A method for compound eye mapping: Dimensions and regional variations in firefly (Coleoptera: Lampyridae) compound eye

Date of Completion

January 2008

Keywords

Biology, Neuroscience|Biology, Entomology|Engineering, Biomedical

Degree

Ph.D.

Abstract

Fireflies, nocturnal insects that rely on their visual systems for communication, were used as a model for morphological and optical analysis of the insect eye. Main goal of this work was to use a conventional optical microscope to study the facet structure on the hemispherical surface of the compound eye. I developed a mathematical model to describe the optical distortions that result when a curved surface is imaged on flat-field and applied my results to the development of a technique to accurately measure the size and distribution of ommatidia facets of the fireflies eyes. A miniature stage goniometer compatible with a conventional optical microscope was developed to study the size and distribution of ommatidia facets across the compound eye of fireflies (Coleoptera: Lampyridae). The goniometer device was used to take sequential overlapping images of the hemispherical surface along longitudinal great-circle arcs of each eye of Photinus carolinus and Photuris sp. fireflies. Images covering the entire eye required 7 great circle scans at increments of 26° rotation. Using the multiple images, we minimized distortions associated with imaging a spherical surface in 2 dimensions and measured surface features with an error of < 1.5%. A polar plot provided a consistent display format to relate facet area with location on the surface of the eye. I tested the functionality of the goniometer on a number of male and female fireflies. It enabled me to make accurate measurements of surface features and address whether there is sexual dimorphism in firefly eyes. Within my sample my results indicated that male eyes were larger than female eyes. In both firefly species the difference between the two sexes in the eye surface area was found significant with P value < 0.01 (unpaired t-tests). Male and female eyes also exhibited regional variation in facet area. The significance of these structural differences was investigated by determining the optical properties of these structures. Measurements such as facets size and number, eye radii, receptors widths, and local focal lengths were obtained from surface scans as well as thin histological sections. Electrophysiology was used as a mean to extract information from the firefly visual neuro-sensory system and relate it to the eye structure. Electrophysiological recordings obtained from optic lobes (OL) of the twilight Photuris fireflies in response to wavelength and region specific stimuli showed that they have a maximum spectral sensitivity in the green spectrum (λcentral = 540 nm). Spectral sensitivity findings suggest that the number of the green-cells and blue-cells is larger in the compound eyes of the firefly Photuris sp. My findings suggest variations of visual performance across the eye of the Photuris sp. firefly. The facet size in male Photuris sp. was roughly correlated with EP amplitudes of region-specific stimulus (r = 0.56). This study illustrates the effectiveness of applying a bioengineering system for analyzing visual performance in the compound eyes which reflects their visual behavior and specialization. ^

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