Dock bug. Close-up of a dock bug (Coreus marginatus), showing its speckled body and one of its large compound eyes (round, upper right). Each compound eye is made up of numerous simple eyes called ommatidia. Each ommatidium sends a signal to the fly's brain, which combines the thousands of images to form a mosaic view of the world. The image is not sharp but is excellent at detecting movement. Dock bugs feed on the leaves and seeds of dock and sorrel plants.