Why do spiders have eight eyes?

Since arachnid information seems to be a popular topic these days, we pursued some of the basics about spider eyes.

Apparently not all arachnids actually have eight eyes. Spiders who have eight of them use them all, yet they still don’t see as well as do those of us critters who possess only two. According to this interesting article from the Australian Museum, “How spiders see the world” (there are closeup photos, so don’t look if spiders make you jump), spiders generally see in terms of shadows and light rather than in crisp images.

Since some spiders hunt in daylight, others at night (while the ones we tend to see most often are around at twilight), each of these species has uniquely adapted sets of eyes.

The Burke Museum in Seattle, WA, has a wonderfully detailed section on arachnology. Here’s Why has previously referred to the museum’s Spider Myths page, hosted by museum curator Rod Crawford, to check out the facts and fiction about these common critters.

We owe this instalment of Here’s Why to Mr. Crawford, because when it came to answering “Why do spiders have eight eyes,” we emailed the expert himself. Mr. Crawford kindly took the time to explain this intriguing subject in detail, responding with the following (excerpted):

… there’s no easy answer to the “why 8 eyes?” question. It almost certainly has nothing to do with the 8 legs. Arachnids in general have variable numbers of simple eyes (never compound eyes like insects, which arachnids never evolved). While 99% of spiders do have 8, almost 1% have 6, and a few have 2 or 0. All harvestmen and solpugids have 2. Pseudoscorpions have 2 or 4. Scorpions (the most ancestral arachnid group still surviving) have 2 to 10. Most spiders seem to have standardized on 8, and if there’s a universal explanation for that, no one has it yet.

The functions of the 4 different eye-pairs vary widely among different groups of spiders. Details would be a whole dissertation in itself.

The only other features that invariably come in eights are those that go with the separate legs. One other feature *can* number eight, the ostia, or blood-intake openings in the heart. Most tarantulas and their relatives have 4 pairs of ostia (=8). One very primitive group has 5 pairs while “higher spiders” mostly have 3 or 2 pairs.

As I mentioned somewhere on my site, it really makes more sense to say “4 pairs of” legs or eyes. When a bilaterally symmetric animal has something on one side, it just about always has another on the other side …

Perhaps you’d like to visit one of my other web sites some time, such as the spider collector’s journal http://crawford.tardigrade.net/journal/ (the most complete and best illustrated entries are the most recent).

—Rod Crawford, Burke Museum, Seattle, USA

Many thanks to Mr. Crawford!

Readers are encouraged to visit the sites mentioned above. It’s possible that you’ll see these little critters in an entirely different way once you get to know them a bit.

[Editorial update: The following note arrived in HW’s inbox this evening from Burke Museum’s arachnid curator Rod Crawford.]

I should point out that in that article from the normally unimpeachable Australian Museum, there is one weird error. It implies that crab spiders have good vision, just because they hunt in the daytime. Actually they are “sit and wait” predators, and have some of the tiniest eyes in all spider-dom.
—Rod Crawford, Burke Museum, Seattle, USA

Thanks again to Mr. Crawford for keeping the facts straight!

ADDENDUM Sept. 8, 2010: Read the Burke Museum’s post on house spiders and why we shouldn’t be scared of them, with more information from Mr. Crawford.
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4 CommentsLeave a comment

  1. […] Came across this article: Why do spiders have 8 eyes? […]

  2. Photographer Alvin Chang had some interesting words on this subject, along with a cool photo. Take a look here, unless spider eyes make you faint-hearted: http://alvinchangphotography.wordpress.com/2012/01/03/random-spider-eyes-vs-human-eyes/

    Thanks for your interest, Alvin!

    ~~ Anne

  3. I wondered if the fact that the spiders eyes never evolved past the light and shade type of sensor – did evolution get lazy? Is the spider a case against evolution since the spider has been around for millions of years and yet the comparably new human has evolved a better eye? Just a thought….

  4. is it possible each eye can see something the others cant see such as different colours of the light spectrum ,it seems most likely they all have a job to do other wise most spiders wouldn’t have 8 eyes

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