How many eyes does a spider have?
There have been a lot of movies with giant spiders in them. One of the things that makes them look so creepy is all those eyes they have. But is that just something from the imagination of Hollywood?

 

This St Andrew's Cross spider (Argiope aetherea) doesn't need great eyesight. The web does the main job at catching prey

Most spiders have eight eyes, and the way those eyes are arranged can help scientists work out what kind of spider it is. Of course, that's assuming they're prepared to get close enough.

Spiders usually have some eyes on top of their head* and some more in front. With so many eyes, it's a bit surprising that they usually don't have good eyesight. Instead, they probably just see light and dark shapes moving around. When they’re hunting they make up for that bad eyesight with an excellent ability to sense vibration and taste.

Huntsman spiders have two rows of eyes, with four eyes in each row. The reason why the eyes look so pale in this photo is because this is only a discarded skin.

It gets easier to count the eyes when the spider isn't so hairy. This flower spider
(Diaea variabilis) is a good example. The picture at right is an enlargement of the small rectangle		  

One exception to the bad-eyesight rule is the group of jumping spiders, which see really well. They have very big eyes on the front of their head*. The picture of a typical jumping spider below makes this point pretty clear.


A typical jumping spider

When you look at this picture you'll notice two things:

  • It has a row of big black eyes
  • It's watching me take its photo

Those two things are connected. Jumping spiders see a lot of stuff with those big eyes. They use them to investigate whatever they see moving. That makes them really difficult to photograph because they keep jumping onto the camera lens. When I photograph these guys I almost always end up with the spider either on the camera or on me.

Just to prove that one wasn't unusual, here's another species of jumping spider (below). You can see those big eyes along the front again. But I was able to get a shot of this one when it wasn't staring back up at my camera lens, which makes it possible to see that there are more than just those four eyes.

And here's more of a close-up view to make it easier. Yeah, I know, it's still not all that easy so I've redrawn it showing the eyes in red.

How many eyes do insects have?

*I talked about eyes on the front of spiders' heads. Well, maybe I shouldn't have said that. Unlike insects, which have a head, a thorax (the bit in the middle) and abdomen (end bit), a spider's head and thorax are combined into one big part called the 'cephalothorax'. So I should have talked about those eyes being on the front of its cephalothorax instead. Now don't you just love that kind of accuracy.



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