Human eyes (and presumably those of all chordates) can be removed from a socket and look generaly ball-shaped.
Assuming you could get one to hold still without smashing it flat or incinerating it to ash, could you do the same thing with a spider?
I’ve always thought spider eyes were little domes housing only rudimentary nervous structures. However, considering arachnids have been around more than 200 million years longer than mammals, and that arthropods have been around MUCH longer than chordates, it seems more sensible to allow the eyes of a spider are* far more *advanced than our own. A bit of preliminary googling seems to back this up as it turns out spider eyes are not just shiny little photoreceptors, but that at least one of the pairs is dedicated to detecting polarization of light! So, the questions:
Are spider eyes simply lidless windows into hell’s abyss or do they move around in sockets in the way our own do?
What other bizarre things do spider eyes do?
What are the structures of a spider’s eyes? Do they have lenses and irises and such? Or do they have other things unknown to those of us who would prefer to smash a spider as soon as look at it?
Evolution isn’t that “sensible,” and evolved characteristics have nothing to do with longevity of a species’ ancestors. If a spider’s eyes serve it adequately, and no better alternative happened to occur . . . or if a spider with more “advanced” eyes never lived to pass it on . . . then the eyes could very well not have evolved in a long, long time. Comparing the longevity of arthropods vs. chordates is meaningless.
I see your point, but it also makes sense that an additional 200 million years of nature’s trial and error allows for more opportunities for improvement. In that regard the age of a particular line seems relevant.
Slightly off-topic nitpick, but arthropods and chordates first appear at roughly the same time in the early Cambrian period, around 540 million years ago. While it’s true that arachnids predate mammals by a long time, the earliest spider-like fossils “only” predate mammals by about 100 Ma and those might not really be spiders as we think of them today.
You’re thinking about this a bit wrong - the common ancestor of chordates and arthropods was some wormlike thing with eyespots. That is, the evolution of what is the eye in spiders and what is the eye in humans has a common origin, so human eyes have been evolving just as long as spider eyes.
Think about it this way, mammals have only been around for 50 mill years, but that’s not to say that we don’t have ancestors if you go back 60 mya. We do, and they had eyes, and our eyes are the product of their eyes, along with whatever evolution has done in the time since.
Another thing to remember is that mammal eyes are not demonstrably “better” in some objective way than spider eyes. Mammal eyes move around in a socket so they can see a wider range of things around them. Many spider eyes are, as the OP noted, dome-like. Think of this like security cameras: you have some mounted on a swivel that move back and forth to survey the area, then you have ‘dome’ like cameras that film the entire area at once. It could be argued that the ability to see in 360 degrees is “better” in some ways than having to scan that same area. Of course, for our arboreal and predatory ancestors, being able to see straight ahead in 3D was more important than being able to see all around us at once. For a spider, being able to see a predator coming at you from any direction and allowing you to quickly move away is more important.
Imagine having having stationary eyes all around your head like that little gal (for scale, minus legs she is around than 1mm long)
She is a hunting spider, so the six eyes on the front are for movement where the two on the side are purported to actually see in color.
I am sure you have seen the more popular Jumping spider eyes, they do have good stereo vision, some with color and almost 360 degrees of motion detection.
Some spiders (wolf spiders IIRC, but the book this is in is packed away at the moment, so I can’t check) have a special pair of eyes with banana shaped retinas. These appear to be specifically designed for detecting the legs of other spiders of the same species. The bent shape of the retina, in effect, matches the shape of a spider leg. Basically, they are there to tell the spider, if the legs aren’t there (on the little blobby thing in front of me), try to eat it, if the legs are there, try to mate with it. (They have other eyes too, for other purposes.) [Land, M. (1990). “Vision in Other Animals.” In Barlow, H., Blakemore, C., & Weston-Smith, M. (eds.), Images and Understanding. Cambridge: Cambridge University Press.]
Hmm, are you sure about that? Humans ultimately evolved from an ancestor we’d call a “hagfish” if we saw it. Some of those 200 Ma old hagfish went on to rule the world in the form of humans. Others stayed in their original niche relatively unchanged (see also the “if humans evolved from monkeys why are there still monkeys” fallacy).
It would not necessarily be incorrect to call humans a sub-group of hagfish
According to this site, the two large eyes that jumping spiders have in the front do have lenses, but the lenses are fixed in place and not movable. The retina is suspended by muscles, and the spider can shift its field of view by moving the retina around behind the fixed lens. So not an ‘eyeball’ as such, but something quite different for performing the same job.
