Survival is crucial for tiny crawling creatures like the spider. And their more than 35,000 species worldwide have survived the ravages of time, adverse climate conditions, and the threat of predators, including insects, small animals, even humans. They have learned to adapt and change over thousand of years. And one of the observable habits that some spider species has adapted to is the ability to move about and hunt at night. But, can spiders see in the dark?
Can spiders see in the dark?
Most spiders (99%) have eight eyes. So, you’d think that they see clearly. But the news (good or bad?) is, they don’t. Most spiders don’t see clearly, they have blurred vision, even during the day. But not all. Some hunters have clear daytime vision. But how about during the night or in the dark — Can spiders see in the dark or at night? Hardly. But not all — there are spiders that see in the dark.
How spiders see in the dark.
Most spiders who move about at night don’t rely on their sense of sight. Spiders who go out at night, often to hunt for food, sense vibrations on the ground and in the surrounding air to detect movement. Those who don’t go out at night but stay on their webs also sense tiny vibrations on their webs to detect movement. When they detect vibrations, that’s the only time they move to go out and check. It’s a sign to them of (possibly) food. It’s like when your doorbell rings — it might be the pizza delivery guy!
Most spiders sense vibrations both on the ground and in the air through fine microscopic hairs on their legs. Their hairy legs act as both their eyes and ears, especially at night. The hairs (or trichobothria) on their legs react to intensity of vibrations and gives the spider an idea of what the moving object might be. Spiders might not be able to see clearly, but their hairy legs can tell that a winged creature (bird or flying insect) is coming, through the changes in the surrounding air. Also, spiders use their hairy legs to smell. They also use their sense of smell to help them navigate in the dark, and detect prey or predator, A spider that shaves her legs is a deaf and blind spider!
Most spiders can see during the day, albeit some need eyeglasses. But spiders’ eyes are not arranged in the same way for all spiders. It’s very fascinating to look at pictures of different species of spiders to see how their eyes line up. Some have four on top and four on the bottom of their face (e.g. Theridiidae, or cobweb spider; and Dictynidae, or mesh web spider). Some have six in front and one on either side of its head (e.g. Philodromidae, or crab spider). There is a species of spider who even has a pair of eyes near the back of its head (e.g. Oxyopidae, or lynx spider). And there are spiders who have two big eyes in front (e.g. Salticidae, or jumping spider) and two or four underneath (e.g. Lycosidae, or wolf spider), plus one on each side of its head. But their eyes are fixed, they can’t move their eyeballs or roll their eyes. So if you force a spider to agree to something it doesn’t like — it can’t roll its eyes!
Spiders, e.g. wolf spiders, who have two big eyes in front of their faces, see more clearly than the others. These are daylight hunters. They are also great night time hunters. These spiders use their big eyes to focus on their prey. They also have two to four eyes underneath the two big eyes to give them an idea how far the prey is. In the case of these jumping spiders, knowing how far its prey is, is important for precision. It needs to visually measure how far it needs to jump to get to the prey. Slam dunk! Their big eyes are also useful to them in the dark of night.
Their big eyes also see in the dark. They adjust to low levels of light and are therefore good night hunters. Their eyes are lined with a tissue (tapetum lucidum) that reflects light. This is how they see in the dark. The tapetum reflects whatever light is gathered by the eye back to the retina to give it night vision. It boomerangs light back to the image perceived by the retina, like a secondary spotlight producing a clearer image. Cats, dogs, and many other night prowlers, like frogs and bats, have tapeta (plural of tapetum) in their eyes. If you shine a flashlight or if the headlight of your car hits a cat’s eyes, you’ll see it reflect back to you, in green, or red, or yellow. Try to shine a flashlight on your garden, do you see tiny reflections of green light returning to you? They’re probably wolf spiders — the night hunters.
There is a family of spiders (e.g. Deinopidae) that developed a pair of huge eyes for extreme night vision. Its name deinopidae is derived from the Greek word meaning ‘fearful appearance’. Because of their huge over-sized eyes they have been called the ogre-eyed spider. They really look ugly. Deinopidae have the largest pair of eyes and the most powerful night vision in the spider world. Their night vision is 1,000 times more powerful than human eyes. Also, they’re more powerful than cats’ or owls’ eyes. And their eyes glow in the dark.
The light-sensitive membrane that powers their night vision dissolves and disappears every morning. And every night for two hours it produces a new membrane to replace it. Then it goes hunting for prey. It’s the Deinopidae’s daily life.
Deinopidae are net-casting spiders. This means, they cast nets in order to catch their prey. They’re like fishermen who cast their net into the sea to catch fish. Only in this case, its a spider net. The net is woven from the same thread that spider webs are made. But they are thicker (10x) and naturally harder to break than the thin spider web. When the net is finally finished and ready for use, the deinopidae puts bait on the ground and hangs itself above the area holding the four corners of the net with four of its legs — ready to fall on its prey. Once an insect or a bug feeds on the bait, the deinopidae quickly falls on it and catches it with its net. It then wraps it all around with the net and injects its prey with venom and digestive enzymes. This all happens in the dark!
The blind spider.
Then there’s the blind spider. This spider (Sinopoda scurion) has no eyes at all. Absolutely no eyes! They live most of their lives in dark caves and lightless places. “Who needs eyes?!” they say. They have mastered the art of motion detection, with enhanced sensors for vibrations on the ground and in the air, and a very keen sense of smell. Another rare species of this eyeless spider is the Braken Bat Cave Meshweaver (Cicurina venii). It’s so rare it was last spotted in the 1980s.
The controversial rare spider.
The Braken Bat Cave Meshweaver hit the news channels in 2012 when a huge $15M underpass construction project in San Antonio, Texas, was stopped temporarily because they were building it on top of the spider’s home. Poor blind spider would have lost its home. But thanks to caring humans (biologists and conservationists) they were able to save the helpless spider. It was only the second time in history that this spider was seen by human eyes, after 1980.
Seeing ultraviolet (UV) light.
Spiders generally see only two color wavelengths, viz. green and UV (ultraviolet). Humans see three, viz., red, green, and blue. You’ve probably seen it before, a sticker on your new color TV with the letters RGB (red, green, and blue). This means that your TV can mix these three color wavelengths to produce all shades of color, including white and black. Colors that the human eye normally sees. Unless you are color blind. And spiders are generally color blind. But they see UV light. Humans can’t.
Some species of spiders depend on UV light for survival. They make use of UV light in their mating rituals. This guarantees that they will reproduce more of their kind. The jumping spider, Consmophasis umbratica, uses UV light in their mating rituals. And both the male and female show physiological responses to the UV glow that they show each other. They get excited! But when scientists used UV blocking light during their mating rituals, both male and female C.umbratica showed no interest in each other. Talk about a wet blanket! It’s complicated.
Night life has become a habit of spiders. One of the main reasons for this behavior is to avoid being seen. They are such socially awkward creatures. What kind of furry creature with eight arching legs, eight dark eyes (two sometimes bulging), and hanging fangs would want to be seen in public during the day?! Funny, no? But more than that, spiders avoid daytime predators, like birds and cats. Hunters like jumping spiders and wolf spiders are more intrepid and dare do their hunting during the day. It’s a risk they’re willing to take, for the thrill of the hunt and the prize. But most spiders are timid and couldn’t imagine being hunted down by predators, so they do most of their crawling and hunting at night. Besides, they don’t have the best eyesight, they argue. And they’ve developed improved sensors and a keen sense of smell to support their nocturnal hunting and feeding habits.
For most spiders, it’s a matter of survival.