Those who follow this page closely will know that I have a bit of a thing for frog eyeballs. I did an entire post on them a while back.

But all this time I’ve been getting distracted by frog eyeballs, when perhaps I should’ve been paying attention to frog eyelids…

This is an image of what is referred to as a nictitating membrane. It’s an extra eyelid that some animals have in order to add an bonus layer of protection to their eye while still being able to see. Unlike regular eyelids, the nictitating membrane moves horizontally across the eyeball, acting like a windscreen wiper clearing away dust or debris (fun fact – nictitate is another word for blink). In some animals, like this Rusty Tree Frog, the membrane is semi-transparent, allowing the frog to maintain vision whilst also aiding with camouflage.

But the interesting eyelid facts don’t stop there! Are you ready to have your mind blown? 👀

Go look in the mirror. Do you see those little bits of pink flesh in the corners of your own eyes?

Those are the remnants of the nictitating membrane that our evolutionary ancestors used to have. At some point in history, we outgrew the need for them, and the membrane slowly evolved away – but it hasn’t disappeared completely, and that small bit still remains.

So if you ever need to provide someone with evidence for evolution, you can find it, quite literally, before your very eyes!

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Rusty Tree Frog (Boana boans), Northern Ucayali River, near Nauta, Peru