Amazing eyes: 17 vision champions
From eyes the size of a dinner plate to 360-degree vision, these animals boast extraordinarily efficient ways of seeing.
Meet the creatures with the most crafty, strange and sophisticated eyes in the animal kingdom.
1. Tarsiers (Tarsiidae)
These small primates have the largest eyes of any mammal, relative to their body size. Each eye is about the same volume as the animal's brain.
2. Trilobites (Trilobita)
Trilobites lived between 521 and 252 million years ago. They were one of the first groups of creatures to develop complex, compound eyes, where many individual lenses focus light onto groups of photoreceptor cells.
This group of hard-shelled prehistoric creatures had crystal lenses made of calcite, which helped them see very clearly - even in dark ocean water.
3. Dragonflies (Anisoptera)
Some species of dragonfly have more than 28,000 lenses per compound eye, a greater number than any other living creature.
And with eyes covering almost their entire head, they have nearly 360-degree vision too.
4. Geckos (Gekkota)
Nocturnal geckos have superb vision in dim light - their eyes are 350 times more sensitive to colour at night than a human's (see for yourself with our animal vision interactive).
Lacking eyelids, they use their tongue to keep the eye clean.
5. Colossal squid (Mesonychoteuthis hamiltoni)
Researchers believe the collosal squid's eyes are the largest of any living creature, measuring over 27 centimetres in diameter - the size of a football.
6. Reindeer (Rangifer tarandus)
The eyes of Arctic reindeer change colour with the seasons, from gold in the summer to blue in the winter. The technique allows them to make the most of changing light levels in their extreme habitat.
7. Four-eyed fish (Anableps anableps)
Able to see above and below the water line at the same time, the four-eyed fish has eyes that are split in half horizontally.
Each half has its own pupil and retina (the light-sensitive layer of tissue), allowing them to function separately.
Living in the deep sea, these ostracods (a kind of crustacean) never see sunlight. They have the best night vision of any known animal and probably use their eyes to search for bioluminescent animals to prey on.
Gigantocypris has a pair of concave mirrors in each eye, rather than a lens, to focus light onto its photoreceptors.
9. Acanthodes bridgei
A 300-million-year-old fossil of this extinct fish provides us with the oldest direct evidence of colour vision.
The fish's retina has rods and cones, the photoreceptor cells that allow living vertebrates to see in colour. Rods are light-sensitive cells, and cones are sensitive to colour.
10. Ostrich (Struthio camelus)
An ostrich's eye is bigger than its brain, and is the largest eye of any living land animal, measuring five centimetres across.
11. Cock-eyed squid (Histioteuthis species)
These squid have one small, blue eye and one large and yellow, bulging one, creating a bizarre silhouette.
Scientists think the larger eye scans the water above the squid for prey silhouetted against the light, and the smaller eye searches the murky depths below.
12. Chameleon (Chamaeleonidae)
Chameleons have some of the strangest eyes on the planet, which are able to move independently of each other. This results in almost 360-degree vision.
The reptile can also switch between monocular vision - when both eyes are used separately - and binocular vision, when both eyes are used to look at the same scene.
13. California purple sea urchin (Strongylocentrotus purpuratus)
The California purple sea urchin is so packed with photoreceptors on its surface that its body has been described as a single functional eye.
Because nearly the entire sea urchin is sensitive to light, it can 'see' in every direction. Researchers have suggested the sea urchin uses its spines to block out some of the light hitting it, to help focus the light for better vision.
14. Mantis shrimps (Stomatopoda)
Mantis shrimps probably have the most sophisticated vision in the animal kingdom. Their compound eyes move independently and they have 12 to 16 visual pigments compared to our three.
They are the only animals known to be able to see circular polarised light. Experiments suggest they may use this to send messages to each other - for instance, 'this burrow is occupied'.
Camels have eyes that are specially adapted to their hot, dry surroundings.
Two eyelids with very long lashes keep sand from blowing into a camel's eyes. A third eyelid sweeps across from the corner of the eye to help clean the surface.
16. Male striped horsefly (Tabanus lineola)
As well as having massive eyeballs that join at the top of the head, these horsefly males have two different sized ommatidia (the photoreceptor units that form insects' compound eyes).
The larger ommatidia at the top of the eye are more sensitive to UV light, and help him spot the fast-flying female flies, while the smaller ommatidia help him hone in on details.
Read more about the interesting eyes of these much maligned flies in our Curator of Diptera blog.
17. Bacterial cells
Bacteria aren't actually animals - they're single-celled microbes. But they deserve a mention because they are probably the world's smallest and oldest example of a camera-type eye. Camera-type eyes use a single lens to focus light onto a sensitive membrane or retina.
Recent research has shown that the tiny cells of some cyanobacteria (bacteria that get their energy from sunlight) act like a lens. Their spherical body bends incoming light rays, focusing them on the opposite surface. Their entire body effectively becomes an eyeball.
A bacterial cell can sense the direction the light is coming from, and move towards it.