We all know spiders have eight eyes. In predator species (those that hunt for prey), spiders have four eyes that have an iridescent layer behind their retinas. This layer is the same as the one that is found in cats. When you look at a cat’s eyes in partial darkness from a certain angle, the eyes give off a glow-in-the-dark effect. This layer is called the tapetum, and is found especially in animals that have evolved to hunt or forage at night. Some more examples of animals that have a tapetum include owls, dogs, wolves, and raccoons.
In the spider world, it is easy to figure out if a spider has a tapetum. Spiders that hunt insects at night, like the wolf spider, all have one to help them see better. In your backyard and night, shine a flashlight into the darkness. If some wolf spiders are hiding out in the grass, you will see a bunch of tiny glow in the dark eyes staring back at you. Some may find this be quite scary!
This funnel web wolf spider uses its excellent night vision to see prey as it scurries by
The reason why this glow in the dark effect happens is that when light shines into the eye, it reflects off of the tapetum and back onto the eye’s retina. This allows the animal to see better in the dark, since more light hits the retina. It is kind of like using a mirror to direct all available light into a camera lens. More light hits the retina, allowing the eye to get the most out of every photon. This way, it can see in the dark much better than animals without a tapetum.
The yellow sac spider is a species of spider that has a tapetum because it hunts insects using its sight, not a web
The glow of the spider’s eye in the dark is so strong it can be seen from 15 feet away! It is important to know that only bright green dots are the eyes of wolf spiders. Any red dots you might see are probably june bugs or moths.