Why Do Wolf Spider Eyes Glow? Eyeshine and Night Vision

What makes wolf spider eyes shine?

Most of a wolf spider’s eyes contain a tapetum, a layer of reflective crystals behind the retina. Light that passes through the retina without being absorbed hits this layer and is reflected back through the light-sensitive cells a second time. This gives the eye a second chance to capture each photon, which makes it much more sensitive in dim light.

Some of the reflected light escapes back out through the lens. Because the eye works like a tiny retroreflector, the light returns almost exactly in the direction it came from. That is why you see the glow only when the light source is close to your own eyes, such as a headlamp or a flashlight held at eye level. Someone standing a few steps to the side may see nothing at all.

The eyes do not produce any light of their own. It is pure reflection, the same principle as the eyeshine of cats, deer and crocodiles, although spiders evolved their reflective layer completely independently from vertebrates.

How are a wolf spider’s eight eyes arranged?

Wolf spiders have eight eyes in three rows, a pattern that helps identify the family:

The two small eyes in the middle of the front row are the principal eyes, and they have no tapetum. The other six are secondary eyes, which carry the reflective layer and are specialized for detecting movement and working in low light. The big pair in the middle row produces the most noticeable eyeshine.

Together the eyes give wolf spiders a wide field of view, covering the ground in front, to the sides and partly behind them. That helps a spider waiting at its burrow notice prey or a predator approaching from almost any direction.

How wolf spiders use their eyes to hunt

Wolf spiders hunt mostly by sight and by touch, without a web. A spider sitting at the mouth of its burrow at dusk watches for movement and rushes out when an insect comes close. Among spiders, wolf spiders are thought to have some of the best eyesight, bettered mainly by jumping spiders and huntsman spiders.

The trade-off is sharpness. A reflective layer improves sensitivity but blurs the image slightly, because reflected light spreads within the eye. Jumping spiders, which hunt by day, have instead evolved very sharp principal eyes with no tapetum. You can compare them in how well can jumping spiders see?

Wolf spiders combine their eyesight with other senses. Slit-shaped sense organs and fine hairs on the legs pick up vibrations in the ground and air, and in many species males court females by drumming or tapping signals that the female feels through the soil. Studies of the European tarantula wolf spider, Lycosa tarantula, show that it also uses its eyes to find its way back to its burrow after hunting trips.

Spotting wolf spiders at night with a flashlight

Eyeshine makes wolf spiders easy to find. On a warm night, hold a flashlight or headlamp next to your eyes and sweep the beam slowly across a lawn, a field edge or a gravel path. Tiny points of light that twinkle like dewdrops are usually wolf spiders, and in good habitat there can be many of them in a small area. Arachnologists use exactly this method to locate and count wolf spiders.

If you walk toward a sparkle, you will usually find a spider sitting still or waiting at a burrow entrance. In North America a large one may be a Carolina wolf spider, the biggest wolf spider on the continent. Watch from a distance and do not try to pick it up; wolf spiders are not dangerous, but they can bite in self-defense, as explained in are wolf spiders dangerous?

Other small things can reflect light too. Dew, glass and the eyes of some moths can also shine, but moth eyeshine usually looks orange or red, while spider eyeshine is typically bright green or blue-white.

Why night vision matters to wolf spiders

Many wolf spiders, including the Carolina wolf spider, are mainly nocturnal. Hunting at night lets them avoid the heat of the day, reduce water loss and escape many daytime predators such as birds and lizards. In return they need eyes that work in very little light, which is exactly what the tapetum provides.

Not all wolf spiders are night hunters. Small, fast species such as the thin-legged wolf spiders of the genus Pardosa are often active in sunshine on lawns, riverbanks and mountain slopes. Night-active habits are common across the animal world, as you can see in the list of nocturnal animals.