More doesn’t always mean better. One good example of that is spiders and their eyes.
Most spiders have eight eyes, but most don't have good vision. Instead, spiders rely on their vision along with their other senses, like taste, touch and vibration.
More than 34,000 different spider species exist, and one of the ways they are categorized into families is by the number of eyes they have and how they are arranged. While most spiders have eight eyes arranged in pairs, not all do. Some spiders do not have any eyes, while others have as many as 12. Spiders that have fewer or even no eyes are still able to hunt and stalk prey, but they have different adaptations to help them find food.
For the most part, when spiders have more than two eyes, all their eyes are used for vision. However, not all eyes see in the same way. Typically, the eyes most central to the spider's face are used to detect the size, shape and color of nearby objects. Eyes further to the sides of the head detect motion.
While many spiders have poor vision despite their many eyes, some do have good eyesight. These spiders rely on their good vision to hunt for and capture their food, as well as recognize potential mates and rivals. Among the spiders with good eyesight are wolf spiders, flower spiders, jumping spiders and net-casting spiders.
Some spiders that are active at night, including wolf spiders, have eyes specially designed to sense movement in the dark. A wolf spider's eyes even shine or glow when light shines on them.
Jumping spiders, which are often active during the day, rely on their eyesight to hunt for prey, rather than catching it in a web like some spiders do. Because of this, one of their pairs of eyes on the side of their heads is important for what is called a "looming response." These eyes detect motion, which helps the spider sense danger. Humans and other animals have a looming response too — it's why you duck when someone throws something at you. However, we have just one pair of eyes to collect and provide visual input, while spiders use multiple eye pairs.
Another kind of spider with specialized eyes is the net-casting spider. These spiders have two large eyes toward the back of their heads that are used for night vision. Their eyes are large to both allow as much light in as possible and to provide for a wide field of vision.
Eyes aren't the only thing spiders have eight of. Spiders also have eight legs arranged in four pairs. The four pairs of legs are what separate spiders and other arachnids — including ticks and scorpions — from insects, which have three pairs of legs.
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