Not the most serious of questions, but an important question none the
less. Theres a bet riding on it.
A friend has claimed that we have light receptors on the back of our
knees, I think it was for some evolotional reason, something to do
with detecting when it was light when sleeping on your front or
something. But I can't quite believe this. Clearly skin reacts to
light, and I'd imagine that a completely blind person standing in the
open would be able to tell if they where in reasonable sunlight due to
the warmth on the skin. But specific light receptors on the backs of
our knees, seems unlikely.
However I then contended with an equally ridiculous claim, that we
have taste buds on our anus'. Though I think I did hear that claimed
somewhere else once, but I've no idea if its true. I guess they could
be used in detecting something useful about the content of our more
lovely excretions, just to let the body know everythings coming along
nicely. Tenuous? Maybe. But whats important here is that theres is a
bet at stake, and I'd like to win. Can anybody shed any light on the
situation?

The truth of the matter is that some folk have light bulbs fitted at
the back of their knees, so that they can see what's behind them in
the dark.

Not a bad idea!

Having taste buds fitted in your backside is decidedly in bad taste.

But surgeons could no doubt oblige if you can meet their bills.

Check this out:

http://www.genomenewsnetwork.org/articles/08_02/bright_knees.shtml

I don't think there are any taste receptors in the anus (hope not,
considering what the area probably tastes like). However, you might be
interested to know that turtles may be able to breathe through their
butts:

"Aquatic respiration. As far as in known, all chelids have cloacal
bursae (Smith and James, 1958). These are two large diverticula
arising from the side of the cloaca. The cloacal bursae are almost
certainly an organ of gas exchange in water. In other words, the
turtles can respire through their bums. All chelids are probably able
to meet at least some of their gas exchange requirements (oxygen in,
carbon dioxide out) through cloacal respiration, but none can meet all
of their requirements this way. In addition to cloacal repiration,
chelids can also breath though their lungs and probably through their
skin. The morphology of the interior lining of the cloacal bursae is
probably a rough guide as to how much of their total repiration is
carried out through the bursae. Some species have relatively smooth
linings whereas others have highly evaginated linings, the projects
providing a very extensive surface area for gas exchange."

http://www.genomenewsnetwork.org/articles/08_02/bright_knees.shtml

Not only are there light receptor in the skin, I heard a talk a few
years back that showed light receptors in the liver.

Well, many people can tell the next morning if they ate too spicy food
the evening before, which might account for the claim, but I don't
believe that counts as taste buds.  Capiscum (chili pepper) is an
ingredient in hot rub ointments and the like, but I never considered
that I tasted it.  Of course, I don't have any taste buds on my lower
back.

There was a study published in 1998, about light therapy being as
effective for seasonal affective disorder when the light was shone at
the back of the knees as when used directly in the eyes. It received
much publicity as this method is easier, cheaper, less disruptive etc.
Unfortunately it has never been replicated convincingly. Users who
swear by its efficacy, are believed to be responding to the placebo
effect.
The researchers chose the back of the knees for convenience rather
than any theory of light receptors.
BTW ever thought about why mothers have eyes at the back of their heads?!