Two chiral types of randomly rotated ommatidia are distributed across the retina of the flathead oak borer, Coraebus undatus (Coleoptera: Buprestidae)

Abstract

Jewel beetles are colorful insects, which use vision to recognize their conspecifics and can be lured with colored traps. We investigated the retina and coloration of the flathead oak borer, Coraebus undatus, using microscopy, spectrometry, polarimetry, electroretinography, and intracellular recordings of photoreceptor cell responses. The compound eyes are built of a highly unusual mosaic of mirror-symmetric or chiral ommatidia that are randomly rotated along the body axes. Each ommatidium has eight photoreceptors, two of them having rhabdomeres in tiers. The eyes contain six spectral classes of photoreceptors, peaking in the UV, blue, green and red. Most photoreceptors have moderate polarization sensitivity with randomly distributed angular maxima. The beetles have the necessary substrate for complex color vision, required to recognize the conspecifics and suitable for a targeted design of color traps. However, the jewel beetle array of freely rotated ommatidia is very different from the ordered mosaic in insects that have object-directed polarization vision. We propose that ommatidial rotation allows to cancel out the polarization signals, thus allowing stable color vision, similar to the rhabdomeric twist in the eyes of flies and honeybees.