Spiders! They are definitely among the most interesting and most diverse creatures in the world of bugs (although not technically insects – they are a separate family). Many would agree they are also the most terrifying.
There are so many interesting things to talk about when it comes to spiders: the way they move, the way they build their nets, all the different places they live in… but today, we’ll talk about something more specific: spider eyes.
So why do spiders have 8 eyes? The short answer is: evolution. Just like every living being on Earth, spiders developed their eyes over time to fit their way of life. Most spiders have 2 large front eyes that help them see shapes and colors. The additional small eyes allow them to detect motion, thus helping the spiders hunt at night.
But do all spiders have 8 eyes? Actually, no. Keep reading if you want to find the answer to this question as well as more curious facts about spider eyes.
How Many Eyes Do Spiders Have?
Well, the question from our title is statistically correct: the majority of spiders have 8 eyes.
However, some families of spiders also have 6 eyes. These include Dysderidae (known as woodlouse hunters), Sicariidae (some sand spiders native to Africa belong to this family), and Oonopidae – the goblin spiders.
Although far less common, some spiders can have 4, 2, or even 0 eyes. The very rare species who lives in caves – Kauaʻi cave wolf spider (Adelocosa anops) actually has no eyes. Just a smooth head.
But why this seemingly random number of eyes? And why 8 in any case? Well, it’s hard to answer exactly why but the reason for the differences is definitely the sheer diversity of spiders.
Currently, there are 49622 species of spiders known to man, classified into more than 100 families. This is a whole lot of different species and not all of them have the same lifestyle. Some are hunters, and some are web-casters. Most hunters hunt during the night, but some also live in areas where there is no light at all – like the spider without eyes we mentioned.
Do House Spiders Have 8 Eyes?
Well, it depends on what kind of spiders you have in your house! If you are thinking about the “daddy long-legs”, also called the cellar spider (officially they are the Pholcidae family), then yes – they most likely have 8 eyes (although some have 6).
These thin spider-guys that like to hang around the corners of houses usually have 2 groups of 3 eyes + 2 additional. However, since they are so small and skinny, you will probably not be able to clearly discern all of their eyes with the naked eye.
Moreover, there are quite a few other species of spiders you might encounter in your home (depending on where you live).
Among the most frequently-encountered is the species called ‘common house spider’ or ‘American house spider’ (Parasteatoda tepidariorum) also have 8 eyes.
Do Jumping Spiders Have 8 Eyes?
Yes, jumping spiders do have 8 eyes.
Jumping spiders also have very good eyesight. Despite the number of eyes, many species of spiders actually don’t have a very ‘high-definition’ eyesight, but jumping spiders are definitely an exception.
They are the perfect example of the potential use of the multiple pair of eyes. Jumping spiders have 4 pairs of eyes, each with a different function.
Jumping spiders are excellent hunters, and they usually hunt during the day – which is why they need such great eyesight. Like most spiders, jumping spiders have a pair of large eyes in the front which let them discern shapes and colors. Another pair of large eyes help this function – but perhaps the most interesting are the ‘posterior lateral eyes’ (PLE). This pair of eyes is set to the side, behind the main eyes and they effectively allow jumping spiders to detect motion with an almost complete 360-degree range.
Why Do Spiders Have So Many Eyes?
As we have already mentioned, spiders have evolved this peculiar eye configuration to fit the environment they live in and the way they live.
One of the main tasks of a spider is to catch food, but not all spiders do it in the same way. For example, some spiders are hunters (like jumping spiders) and some build webs and wait for the prey to come to them. That’s why not all spiders have the same eye configuration, and not even the same number of eyes.
But why exactly 8 eyes? Why not two like humans? Well, one way to think about it is to understand that none of the spider eyes are as effective as the two eyes us humans have. However, when working together, the 8 eyes can really give spiders some special abilities.
For example, the secondary eyes of a spider often let them detect motion amazingly well. Sometimes they can also detect polarized light (which tells them where they are in relation to the sun). Spiders that hunt at night also often have very good night vision.
However, the effectiveness of the eyes relies on the fact that they work together. Different pairs of spider eyes have different functions, and that’s (at least partially) the answer to the question as to why they need 8 eyes.
Types Of Spider Eyes
Yes, spiders have 8 eyes, but not all of their eyes are the same. First of all, it’s possible to differentiate between primary (also called principal) eyes in spiders and secondary eyes.
The primary eyes are the two big eyes on the front of the head.
Secondary eyes are usually smaller and can be arranged in different patterns. The arrangement of the eyes depends on the species of spider. However, they are always arranged in two rows – top and bottom. We can also differentiate between median (middle) eyes, and lateral eyes that are set to the side.
The primary eyes, often also referred to as principal eyes are almost located at the middle of the head (where you would expect eyes to be). These eyes are usually (although not always) the largest and most prominent. Primary eyes always look straight forward and they form the basis of spider vision allowing them to discern shapes and various details. The primary eyes of some spiders allow them to see the world in exceptionally sharp detail, why others essentially see blurry blobs.
Besides the primary eyes, spiders can have up to 3 additional pairs of secondary eyes (hence 8 in total). Just like the primary eyes, the secondary eyes of different spiders can have different characteristics.
However, there is one important way in which secondary eyes greatly enhance spider vision. Namely, the secondary eyes usually have something called tapetum lucidum. Located just behind the retina, the tapetum reflects the light back through the retina.
The primary eyes do not have the tapetum which is why they would appear black even if you would shine a light into them in the dark. The secondary eyes, on the other hand, appear shiny and silvery when turned towards a light source.
Due to this feature, some spiders have excellent night vision. In addition to this, the secondary eyes can allow spiders to better detect movement and have a wider range of vision.
How Spiders See The World: Final Thoughts
Once you start thinking about it, you quickly realize that is really hard to imagine how the world looks like if you are a spider. What does it feel like to have eyes on the side of your head? What is it like to have 8 eyes? What about night vision? Or 360-degree coverage?
Of course, not all spiders have these abilities. Typically, the hunting spiders (especially jumping spiders, for example) have amazing eyesight, while the web-casting spiders tend to rely on other spidy senses. Some spiders can also sense vibrations extremely well which helps them ‘see’ the world in a sense.
For more about these amazing creatures, check out the links below:
Curious Kids: Why do spiders need so many eyes, but we only need two?
Spider Anatomy 101: A Look At The Different Parts Of A Spider
Land M.F. (1985) The Morphology and Optics of Spider Eyes. In: Barth F.G. (eds) Neurobiology of Arachnids. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-70348-5_4
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