What Do Spider Eyes Look Like? How Many Eyes Does a Spider Have?
The Basics About Spiders
With over 43,000 different species, spiders are one of the most especially diverse organisms on earth. Spiders belong to the order arachnid, which includes spiders, ticks, and scorpions.
Arachnids differ from insects because they have eight legs, while insects only have 6.
To differentiate spiders from other arachnids, scientists look at the animals waistline. Spiders have very tiny waists compared to other arachnids.
How Many Eyes Do Spiders Have?
The answer is - well, it depends on the species of spider, and the number of eyes can even vary in the different species.
The majority of spiders - 99% - have eight eyes. Some spiders have six, four, two, or even no eyes at all!
The arrangement of a spider's eyes can be helpful in identifying what family it belongs to. The number and arrangement of spider eyes does vary greatly within the species.
An excellent article detailing the arrangement and number of eyes related to different spiders is found at BugGuide.net. It not only has great pictures, but the site also has detailed illustrations of the various placement of eyes.
Why Do Spiders Have so Many Eyes?
Spiders' eyes are fixed, meaning they cannot move them to shift their vision. The lens inside the eye shifts slightly.
The front eyes are used for hunting prey, but the side eyes are believed to be used for detecting motion. Being able to detect motion is vital to the spider's safety.
Scientists did an interesting experiment in which they painted over different corresponding pairs of eyes of jumping spiders. When the side eyes were painted, the spiders were unable able to detect motion and did not react to a motion stimulus. When a spider had its front eyes painted, it reacted to motion in the same way a normal-sighted spider does.
The paint was later removed from the eyes with no ill effects to the spider. The scientists surmise that the side eyes are an evolutionary change that keeps spiders from becoming prey themselves.
How Well Do Spiders See?
Most spiders are nocturnal hunters and have poor vision. Their sight is limited to seeing different shades of light and dark. Most spiders have an excellent ability to feel vibrations. They only need to see well enough to build their webs, move around, or to sense danger coming at them.
There are exceptions, though. The free-roaming species of spiders have excellent vision. This group includes jumping spiders, wolf spiders, net-casting and flower spiders.
Daylight Hunting Spiders and Their Eyes
Daylight hunting spiders rely on eyesight instead of sensing prey by detecting their quarries vibrations or smell. They need excellent vision to catch unsuspecting prey unaware.
Some daylight hunters have eyesight that is almost as good as human eyesight. Jumping spiders have much better vision than dragonflies, which are recognized as having the best vision in the insect world.
Researchers believe daylight hunting spiders use their two pairs of "side" eyes to detect the motion of their prey. The spider then uses the two most central eyes to focus on their prey. The pair of eyes next to these are used for depth perception. When the target is close enough, these eyes let the spider know when to strike.
Wolf Spider's Eyes
Wolf spiders hunt at dusk or in the moonlight. Their four large posterior eyes are much like a cat's eyes; their eyes are very sensitive to low-light levels, and their eyes reflect when light is shined on them. This gives the wolf spider a great advantage when hunting in dim light.
Net-Casting Spider's Eyes
Net-casting spiders, Deinopidae, have two huge rear eyes that are very efficient at seeing in low light. Every night a new light-sensitive membrane is produced in the eye, and the membrane is destroyed every morning. The net-casting spider uses its extraordinary eyesight to track and "net" its prey.
The appearance of the two large rear eyes give this spider another nickname - the ogre-faced spider.
There are few species of spiders, like Sinopoda scurion, which have no eyes at all.
These spiders live in caves and other lightless environments. Since they have no need to see, they have evolved into eye-less arachnids. They hunt by feeling vibrations and using their keen sense of smell.
The Braken Bat Cave Meshweaver is a rare, endangered eyeless spider that stopped a highway construction project in Texas. Construction will resume when new plans have been made which will preserve the spiders' habitat.
Awesome Spider Links
- Spider Eye Arrangements - BugGuide.Net
An online resource devoted to North American insects, spiders and their kin, offering identification, images, and information.