Compound eyes are found in arthropods and are comprised of thousands of tiny structures called ommatidia each of which is an independent photoreceptor unit with its own lens and cornea, the image as perceived by the organism in question is an aggregate of each of the images generated by the individual ommatidium, each of which is orientated slightly differently. Compound eyes provide organisms with the ability to detect fast movements and while provisding poor resolution compared with human eyes yield a very large viewing angle in many cases allowing the organism a complete 360 degree view around its body - the starfish Ophiocoma wendtii has ommatidia all over the surface of its skin making its body in effect a giant eye.
Compound eyes are divided into two main types, Apposition and Superposition.
Apposition
Apposition eyes generate a multitude of single incomplete images and can be further classified into two grourps. The first group possesses ommatidia with a lens which focuses light from a single direction onto a structure at the centre of ommatidium called a rhabdom while light from all other directions is absorbed by the wall of the ommatidium. The second group are only found in the order of insect strepsiptera and in this case each lens forms an image which is then combined in the organisms brain.
Superposition
The superposition eye is further divided into three subtypes, the reflecting, refracting and parabolic. The reflecting superposition eye is found only in crustaceans such as prawns, crayfish, lobster and shrimp and comprises a series reflective surfaces or mirrors which focus incoming light onto the rhabdom. The refracting superposition eye is found in nocturnal insects and is of a similar design to the reflecting variety; however, the mirrors are replaced with lenses which focus light onto the rhabdom. The parabolic eye design is found in many arthropods the most common example being the mayfly, parabolic surfaces located on the inside of each individual facet focus light from a reflector lens onto a photosensitive area.
Disclaimer: The author is not a medical doctor, optician, ophthalmologist or any other medical professional, this article has been compiled using a variety of internet reference sources and while every effort has been made to ensure accuracy this cannot be guaranteed. No preference for product or brand is inferred or intended and the contents of this article are not to be used in whole or in part to inform a decision regarding any aspect of contact lens use.
