Have you ever looked out into your backyard at night and wondered how many spiders are lurking out there? If you have a flashlight, you can spot them by the creepy green glow of their eyes.
Most spiders have eight eyes. In some species -- mostly those that hunt for their prey, like wolf spiders -- four of those eyes have a iridescent layer behind their retinas, called a tapetum. It's a common structure in the animal world, especially in animals that have evolved to hunt or forage at night, like cats, dogs, deer, lemurs, raccoon, and even some bird species.
When light shines into the eye, it reflects off the tapetum and back onto the retina. More light hitting the retina means that the animal can get the most out of every photon, improving its ability to see in the dark. The reflected light also creates what's called "eyeshine," which is why cats' eyes seem to glow in the dark.
It's also why you can see spiders' eyes glowing bright green in the night, if you know how to look: light reflects off their tapeta, and they're looking in your direction. Enjoy thinking about that.
Follow the Eyeshine
I caught this one myself.
The Boy Scouts have taught this trick for years; they call it "spider sniffing," but professional biologists also use it to find and study spiders that hunt at night. Here's how it works:
Hold a flashlight level with your eyes. You'll look a little silly doing this, but it means the light will reflect from the spiders' eyes at an easier angle for you to spot. Sweep the light across the grass about 15 or 20 feet away. If you see a bright point of eerie green light, that's your flashlight beam reflecting off a spider's eyes. Feel free to walk over and check it out; you'll probably be surprised at how bright the eyeshine from even a tiny spider can be.
In most of North America, you'll probably find mostly wolf spiders, whose venom isn't dangerous to humans, but you should still be careful. You may also see some red reflections out there in the dark; they're probably June bugs or moths.
This one's just minding its business, not staring at you at all, honest. Image: Wikimedia Commons
Looking at You?
Hi, there! Image: Wikimedia Commons
Spiders have eight eyes, but they don't actually see that well. Wolf spiders have the second-best vision in the class Arachnida, after jumping spiders -- but even jumping spiders see only about as well as cats. Although cats have excellent night vision, and they're great at seeing even the slightest motion, their vision isn't actually very sharp. The world looks brighter, but blurrier, to a cat or a jumping spider.
The world looks even blurrier to a wolf spider. To find prey and dodge predators, they rely more on their ability to sense vibrations. That means, of course, that they still know exactly where you are. Especially the one on your shoulder.
Top image: Wikimedia Commons