Compound eye

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Compound eye of a dragonfly

Compound eye of Antarctic krill as imaged by an electron microscope

Drawing from Robert Hooke's, Micrographia of the compound eye of a grey drone fly.

A compound eye is a visual organ found in certain arthropods such as insects and crustaceans. It consists of between 12 and 1,000 ommatidia which are tiny sensors that distinguish between brightness and darkness, and sometimes can detect color. The image perceived by the arthropod is a combination of inputs from the numerous ommatidia, which are oriented to point in slightly different directions. In contrast to other eye types, there is no central lens or retina, resulting in poor image resolution; however, it can detect fast movement and, in some cases, the polarization of light.

Each ommatidium consists of a lens and a rhabdom, which consists of several visual receptor cells parallel to each other or slightly twisted. In some species which can see polarization, the cells undergo a half-twist along their length, except for the polarization sensor, which is shorter.

There are two basic types of compound eyes:

The apposition eye which can be divided into two groups. The typical apposition eye has a lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the ommatidium. The other kind of an apposition eye is found in Strepsiptera, in which each lens form an image, and the images are combined in the brain. This is called the schizochroal compound eye or the neural superposition eye (which despite its name, is a form of the apposition eye).
The second type is named the superposition eye. The superposition eye is divided into three types; the refracting, the reflecting and the parabolic superposition eye. The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to the same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This kind is used mostly by nocturnal insects. In the parabolic superposition compound eye design, seen in arthropods such as mayflies, the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied decapod crustaceans such as shrimp, prawns, crayfish and lobsters are alone in having reflecting superposition eyes, which also has a transparent gap but uses corner mirrors instead of lenses.

There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the mysid shrimp Dioptromysis paucispinosa. The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialised retina. The resulting eye is a mixture of a simple eye within a compound eye.

Another version is the pseudofaceted eye. This type of eye consists of a cluster of numerous ocelli on each side of the head, organized in a way that resembles a true compound eye.

The body of Ophiocoma wendtii, a type of brittle star, is covered with ommatidia, turning its whole skin into a compound eye.

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