocularity area is also represented by special arrangements of columnar neurons, which map the binocularity area of one eye into the lobula beneath the opposite eye. Another type of binocularity neuron terminates in the midbrain. These neuronal arrangements suggest two novel features of the insect optic lobes and brain: (1) Marginal neurons that directly connect the left and right optic lobes imply that each lobe receives a common input from areas of the left and right eye, specialized for detecting the pattern of polarized light. (2) Information about the e-vector pattern of sky-light polarization may be integrated with binocular and monocular pathways at the level of descending neurons leading to thoracic motor neuropil.