Structure of the Compound Eye
The compound eyes are bean-shaped structures located below the lateral spines of the prosoma. The eyes have nearly 1,000 facets in the adult limulus, and have a distinct "pseudopupil" that varies in size with light and dark.
The underside of the compound eye cuticle consists of cones, which are added at the anterior edge of the eye as the horseshoe crab molts. These cones serve to concentrate the incoming light toward the blunt tip and the underlying ommatidium.
