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The compound eyes of butterflies consist of thousands of ommatidia, each functioning as a compact optical unit that precisely routes incident light through its distal optics. In each ommatidium, a corneal lens and a gradient-index crystalline cone guide light into the rhabdom, which acts as a photoreceptive waveguide. A fraction of the non-absorbed light is reflected by a basal reflector and relaunched toward free space as eyeshine. Here, we investigate how optical fields evolve as the light propagates through the distal optics. We examine how field transformations depend on wavelength, geometry, and refractive-index profiles, and assess potential directional symmetry of forward and backward propagation. The results clarify how the light entering the photoreceptors is shaped, and how the back-propagated fraction emerges as eyeshine, transitioning into free space from Fresnel to Fraunhofer diffraction regime.