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Hi, Amos. Thanks for your excellent question.
The first thing we should say is that while it's true that some spiders have six eyes, most actually have eight.
The short answer to your question is that animals have evolved different eyes that best suit the lives they lead.
Humans have two eyes that face forward. Our eyes are very good at seeing colours and shapes. Having two big eyes in the front of our head means they can work together to guess how far away something is (we call this "judging distance"). That makes it easier for us to catch another animal so we can eat it.
Spiders are also hunters and they need eyes that help them find and catch their food. In fact, most spiders can't see very well, and use touch and taste to explore the world. But the kind of eyes they have tells us something about the food they eat and the lives they live.
Spider eyes for spider lives
Jumping spiders are active hunters, like tiny lions chasing down their prey (bugs). They usually have eight eyes: two very large front eyes to get a clear, colour image and judge distance, and extra side eyes to detect when something is moving. Here's a picture of an Australian jumping spider.
Some spiders make nets to catch their prey. These net-casting spiders also need to see clearly and judge distances. Some have developed huge, scary-looking black eyes that stare straight ahead, so they are nicknamed ogre spiders! These gigantic eyes help the spider to see a wide area and accurately throw down its spider web net to catch its prey. Here's a picture of a net-casting spider.
Some spiders live in caves that are completely dark, where eyes are no use at all. They have to rely on other senses to find their food in the dark. To save energy making eyes, these spiders lost their eyes during evolution, so now some of them have no eyes at all. You can see a picture of a spider like that here.
So why did most spiders end up with so many eyes?
Both human and spider eyes are the result of slowly evolving to help us survive in our different environments. One reason our human eyes are different from spiders is because our bodies and brains are also built differently.
For example, spiders don't have necks. So they can't turn their heads to look at things like we can. Having extra eyes around their heads is one way that spiders see more of the world around them, helping them to quickly spot prey or a potential predator.
Human eyes and spider eyes also do different jobs. Our two eyes are very complex and are good at doing many jobs at once, while spiders have different sorts of eyes that do different jobs.
For example, the large central eyes of jumping spiders are best for seeing shapes, but the simple side eyes have the important job of watching out for predators.
So a two-eyed spider or even an eight-eyed human isn't impossible. But the two eyes we have and the eight eyes most spiders have are perfectly suited to help each of us live our lives just the way they are.
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Authors: Samantha Nixon - PhD, The University of Queensland | Andrew Walker - Postdoctoral Research Fellow, The University of Queensland