Two forward-facing eyes work well for us, but not for everything else. These animals have some bizarre and beautiful eyes that allow them to see the world in very different ways.
Graceful Wandering Spider, Brazil
It seems that spiders don’t like odd numbers. Two body segments, eight legs, one to four pairs of spinnerets and always an even number of eyes. Most spiders, like this graceful wandering spider, have eight simple eyes, but some have six or two.
The way a spider catches its food strongly influences the way these eyes are arranged. Web-building spiders, for example, tend to have smaller eyes that are evenly spaced, but the more centrally placed eyes of visual predators, like our friend here, are often enlarged. All the better to sample a tasty snack with!
There are main eyes (here the bottom pair) and secondary eyes (the rest), which differ in structure and function. Main eyes have sharp vision, along with retinas that can move behind their fixed lenses, allowing the spider to track its prey. Meanwhile, the secondary eyes work together to provide a wider field of view and identify objects and movements of interest.
Red-Eyed Tree Frog, Costa Rica
The red-eyed tree frog likes to nap all day and cling to the underside of a tropical leaf. To go unnoticed, they cover their lurid blue and yellow side stripes with their folded limbs and tuck their bright orange feet under their bellies.
A transparent lower eyelid, also called the nictitating membrane, crawls over the eye to camouflage it and keep it moist. When a hungry bird or snake approaches, the membrane lets in just enough light for the amphibian to become aware. Then it’s show time! Eyes open, stripes out, orange legs ‘in your face’ – the frog does its best to blind the predator.
It’s a bold bluff, because this frog isn’t poisonous or combative. Its eyes are an evolutionary ruse to deter predators. The hope is that the attacker will startle long enough for the frog to jump away.
Peacock mantis shrimp, Indonesia
Why stop at three kinds of color receptors, like humans, when you could have a dozen, like the mantis shrimp? It’s just one of the many quirks of this crustacean’s visual system. Its compound eyes sit on stems, where they are in permanent motion and spin independently. They can see colours, but also ultraviolet and infrared. Each eye has an independent depth perception, along with three black slits or ‘pseudopupils’.
Unlike ordinary pupils, which are anatomical features, pseudopupils are an optical effect created by the structure of the compound eye. These crustaceans build a picture of their environment by moving their eyes up and down as they scan sideways across a distant view. It’s a bit like a scanner taking a picture, but faster and more reliable!
Pink lady mayfly, USA
Male pink lady mayflies, like this one, usually have bigger eyes than their female counterparts, helping them find a mate amid a frenzied swarm. Each of its compound eyes is made up of thousands of lenses, all pointing in slightly different directions.
The eyes can detect movement and color, albeit on the blue and ultraviolet end of the spectrum. “This is important for seeing the sky,” said Dr. Luke Jacobus, a freshwater insect expert at Indiana University. “It also helps them orientate themselves to get out of the water as they molt from the nymph stage to the next instar.” In addition, mayflies have three much smaller simple eyes: one in the center and two on either side of their faces. Known as ocelli, these detect light and dark, and possibly day and night.
Elegant shell, Philippines
The elegant conch lives in the shallow, well-lit tropical waters of the western Indo-Pacific. The herbivorous mollusks have developed large eyes and excellent vision. “The conch’s vision is similar to that of worker bees, which use their eyesight for complex flights,” said Alison Irwin of the Natural History Museum. “It’s really great.”
Their eyes are focused on stems, which increases their field of vision, and their retina contains six different cell types, which is more than has been found in any other gastropod. Irwin has played shell videos designed to mimic a rapidly approaching predator, and found that the animals are highly responsive to even the smallest threats. They swim away from predators, such as turtles and crabs, by making large jerky movements, but this takes a lot of energy.
It makes sense that they’ve developed the ability to see fine detail, Irwin says, so they can accurately detect problems at a distance and not waste resources responding to false alarms.
Striped Guitar Fish, Mexico
Q: How can you tell if a guitar fish can see in color? The answer, according to Prof. Nathan Hart of the University of Queensland, is by training these rays to choose between different colored cards in exchange for a reward for mashed shrimp. “Unlike sharks, which appear to be completely color blind, guitarfish seem to be able to see color,” he says. “They’re dichromats, so they see the world a bit like a red-green ‘color-blind’ person.”
The silvery part of the eye is the iris, with its three-lobed flap partially covering the jet black pupil. This can retract or advance to alter the amount of light entering the eye, and the cryptic shape is thought to aid in camouflage. With their pebble-colored hues, these bottom dwellers blend in well with the ocean floor, but their eyes can be a giveaway.
Dimming the pupil makes the guitar fish harder to spot, while the iridescent iris acts like a mirror, blending the beam seamlessly into its surroundings.
Spiny pink scallop, Canada
The spiky pink scallop has hundreds of tiny eyes that run along the edge of its two shells. Each eye contains a pupil, a lens, not one but two retinas, and a concave spherical mirror made of guanine crystals. The setup has been compared to the optical systems of advanced telescopes, but the final results are less than stellar.
The scallop sight is lousy. The mobile bivalve mollusk cannot distinguish objects, but at least it can sense the difference between light and dark. So if a large, predatory octopus shows up nearby, it can make its shells clatter open and shut like a pair of false teeth, and comically swim away.
Some scallops can also detect the size and speed of particles moving past them in the water. Scallops are filter feeders, so this allows them to watch the water for food opportunities and then “open wide” at the right time.
Panther Chameleon, Madagascar
These are the best boggly eyes in the animal kingdom! The colorful panther chameleon’s eyes are located in two cone-shaped turrets on either side of the head. The upper and lower lids are joined, leaving only a small hole for light to enter. Under the eyelids, the eyes can rotate and focus independently, allowing this reptile to see nearly 360° and see different objects on either side of its head.
When a succulent insect is spotted, the reptile can switch from monocular to binocular vision, turning its head so that both eyes can focus on the bite. It’s a move that improves depth perception, and in less than a hundredth of a second, a sticky tongue has popped out, grabbed the prey and gobbled it up. Compared to other reptiles, their vision is particularly sharp. Panther chameleons can spot small insects up to 10 meters away.
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