I like this turtle because he looks just a little deranged. Image by Flickr user audreyjm529, via CC.

The daughter of reader Amie Miller wants to know, “Do turtles have eyelashes?”

Short answer: No.
Eyelashes are,
in general, kind of a mammal thing–what with being made out of
hair and all. Besides their usefulness in pseudo-comic flirting
and as a habitat for freakishly awesome mites (as well as their
importance as an advertising revenue stream for Hulu),
eyelashes also work as a trap, catching bits of dust and other
scrapey-ouchy particles before they can reach our
eyeballs.

But as we behold the
motes eyelashes protect us from, we have to consider the beams
that surely must be getting into the eyes
of creatures unfortunate enough to be lash-less.* Poor, little
turtle.

Or not. Non-mammals have their own
way around this problem. They protect their eyeballs with a
nictitating membrane–basically a third eyelid that slides in
horizontally from the side. Besides having a great name, the
nictitating membrane also has some pretty cool features
eyelashes can’t claim. For instance, nictitating membranes are
translucent. Turtles can close their third eyelid completely,
but still see. It’s a skill that’s particularly useful when you
spend a lot of time swimming around with your head mostly below
the waterline–kind of like having built-in goggles. Amphibians
and reptiles have nictitating membranes. So do birds, who use
them to protect and moisten the eye during flight much the same
way that turtles use them in the water.

But
mammals aren’t wholly without nictitating membranes. In fact,
you have the vestigial remnants of one. Called the plica
semilunaris, it’s that little lump on the inside of your eye,
next to your nose. In animals that still use them, nictitating
membranes are associated with glands that secrete
eye-moistening goo. So it’s no coincidence that the “sleepies”
you pick out of your eye in the morning come from the same area
as your plica semilunaris.

For humans,
nictitating membrane may be a thing of the past, but other
mammals still get some use out of it. Harbor seals, which spend
plenty of time underwater, have functioning nictitating
membranes. So do camels, who use the third eyelid for added
protection during desert sand storms. Aardvarks, awesomely,
actually use the thing to keep termites from biting their eyes
while they (the aardvarks) are trying to eat them (the
termites).

Cats and dogs have nictitating
membranes that are somewhere in-between. Their membranes still
exist–and still work–but our pets can’t control them the way
turtles and other creatures can. Instead, nictitating membranes
only show up when the cat or dog is sick, or otherwise messed
up in the head. Case in point, I last saw my cat, Red’s,
nictitating membranes on a drive from Alabama to Minnesota, for
which the vet had prescribed a slightly higher-than-necessary
dose of kitty Valium.