How Many Eyes Does a Spider Have? What Do Spider Eyes Look Like?
With over 43,000 different species, spiders (Araneae) comprise one of the most diverse orders on earth. Spiders belong to the arachnid class (Arachnida) which also includes ticks, mites, and scorpions. Arachnids differ from insects in that they have eight legs instead of six and two body segments instead of three.
To differentiate spiders from other arachnids, scientists look at the animals' waistlines. 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. The majority of spiders—about 99 percent—have eight eyes. Some spiders, however, 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. An excellent article detailing the arrangement and number of eyes possessed by different spider species is available at Bug Guide. It includes closeup photos of the eyes of a variety of species as well as detailed illustrations of the various eye arrangements seen in different spider families.
Why Do Spiders Have so Many Eyes?
Spiders' eyes are fixed, meaning they cannot move them to shift their vision. The lenses inside spiders' eyes can, however, shift slightly. Generally, front-eyes are used for hunting prey, while side-eyes are believed to be used for detecting motion. Being able to detect motion is vital to a spider's safety.
An interesting experiment was performed in which researchers painted over various corresponding pairs of jumping-spider eyes. Spiders whose side-eyes were painted were unable able to detect motion and did not react to a motion stimulus. Spiders whose front-eyes were painted reacted to motion in the same way a normally sighted spider would.
The paint was removed from the spiders' eyes with no ill effects. The researchers who conducted the study surmised that spiders' side-eyes are an evolutionary development that helps them detect potential predators.
How Well Do Spiders See?
Most spiders are nocturnal hunters and have poor vision. Their sight is limited to the ability to see 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, and sense potential danger in their vicinity.
There are exceptions, though. The free-roaming spider species tend to have excellent vision. This group includes jumping, wolf, net-casting, and flower spiders.
Daylight Hunting Spiders and Their Eyes
Daylight hunting spiders rely on eyesight rather than web vibrations or smell to capture their prey. They need excellent vision to catch unsuspecting insects without being detected as they approach.
Some daylight hunters have eyesight that is almost as good as humans'. Jumping spiders have much better vision than dragonflies, which are recognized as having the best vision in the insect class.
Researchers believe daylight hunting spiders use their two pairs of side-eyes to detect the motion of their prey. The spiders then use their 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 Spiders' Eyes
Wolf spiders hunt at dusk or in the moonlight. Their four large posterior eyes are much like a cat's eyes—they are very sensitive to low light levels, and they reflect brightly when light is shined on them. This gives the wolf spider a great advantage when hunting in dim light.
Net-Casting Spiders' Eyes
Net-casting spiders—family Deinopidae—have two huge rear eyes that are very efficient when it comes to seeing in low light. Every night, a new light-sensitive membrane is produced in the eye, and every morning, the previous night's membrane is destroyed. The net-casting spider uses its extraordinary eyesight to track and "net" its prey. The appearance of the two large rear eyes gives these spiders another nickname—the ogre-faced spiders.
Do you like spiders?
There are a few species of spiders—like Sinopoda scurion, a cave-dwelling huntsman—that have no eyes at all.
Eyeless spiders usually live in caves and other lightless environments. Since they have no need to see, they have evolved into eyeless arachnids. They hunt by feeling vibrations and using their keen sense of smell.
The Braken Bat Cave mesh-weaver is a rare, endangered eyeless spider that stopped a highway construction project in Texas. Construction will resume when new plans have been made to preserve the spiders' habitat.