Sense Organs

The compound eye

The cockroach carries a pair of large, kidney-shaped (reniform) compound eyes, one on either side of the head, occupying much of its dorsolateral surface. Each compound eye is not a single lens-and-retina structure like a vertebrate eye but a mosaic of thousands of tiny, elongated visual units called ommatidia packed tightly together like the cells of a honeycomb, each ommatidium pointing in a very slightly different direction and each contributing one "dot" to the overall mosaic image.

Structure of an ommatidium

Each ommatidium is a long, narrow cylinder running from the eye's outer surface inward to the optic nerve, made up of the following parts in sequence:

• Corneal lens — the outermost part, a transparent, biconvex thickening of the cuticle itself, which refracts (bends) incoming light; it is secreted by a pair of corneagen cells lying just beneath it.
• Crystalline cone — a clear, cone-shaped body lying just under the corneal lens, formed by four cone cells; together with the corneal lens it focuses light onto the light-sensitive cells below, forming the dioptric (light-bending) apparatus of the ommatidium.
• Retinula cells — typically seven or eight elongated sensory (retinal) cells arranged radially around the long axis of the ommatidium, directly below the crystalline cone. Each retinula cell bears, along the face turned toward the central axis, a fringe of parallel microvilli called a rhabdomere; the rhabdomeres of all the retinula cells of one ommatidium together form a single central rod-like structure, the rhabdom, which is the actual light-absorbing, photoreceptive organelle containing the visual pigment.
• Pigment cells — a sheath of primary iris (pigment) cells surrounds the crystalline cone, and a further sheath of secondary pigment cells surrounds the retinula cells and rhabdom along the length of the ommatidium. These pigment cells contain dark screening pigment that optically isolates each ommatidium from its neighbours, stopping stray light from one ommatidium's cone from leaking sideways into the next.
• Basement membrane and optic nerve fibres — at the base of the ommatidium, the retinula cells' axons pass through a basement membrane and join the optic nerve, which carries the visual signal to the optic lobe of the brain (protocerebrum).

Formation of the image …