Sophisticated eyes evolve because the eye has remarkable powers to help its owner avoid predators, find food, and locate mates. Individuals better at these "big three" are more likely to survive and pass on the traits that have helped in their success. This is the stuff of natural selection, constantly at work to shape an organ over evolutionary time. Several times in the history of animals, very complex eyes evolved with better powers to resolved images of predators, food and mates (See our essay on this topic).

The insect and the octopus are two types of animals that have evolved sophisticated eyes. As in all animal groups, there is not just one refined eye structure found in all species in the group. Instead, insect eyes vary in their abilities to resolve images, just as do the eyes of different species cephalopods, the larger group to which octopus belong. There are insects with no eyes, insects with minimal eyes, deep water octopods with no eyes, and the Nautilus, an octopus relative, has eyes that completely lack a lens. As to which eye is more sophisticated, a highly refined insect eye or octopus eye, this is a bit like comparing a good orange with a good apple. As an examination of their biology reveals, both eyes perform impressive feats of imaging, but with very different eye structures, that yield very different biological results.

The Compound Eye:

If you ever held a dragonfly or fly on your finger long enough to closely examine its head, no doubt you've noticed their eyes are quite large and remarkably unlike our own. These eyes are made up of many divisions, giving the eye a net or mesh like appearance. It is this multitude of divisions that gives it the name: compound eye.

This mesh we see is the cornea, split up in to many facets.

Each facet represents a single visual element, called an ommatidium, that descends into the core of the eye. Each individual ommatidium has:
• A corneal lens: a hexagonal facet with a transparent cuticle that forms a lens.
• Crystalline cone: a group of four cells.
• Pigment cells: that surround the crystalline cone and the retinula cells and separate one ommatidium for the next.
• Retinula cells: visual cells at the base of each ommatidium with a photoreceptor structure called a rhabdom.

Limited Range:

Compound eyes generally allow only a short range of vision. Flies and mosquitoes are very near-sighted, and can see only a few millimetres in front of them with any degree of resolution. Also, although the insect depth of focus is very short, it is nevertheless very broad. The near-sightedness of insects is so extreme that they see detail where we would need a microscope to see. On the other hand, in the human eye, the fovea, or area of sharpest focus, is only as big as our thumb. Unlike the mosquito, we can see extreme detail by focusing in on only a tiny fraction of our field of view. But for a mosquito to have the wide distance vision we have, its compound eyes would need to be roughly 3 feet wide! Tradeoffs!

Insect Eye Diversity

As insects are by far the most diverse and plentiful group of animals on the planet, probably numbering in the millions of species, it's clear they have many different levels of visual acuity. Many insects have incredible bright-light and color vision. Others are color blind. Butterflies rely on color to find food from flowers, and they have color vision that is more enhanced than our own. Dragonflies have one of the most elaborate eyes of any insect, huge domes on the front of the head capable of pinpointing the motion of a small flying insect from several yards distance, and all this while flying at a rapid speed. Honey bees are able to see beyond the spectrum of visible light. They use ultraviolet (UV) light to perceive patterns on nectar-laden flowers that are invisible to us. Many insects including bees can also detect polarized light. Light becomes polarized as it enters the atmosphere at low angles. In this polarized light, vibrating waves are restricted so that they have different amplitudes in different planes. By sensing the different planes of light waves in relation to the sun and earth, flying insects can navigate through an otherwise overwhelmingly large spatial realm.

Some insects have secondary visual perception in addition to their pair of compound eyes. The secondary eyes are simple ocelli, which can sense movement and changes in light.

Compound eyes are by no means a new evolutionary invention. For example, trilobites, arthropods known from the fossil record of 500 million years ago, also had compound eyes.

The Cephalopod Eye:

The octopus eye and the vertebrate eye are extraordinarily similar. Each has a cornea, an iris, an accommodating lens, a fluid-filled vitreous humor, a retina, and so forth. However, there are major differences. First, the photoreceptor cells in the cephalopod eye point forwards toward the incoming light. Our own visual cells point backwards and absorb light bouncing off the back of the eye. Secondly, the cephalopod eye, like other invertebrate eyes, develops as an invagination, or in-pocketing, of the skin. All vertebrate eyes develop as extensions of the brain.