For most people, they're the stuff of nightmares.

But there's a lot more to them than just slimy bloodsuckers.

Now, the leeches in this tank are medicinal leeches, and

they've got this name for a really good reason.

So for about 2,000 years, up until quite recently, it was

thought that the body's health was kept in balance by

substances called humours which flowed through the body.

Now blood was one of those humours, and if you had bad

blood or too much blood, then it needed to be drained.

And that is where these guys came in.

Draining the blood from the body was known as

bloodletting.

And using leeches to do this was the height of fashion in

Victorian times.

In fact, millions were used every year across Europe.

Leeches are actually worms related to the ones you find

in your back garden.

But these are parasites that suck the

blood of other animals.

And they find a host by detecting shadows and movement

in the water.

So if I dip my fingers in the tank and wiggle them around,

the leeches should come to me.

And there we are.

This is Larry the leech--

or it could be Lucia.

Leeches are hermaphrodites.

They've got both male and female body parts.

Now this one is hungry.

You can see it's desperately trying to find

somewhere to attach.

So I'm going to let it feed on my arm.

When leeches are feeding, they inject anti-inflammatories and

an anticoagulant called hirudin that stops the blood

from clotting.

And it's because of these that they're still

used in medicine today.

They drain blood clots and promote blood

flowing skin grafts.

People say that leeches inject a kind of anaesthetic so the

host can't feel it feeding.

Now, there's actually no good evidence to support this.

And I can actually feel this.

It feels a bit like a mild nettle sting.

And what it's actually doing is using three rows of teeth

in a kind of Y shape.

There's about 100 teeth in each row, and it's using these

to slice through the skin to suck the blood.

It could take up to a couple of hours until it's fully

engorged, so all we can do is sit and wait.

The leech has been feeding for about two hours.

And as you can see it's absolutely massive, but it's

showing no sign of stopping yet.

Now, I don't want to try to pull it off, because if the

leech feels threatened it might regurgitate is stomach

contents back into the wound.

So after nearly three hours the leech has

finally dropped off.

And the wound is going to continue to bleed for at least

two hours afterwards.

Now the leech can last on this single

meal for over 12 months.

So I'll see you the same time next year.

The human volcano--

fire breathing.

It's the most dangerous stunt in the fire eater's

repertoire.

But how does it work?

Well, it all hinges on the oxygen in the air.