The surface of a compound eye is seen to be covered with a large number of hexagonal corneal facets, each of which overlies an ommatidium or series of cell elements (fig.
The lateral eyes of Scorpio consist of groups of separate small lenses each with its ommatidium, but they do not form a continuous compound eye as in Limulus.
The compound eye of the king-crab (Limulus) is the only recognized instance of ommatidia in their simplest state.
B, Section through compound eye (after Miall and Denny); C, organs of smell in cockchafer; (after Kraepelin); D, a, b, sensory pits on cercopods of golden-eye fly; c, sensory pit on palp of stone-fly (after Packard); E, sensory hair (after Miall and Denny); F, ear of long-horned grasshopper; a, Front shin showing outer opening and air-tube; b, section (after Graber); G, ear of locust from within (after Graber).
At first the eyes consist of a few distinct facets on either side of the head; they increase in number as growth proceeds, and become aggregated to form the curious compound eye of the adult.