Two forward-facing eyes work well for us but not for everything else. These animals have bizarre and beautiful eyes that allow them to see the world in very different ways.
Ornate wandering spider, Brazil
It seems 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 ornate wandering spider, have eight simple eyes, but some have six or two.
The way a spider grabs its food has a strong influence on the arrangement of its eyes. Web-building spiders, for example, tend to have smaller, evenly spaced eyes, but the most central eyes of visual predators, like our friend here, are often enlarged. To better glimpse a tasty bite with!
There are primary eyes (the bottom pair here) and secondary eyes (the rest), which differ in structure and function. The main eyes have sharp vision, along with retinas that can move behind their fixed lenses, helping the spider track its prey. Meanwhile, the secondary eyes work together to provide a wider field of vision and identify objects and movements of interest.
Red-eyed tree frog, Costa Rica
The red-eyed tree frog likes to sleep all day clinging to the underside of a tropical leaf. To avoid detection, they cover their blue and yellow side stripes with their folded limbs and place their bright orange paws under their bellies.
A transparent lower eyelid, known as the nictitating membrane, slips over the eye to camouflage it and keep it moist. If a hungry bird or snake approaches, the membrane lets in just enough light to make the amphibian aware of it. So it’s show time! Open eyes, stripes, orange paws “in your face” – the frog does its best to dazzle the predator.
It’s a cheeky bluff, because this frog is neither poisonous nor combative. Its eyes are an evolutionary ruse to scare off predators. The hope is that the attacker will jump long enough for the frog to jump.
Peacock mantis shrimp, Indonesia
Why stop at three types of color receptors, like humans do, when you could have a dozen like the mantis shrimp? This is just one of the many quirks of this crustacean’s visual system. Its compound eyes rest on rods, where they are constantly moving and rotating independently. They can see color, as well as ultraviolet and infrared. Each eye has independent depth perception, as well as three black slits or “pseudo-pupils”.
Unlike regular pupils, which are anatomical features, pseudo-pupils are an optical effect created by the structure of the compound eye. These crustaceans construct an image of their surroundings by moving their eyes up and down as they scan a view sideways. It’s a bit like a scanner capturing a photo, only faster and more reliable!
Pink ladyfly, USA
Male pink mayflies, like this one, tend to have larger eyes than their female counterparts, to help them find a mate amid a frantic swarm. Each of his compound eyes is made up of thousands of lenses, all pointing in slightly different directions.
The eyes can detect motion and color, but towards the blue and ultraviolet end of the spectrum. “It’s important to see the sky”, says Dr Luke Jacobus, a freshwater insect expert from Indiana University. “It also helps them orient themselves to get out of the water as they moult from the nymph stage to the next stage.” Additionally, mayflies have three much smaller simple eyes: one in the middle and two on either side of their face. Known as eyespots, they sense light and dark, and possibly day and night.
Elegant Conch, Philippines
The elegant conch lives in the shallow, well-lit tropical waters of the western Indo-Pacific. Herbivorous molluscs have developed large eyes and excellent vision. “The vision of the conch is comparable to that of worker bees, which use their vision for complex flight,” explains Alison Irwin of the Natural History Museum. “It’s really unbelievable.”
Their eyes are on stalks, which improves their field of vision, and their retinas contain six different cell types, which is more than has been found in any other gastropod. Irwin released videos of conches designed to mimic a rapidly approaching predator and found that the animals react strongly to even the smallest of threats. They swim away from predators, such as turtles and crabs, with large jerky movements, but this consumes a lot of energy.
It makes sense that they’ve developed the ability to see fine detail, Irwin says, so they can accurately spot problems from a distance and not waste resources responding to false alarms.
Banded guitarfish, Mexico
Question: How do you know if a guitarfish can see in color? The answer, according to Professor Nathan Hart from the University of Queensland, consists of training these rays to choose between cards of different colors in exchange for a prawn mash as a reward. “Unlike sharks, which seem completely color blind, guitarfish seem able to see color,” he says. “They’re dichromats, so they see the world much like a ‘color-blind’ red-green human.”
The silvery part of the eye is the iris, with its trilobed 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 hues, these bottom dwellers blend in nicely with the ocean floor, but their eyes can be revealing.
The clouding of the pupil makes the guitarfish harder to spot, while the iridescent iris acts as a mirror, helping the ray blend seamlessly into its surroundings.
Spiny pink scallop, Canada
The spiny pink scallop has hundreds of tiny eyes lining 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 arrangement has been compared to optical systems in advanced telescopes, but the end results are less than stellar.
Scallop vision is poor. The mobile marine bivalve mollusk is unable to distinguish objects, but it can at least sense the difference between light and dark. So if a large predatory octopus looms nearby, it can open and close its shells like a pair of false teeth and comically wander away.
Some scallops are also able to sense the size and speed of particles moving past them in the water. Scallops are filter feeders, allowing them to monitor the water for feeding opportunities and then “open wide” at the right time.
Panther chameleon, Madagascar
These are the best boggly eyes in the animal kingdom! The eyes of the colorful panther chameleon reside in twin conical turrets on either side of the head. The upper and lower eyelids are joined, leaving just a small pinhole for light to enter. Below the lids, the eyes can rotate and focus independently, giving this reptile nearly 360° vision and the ability to look at different objects on opposite sides of its head.
When a juicy insect is spotted, the reptile can then switch from monocular to binocular vision, turning its head so that both eyes can focus on the bite. It’s a movement that enhances depth perception, and in less than a hundredth of a second, a sticky tongue swooped out, grabbed the prey and engulfed it. Compared to other reptiles, their vision is particularly sharp. Panther chameleons can spot small insects up to 10 meters away.
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