Early animal eyespots were protein photoreceptors, similar to receptor patches for smell and taste in single cell organisms. Such eyespots could distinguish light from dark, but not direction. Their original function was regulating circadian rhythm.
All animal eyes have a common origin: a proto-eye that could resolve something more than light from dark. Sophisticated optical systems appear abruptly in the fossil record ~540 million years ago during the Cambrian explosion of life forms.
Rapid eye evolution was dictated by competition between predators and prey. Good eyesight quickly became essential. The Cambrian period witnessed nimble innovations in fine-scale anatomy as well as gross morphology.
In the animal kingdom, there are ~38 different body plans, or phyla. Only 6 have eyes. But those 6 body plans account for 96% of all animal species. There are 10 different structural forms of eyes with resolving power: the ability to perceive detailed images.
These different eye layouts vary considerably in architectural aspects. In some instances, similar forms evolved multiple times. But some things about vision are consistent. With rare exceptions animals rely upon sight as a primary sense.
2 eyes are ubiquitous. But starfish have a compound eye at the tip of each arm which can see a low-resolution image.
Of the 10 eye forms, apposition eyes are the most common. The human eye is an apposition eye: individual images from each eye are combined in the mind-brain. Apposition eyes are presumed to be the ancestral form of compound eyes.
Compound eyes are a collection of ommatidia: clusters of photoreceptor cells suffused with support and pigment cells. Each ommatidium is innervated by a single axon, providing the brain with a single picture element.
There are at least 7 distinct forms of optics in the various types of compound eyes. These may form either a single image or multiple inverted images. How multiple images are meaningfully interpreted by the mind-brain is not known.
Arthopods are the phylum that includes slugs, snails, insects, spiders, and crustaceans. There may have been a billion species of arthropod in Earth’s history, with 5 to 10 million still extant.
515 mya the Cambrian seas were home to Anomalocaris, a predator that was a cousin of arthropods. Anomalocaris had compound eyes with snug-fitting hexagonal lenses; 16,000 in each eye. The eyes were on movable stalks. That allowed high-resolution vision equivalent to dragonflies today.
Arthropod species in those same seas had highly advanced compound eyes, each eye with over 3,000 ommatidial lenses and a specialized zone for handling bright light. Such eyes may have had 28,000 photoreceptors, arranged hexagonally, able to provide a full 360° field of vision.
Mental integration of pixelated or partial scene elements, while differing in details, applies to all vision systems. Motion detection is another facet of memory-based visual integration.
Insect eyes and their visual system may account for up to 30% of the body’s mass; far more than most other animals.
Compound eyes are excellent for detecting fast movement, good vision at low light levels, and, in many instances, the polarization of light. Polarized light waves are aligned in a plane. Fruit flies, foraging ants, and bees navigate by the polarization of natural light. Monarch butterflies and locusts migrate thousands of miles, across continents. Even a patch of sunlight on a cloudy day provides sufficient polarization information to navigate.
Detecting polarized light is not solely the province of compound eyes. With simple eyes bats use polarization at sunset to calibrate their internal compass. We have no perception of light polarization.
Cuttlefish are known to be able to detect polarization changes as small as 1°: an incredible acuity. Octopi, shrimp, and other crustaceans manipulate polarized light to send messages.
Flying insects, such as flies and bees, or prey-catching insects, like praying mantis or dragonflies, have specialized zones of ommatidia, organized into a fovea area which affords acute vision. Fovea cells are flatter and the facets larger. Flattening allows more ommatidia to receive light from a specific spot, creating higher resolution.