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So I've got a fantasy race of people with four arms. Their second pair is able to reach behind them like how our arms are able to reach in front of us, and so could work with things behind their backs just as well as in front (I've already got their arm anatomy worked out). The only problem is, they'd need to be able to see what they're doing behind their backs, so I thought of giving them a third (and possibly fourth) eye(s) on the back of their head. The extra eye(s) would be independent from their front eyes. I'm not sure though how this would work anatomically though. I'm also not sure how the new eye additions would affect their vision, both overall and with specific regard to focusing on things.
Thanks!!

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  • $\begingroup$ Why would you need anything special? Just have what's on the front on the back. I don't know what sort of skulls your creatures have, but if it's vaguely humanoid all you need to do is have a third eye socket on the back with nerves feeding to the optical center of your species brain. In humans it just so happens that the latter is located at the back of the brain, funnily enough. Speaking of brains, you'll need to allocate more resources to visual processing for the sensory input of the additional eyes. So your creatures will likely have larger visual centers. That's about it. $\endgroup$ – AngelPray Dec 30 '18 at 1:50
  • $\begingroup$ I wrote a terribly long answer, and I'm going to cut it down to one question: how are the legs arranged? Do they all head in the same direction, like a four-armed centaur? Or do the two pairs of feet face different directions, just as the two pairs of arms are (apparently) arranged? My answer depends on yours. $\endgroup$ – bitter lily Dec 30 '18 at 2:30
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Just as well as anywhere else

There is no reason why such eyes should have any difficulty in working and enabling your fantasy people to see just as well with these backwards-facing eyes as their front-facing eyes.

A scientific study implanted an eye into the tail of tadpoles and found that the tadpoles could learn to see using that eye instead of their normal ones. Granted, the scientists removed the normal eyes that tadpoles usually have to make it much easier to identify if the tadpoles are using the new eye, but there is no reason to believe that it would be any different if the original eyes were not removed.

Given that Earth life which evolved to have eyes facing in one direction can adapt to having eyes facing other directions within one generation, there is no reason to believe that such things would be a problem for these organisms.

A few considerations with the eye(s) in back:

  • Is there body fully symmetric? That is to say, is there a distinct front and back to the creature with some meaningful biological difference between them? The answer to this would greatly change how your species views the world and is something you need to figure out.

  • Whether the species has one eye or two in back would likely be determined by if they're naturally predators or prey. Predator species on Earth usually have two eyes close together so that the animal gets good depth perception, so, if your species is a predator, they'd likely have two eyes in the back of their head. On the other hand, prey species on Earth have their eyes spaced widely apart from each other, so that they can more easily see when predators approach, so if your species is prey, they'd likely only have one eye in back, so they can see approaching predators.

  • If this species evolved naturally, then you'd need pretty specific circumstances for why they evolved the ability to work with things behind them. Earth life nearly invariably focuses on things in front and turns around when they need to work on things behind them, and you'd need a good reason for why your life can't do that. However, if this fantasy race was just created by God/the gods, or you're not explaining the world's history that much, you can skip that part.

  • How good is your species at multitasking? If I were given two extra eyes in the back of my head and two rear-facing arms, they wouldn't actually be that helpful for me because I'm limited in what I can do all at once, so I would seldom be able to use both pairs of arms at the same time. Your species would need significantly more capable brains in order to get around that limitation.

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  • $\begingroup$ +1 for the link to the article on backwards-facing eyes on tadpoles - would never have guessed that had actually been done $\endgroup$ – KerrAvon2055 Dec 30 '18 at 9:38
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    $\begingroup$ As for eye placement, I'd like to point you to jumping spiders. They have four eyes in the front, two at the top of their heads, and two on the sides and slightly to the back, for a full hemispherical vision. I'd argue that having eyes in pairs is so much more likely than an unpaired eye ever in prey species, you just set them further apart. The species might opt for a wide stereoscopic FOV in the front while just barely covering 360° monoscopic FOV, resulting in a pattern similar to that of jumping spiders $\endgroup$ – John Dvorak Dec 30 '18 at 20:04
  • $\begingroup$ You could prioritize two separate cones of stereoscopic vision and get a regular human face plus two eyes in the back, but you need to justify the need for back-facing stereoscopic vision, otherwise the eyes will start to migrate outwards... unless of course, the creature does only have three eyes to begin with. Perhaps the third eye started from a sun-sensing ocellum like today's wasps have, which then migrated lower to the back and got repurposed for proper anti-predator vision? $\endgroup$ – John Dvorak Dec 30 '18 at 20:17
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It might be difficult for a human to imagine how such a world would appear to such a creature, but the fact is that organisms can have functional eyes pretty much anywhere - the brain generally figures out how to handle the information regardless of where it comes from (you can hear things that are in front of or behind you and your brain can figure out where the sound is coming from without you getting confused, there's no reason why you couldn't see things that are behind you just as well.) Dragonflies have full 360-degree vision, starfish have an eye on each arm, chameleons can move their eyes independently, and they all manage just fine.

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    $\begingroup$ Also many animals, for instance horses, manage to have nearly 360 degree visual fields with just two eyes. Wide visual fields are quite common in prey animals, especially those that evolved to live in open plains. $\endgroup$ – jamesqf Dec 31 '18 at 4:41

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